Plectronoceratids (Cephalopoda) from the latest Cambrian at Black Mountain, Queensland, reveal complex three-dimensional siphuncle morphology, with major taxonomic implications

Geological background

The material described here comes from Black Mountain (Mount Unbunmaroo), which is situated 55 km northeast of the nearest town, Boulia, in the Burke River Structural Belt within the southeastern part of the Georgina basin in western Queensland (Fig. 1). Black Mountain, Mount Ninmaroo, Mount Datson and Dribbling Bore make up a faulted and folded belt trending south southeast. The section at Black Mountain has been the subject of numerous faunal studies around the Cambrian–Ordovician boundary interval and has played an important role in regional and global biostratigraphic correlations (Druce & Jones, 1971; Jones, Shergold & Druce, 1971; Shergold, 1975; Druce, Shergold & Radke, 1982; Shergold et al., 1982, 1991; Nicoll & Shergold, 1991; Ripperdan et al., 1992; Shergold & Nicoll, 1992; Zhen, Percival & Webby, 2017).

The earliest occurrence of cephalopods at Black Mountain is within the Ninmaroo Formation, which overlies the Chatsworth Limestone (Fig. 2). The origin of the specimens is indicated by horizon numbers (BMT1, BMT 2, etc.,) given by Mary Wade and written on nearly all specimens as noted in the Queensland Museum collection database. Furthermore, she plotted the numbered horizons on the stratigraphic chart from Druce, Shergold & Radke (1982). The plectronoceratids come exclusively from layer BMT 1, which is situated at around 500 m in Druce, Shergold & Radke’s (1982) section, within the middle part of the Unbunmaroo Member of the Ninmaroo Formation. In their chart, this horizon corresponds to the lower Datsonian regional stage and the Mictosaukia perplexa trilobite zone and Cordylodus proavus conodont zone. Following the revised conodont zonation of Nicoll & Shergold (1991), BMT 1 lies within their Assemblage Zone 3 (Hirsutodontus appressus), as there is no evidence of C. proavus lower than the upper Unbunmaroo Member. This zone lies within the upper part of the Eoconodontus Zone and can thus be correlated with the Cambrooistodus minutus Subzone in Laurentia (Miller, 2020). The base of the Datsonian is defined by the lowest occurrence (= LO; see Landing et al. (2013) for problems of the FAD concept) of C. proavus, so horizon BMT 1 is late Payntonian rather than Datsonian in age. Iapetognathus fluctivagus, the primary marker for the base of the Ordovician is absent from the Black Mountain section, complicating the global correlation of the Cambrian–Ordovician boundary at this locality, although the currently available evidence places it within the Cordylodus lindstromi Zone that also marks the base of the Warendian (Nicoll & Shergold, 1991; Shergold et al., 1991; Ripperdan et al., 1992) or slightly higher (Zhen, Percival & Webby, 2017). In any case, the horizon BMT 1 is dated well into the Cambrian Stage 10, but potentially slightly younger than the cephalopod occurrences from North China, where the coeval Mictosaukia Zone has been found to be essentially devoid of cephlaopods (Chen & Teichert, 1983; Fang et al., 2019). Further evidence for the late Cambrian age of BMT 1 is the fact that Eoconodontus notchpeakensis, its LO below the onset of the HERB carbon isotope excursion being a potential marker species for the GSSP of the Cambrian Stage 10 (Landing, Westrop & Miller, 2010; Landing, Westrop & Adrain, 2011; Miller et al., 2015), occurs between the assemblage zones 2–4 of Nicoll & Shergold (1991). In comparison, the cephalopod-rich Wanwankou Member of the Fengshan Formation in North China correlates with the upper Proconodontus muelleri Zone and lower Eoconodontus notchpeakensis Subzone (Chen & Teichert, 1983). In their global correlation of the Cambrian–Ordovician boundary, Geyer (2019) and Miller (2020) placed the boundary within or at the base of the C. lindstromi Zone, respectively. Plectronoceratids appear to be slightly more common than ellesmeroceratids in BMT1, although we do not know whether this faunal composition is genuine or if it represents a difference in sampling effort. BMT 2 is situated in the lowermost C. lindstromi Zone, i.e., probably very close to the Cambrian–Ordovician boundary but contains no plectronoceratids and is thus not investigated here.

Morphology

Terminology and abbreviations of conch parameters largely follow Pohle et al. (2022, supplementary information) and are listed in Table 1. Measurements of the Black Mountain material were taken with digital callipers. To compare species from China, North America, and Russia, we took the published measurements and ratios from the original species descriptions where available, thus reflecting intraspecific variation as conceived by the authors. We measured additional data points from our own photos or directly from the original illustrations using ImageJ (Rueden et al., 2017). Note that in the Chinese literature, expansion rate is usually given as a ratio, e.g., an expansion rate of 1:5 corresponds to a conch that expands by 1 mm within a length of 5 mm. We calculated angular expansion rates from these values, using the following formula for an expansion ratio of a:b (after Pohle & Klug, 2018):

$$ER=2\times {\tan }^{-1}\left({\displaystyle \frac{0.5\times a}{b}}\right)$$

As expansion differs in lateral and dorsoventral directions in conchs with constant non-circular cross-sections (which is always the case in the material studied here), we differentiate between height expansion rate ER_h and width expansion rate ER_w. For most previously described species, which are only available as longitudinal thin sections assumed to represent the median plane, we use only ER_h.

Despite the previous description of more than 60 species, ontogenetic trajectories of important conch parameters have never been plotted for Cambrian cephalopods; we provide them in this article for the first time, using conch height (ch) as proxy for the ontogenetic stage. Besides expansion rate (ER_h and ER_w), we compared conch width index (CWI), relative cameral length (RCL) and relative siphuncular diameter (RSD). We did not distinguish between the diameter at the septal foramen and the siphuncular segment, because the difference is usually less than 1 mm, making it difficult to obtain accurate measurements, particularly when sections are potentially misaligned, or when the siphuncle has been subject to significant weathering, as is often the case in the Australian material. Ontogenetic trends were tested by using simple linear regression models, taking conch height as the independent variable. Where significant trends could be observed, the linear regression models were incorporated into species diagnoses, providing expected values of conch parameters at certain conch heights. Thus, in the case of significant p-values, intercept and slope describe the direction and position of the linear trendline, while R² represents the proportion of variation that can be explained by ontogenetic variation and σ is the standard error of the regression, which can be thought of as the average spread of values from the trendline. From published material, we usually recorded only a single ontogenetic stage, to weight each specimen equally. The only exception to this approach was made where only very few specimens of a species were available; in those cases, we incorporated multiple measurements per specimen into the regression models. To test if the slopes and intercepts of the ontogenetic trajectories are significantly different from each other, we included the species as interaction terms into the regression models and used analyses of variance (ANOVA) to determine p-values. We exclusively used this approach where conch parameters showed trends throughout ontogeny. Intercepts were only compared where no significant difference in slope was detected, as the intercept (value of conch parameter at conch height of 0 mm) is only meaningful when the slopes are parallel, i.e., the difference is constant throughout ontogeny. Because significant and non-significant p-values of the regression coefficients and intercepts may also result from preparation and preservation biases, sample size or scatter of the data, this approach requires careful interpretation of the results and should not be used as single discriminator between species.

We compare the distribution of maximum phragmocone size and body chamber size (where known) to investigate differences in conch size between populations. The specimens were grouped according to their geographic and stratigraphic occurrences, to show temporal and spatial variation between populations or species.

Species delimitation

As the great majority of Cambrian cephalopod species have been described from China, comparison with these is essential. Unfortunately, most Chinese specimens are only available as longitudinal thin-sections (the original blocks are probably lost) and in most cases, it is difficult to reconstruct the exact orientation of the plane of section relative to the median plane. Examples of the result of misaligned sections have already been shown by Wade & Stait (1998), Mutvei, Zhang & Dunca (2007) and Mutvei (2020). Consequently, the diagnostic value of many characters is questionable, since other species would have to be sectioned exactly in the same plane (which is usually impossible to do) to be comparable. Unfortunately, this renders many Chinese taxa unrecognisable, until more material is studied to clarify the extent of (three-dimensional) variation. Nevertheless, declaring nearly all Chinese species as nomina dubia is undesirable because it would render most Chinese Cambrian cephalopods unusable for taxonomic purposes, thus ignoring a large part of the hitherto known morphological variation. Instead, we adopt a pragmatic approach, identifying which features may vary due to misaligned sections, based on comparisons with the three-dimensionally preserved siphuncles of the Australian plectronoceratids. If it appears likely that diagnostic characters of genera and species merely represent differences in the plane of section, we identify synonymy with other taxa. Additionally, we compare the ontogenetic trajectories to assess inter- and intraspecific variation. This approach shows the variability between species as conceived by their original authors, revealing whether they form distinct groups that can be separated based on conch parameters. While some errors in conch parameters due to non-aligned plane of section are to be expected, we assume that these affect all specimens with a similar probability. Thus, the expected errors are more likely to be random than systematic, although the exact impacts on conch parameters are difficult to predict. As plectronoceratids have been reported from localities widely dispersed across China and other parts of the world, we compare material from different geographical regions to investigate whether there are differences between populations. We make similar comparisons of material from different stratigraphic levels. Additionally, we compare body size distributions from different localities and stratigraphic levels, with particular reference to preserved body chambers and their diameters.

When considering variation due to the plane of section, we refer to a Cartesian coordinate system (Fig. 3). Palaeozoic cephalopods are most commonly investigated using median sections, which corresponds to the yz-plane in our coordinate system. However, this ideal case is, in practice, difficult to achieve, especially for small specimens that are still embedded in matrix. The true plane of section can thus differ from the ideal median plane in a combination of three principal ways:

• x-translation: parallel shift of yz-plane along x-axis to off-centre position (may result in apparent differences in siphuncle size and position).

• y-rotation: rotation of yz-plane around y-axis (may result in apparent ontogenetic changes).

• z-rotation: rotation of yz-plane around z-axis (may result in apparent differences in siphuncle size, septal concavity, or expansion rate).

Cambrian cephalopods have also been investigated based on cross-sections (e.g., Xiaoshanoceras subcirculare Chen & Teichert, 1983, pl. 17, fig. 5) or coronal sections (e.g., Balkoceras gracile Flower, 1964, pl. 3, fig. 15). These two planes correspond to the xy-plane (possible modifications: z-translation, x-rotation and y-rotation; may result in apparent differences in cross-section shape) and xz-plane (possible modifications: y-translation, x-rotation and z-rotation; may result in apparent differences in siphuncle size, shape or ontogenetic changes), respectively.

Based on these considerations, we evaluated which of the characters commonly used for generic and specific diagnoses are likely to be influenced by misaligning the plane of section. Only characters that are unlikely to be affected by sectioning can be used with certainty for species identifications. Those characters that were identified by us to be susceptible to variations in alignment of the plane of section, are only cautiously used for taxonomic purposes. Accordingly, established diagnoses were subjected to the following questions, which guided us in our revised taxonomic classification of plectronoceratids:

• 1)

  Are there any discrete characters that allow for the distinction between species other than those features that can be explained by a variation in alignment of the plane of section?

• 2)

  Are there distinct groupings of conch parameter distributions or ontogenetic distributions that could be used to distinguish between species?

• 3)

  Are the distributions of conch parameters and ontogenetic trajectories different between regions and/or stratigraphic level?

Synonymies proposed herein are intended as a useful reconsideration of previously available data with a view to better understanding of biological speciation. An improved taxonomy of plectronoceratids must include 3D-reconstructions through either µCT scans or serial grinding tomography, ideally of many specimens from the original localities to accurately assess variation of the siphuncle in three-dimensional space.

The electronic version of this article in Portable Document Format (PDF) will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: urn:lsid:zoobank.org:pub:F1C67134-9A19-4D18-AB74-B984B5555D40. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central SCIE and CLOCKSS.

Siphuncle morphology

The siphuncular segments are strongly oblique (Fig. 7D). In combination with the very short chamber length, this means that individual segments are strongly elongated dorsoventrally despite the cross-section of the siphuncle being roughly circular. In transverse section, three or four segments may be visible at the same time (Figs. 7G, 7J). The segments are expanded, resulting in cyrtochoanitic septal necks (Figs. 7K–7M). It is not quite clear how the septal necks join the shell wall ventrally, but from longitudinal and cross-sections they appear to become straight (Figs. 7G, 7J, 7K). It is likely that the septal necks adnate to the shell wall midventrally, so that the foramen is open towards the shell for a short distance. This would explain why most specimens are preserved with the siphuncle exposed along the venter as an external mould, though the question remains whether the connecting ring would be closed ventrally (thus directly overgrowing the shell wall). In internal moulds of the siphuncle, the segments end in a relatively acute angle ventrally (Fig. 7E), which is another indication that the septal necks are unlikely to be cyrtochoanitic throughout.

The deviation from typical cyrtochoanitic septal necks is more obvious in the dorsal area of the siphuncle. Towards the dorsal side of the foramen, the septal necks are elongate adapically, resulting in a triangular shape (Figs. 7A, 7B). Mary Wade referred to this structure as a “septal flap”, a term which is followed here. There is some overlap between successive septal flaps (Fig. 7A), creating a holochoanitic or even macrochoanitic appearance of the septal necks. However, smaller specimens appear to have shorter septal flaps (Fig. 7B), suggesting an ontogenetic trend towards an elongation of the septal flap, although this is currently difficult to assess due to the limited amount of material preserving septal flaps and different states of preservation. We found no indication of clear growth-independent morphological groups of septal flaps and as there are no other distinguishing characters, we regard those specimens as growth stages of one species. Note that in terms of shape, the septal flaps do not represent a transition from cyrtochoanitic to orthochoanitic septal necks, but rather consist of tilted cyrtochoanitic septal necks directed laterally rather than adapically as evident from natural and artificial cross-sections of the siphuncle (Figs. 7G, 7J). Correspondingly, the segments expand laterally, creating disconnected siphuncular bulges dorsolaterally of the septal flap (Figs. 7F, 7G, 7J).

Most specimens are preserved with at least part of the siphuncle exposed, due to its position on the ventral shell margin. In specimens, where the siphuncle is exfoliated (i.e., where an external mould is evident), there are essentially two types of preservation. In the first, the septal flaps are preserved on the dorsal side of the siphuncle and may overlap each other (Figs. 7A, 7B), while the siphuncular segments are visibly expanded into the chambers. In the second type, a small ridge is present middorsally, with traces of the laterally expanded siphuncle but no visible septal flap (Fig. 7C). These different appearances might lead one to conclude that the original siphuncles were structurally different and thus could be used for species discrimination. However, the ridges in the second type likely represent the imprints of the ends of the siphuncular bulges, as they fit very closely to internal moulds of the siphuncle as seen from the dorsal side (Fig. 7F). Here, the converging siphuncular bulges leave a small gap middorsally, which would result in exactly this ridge. This can also be seen in a heavily corroded specimen, where both preservation types occur in the same individual (Fig. 6D).

Imprints of diaphragms can be seen at the adapical end of several phragmocone fragments (Figs. 7H, 7I), as well as in sectioned specimens (Figs. 7J–7M). The diaphragms are convex towards the apex, with a central ridge that is about as wide as the septal flap, but not extending to the venter. While the lateral parts of the diaphragm appear to be parallel with the septum, the central ridge is more perpendicular to the growth axis, thus traversing at least one or two further adapical segments. Consequently, the lateral parts of the diaphragm appear to slope ventraperturally in longitudinal sections (Fig. 7K) while the ridge is directly transverse (Fig. 7M). In another specimen, the ridge is crossed by a narrow furrow along the median plane, passing from the dorsum to about the middle of the diaphragm, thus splitting the dorsal part of the diaphragm essentially in half (Fig. 7H). At the dorsal end, the two halves of the ridge diverge, which leaves a triangular space likely corresponding to the mould of the septal flap. In some specimens, the central furrow reaches across the entire ridge, to the ventral side of the siphuncle (Fig. 7E). The influence of taphonomy is not entirely clear in this case, both specimens are somewhat corroded, allowing for the possibility that the rest of the furrow is simply not preserved, nor is the influence of growth clear, with the two specimens representing different ontogenetic stages. The diaphragms reported here are structurally complex, and their common characterisation as “concave”, “conical”, “directly transverse” or other simple terms are probably not enough to capture their variability, as different structures may appear from differently oriented sections of the same species or even specimen.

Ontogenetic trajectories

We assign 210 specimens from horizon BMT1 to Sinoeremoceras marywadeae sp. nov. The ontogenetic trajectories of conch parameters show continuous variation among the material, making it impossible to use them for species delimitation. The conch width index (CWI) is constant throughout ontogeny, with some variability, which may partly be due to suboptimal preservation or weathering (Fig. 4A). Cameral length is usually less than 1 mm throughout ontogeny so promoting a decreasing relative conch width (RCL) (Fig. 4B). The relative size of the siphuncle (RSD) apparently slightly decreases during ontogeny (Fig. 4C); however, this could be a taphonomic artefact, as larger specimens tend to be more heavily corroded, making it difficult to measure their siphuncle. Weathering and the associated uncertainties in measurements are also the likely causes for the relatively high variation in siphuncle size. Expansion rate decreases with a relatively high variation (Fig. 4D), very likely another consequence of weathering, notwithstanding the difficulties of measuring expansion rate in cyrtoconic specimens and the relatively high error potential when calculating conch angles (Pohle & Klug, 2018).

When compared to co-occurring ellesmeroceratids, the trajectories differ significantly (p-value for two-sample t-test of (constant) CWI < 0.001 and p-values of ANOVA of the slope of the linear regressions of RCL, RSD and ER_h < 0.001), although they are partially overlapping (Fig. 4). Ellesmeroceratids from Black Mountain are easily distinguished by their invariably tubular siphuncle. Ellesmeroceratids have a very slightly wider conch cross-section (Fig. 4A), longer cameral lengths (Fig. 4B), a narrower siphuncle (Fig. 4C) and a slower expansion rate that is only slightly decreasing during ontogeny (Fig. 4D).

To investigate the potential impact of small differences in alignment of the plane of section, we compared the ontogenetic trajectories of both halves of a longitudinally sectioned specimen of Sinoeremoceras marywadeae sp. nov. (QMF 13992) and two thin sections of previously described protactinoceratids assigned to Sinoeremoceras foliosum Chen & Qi in Chen et al., 1979a (NIGP 46128) and Physalactinoceras qiushugouense Chen & Teichert, 1983 (NIGP 73801), respectively (Fig. 9). The sections of QMF 13992 are only about 0.5 mm apart, but the variation seen between them is about as large as the variation seen between the two other species, which supposedly belong to separate genera. While the ontogenetic trajectories of cameral length and siphuncular diameter (Figs. 9B, 9C) are very similar in all four specimens (counting the two halves of QMF 13992 as separate specimens), the expansion rate is higher and decreases more slowly in the Chinese specimens than both halves of the Australian specimen (Fig. 9D). Applying ANOVA to the regression coefficient of the conch parameters reveals that the slopes of RCL and RSD in the Chinese and Australian specimens are significantly different (p-value < 0.001), but not for ER_h (p-value = 0.64), which is likely caused by the large scatter of ER_h in comparison to RCL and RSD.

In general, the ontogenetic patterns of Cambrian plectronoceratids from elsewhere in the world are congruent with those seen in S. marywadeae sp. nov., i.e., constant CWI, decreasing RCL and ER, and constant RSD, although the latter is somewhat more variable (Fig. 10). Ontogenetic trajectories show some overlap between different localities and stratigraphic ages, but the inferred slopes and intercepts of RCL and ER are distinct, though not always significantly (Tables S1–S4). Stratigraphically older specimens, e.g., from the Ptychaspis-Tsinania or the Quadraticephalus zones in North China, show a more rapid decrease in RCL and lower ER at comparable conch diameters. However, when including geographic comparisons, the picture becomes more complex. Specimens of Plectronoceras cambria (Walcott, 1905), the oldest cephalopod, are relatively similar to each other in their conch parameters regardless of their provenance. In fact, some of the differences may even be attributable to rounding errors. Comparing between regions in the next youngest Quadraticephalus Zone is challenging because only two relatively poorly preserved specimens of one species have been described outside northern Anhui, S. (?) shanxiense (Chen & Teichert, 1983) from Shanxi Province. In terms of conch parameters, the specimens from Shanxi fall within the variation seen in the material from Anhui, although they are more strongly curved. Regional differences are much more obvious in the Wanwankou Member of the Fengshan Formation and its equivalents, which represents the highest and most fossiliferous (in terms of cephalopods) plectronoceratid-bearing horizon in China. Variability is highest, both between and within regions, including occurrences in Laurentia and Siberia, which are assumed to be roughly equivalent in age (Flower, 1964; Fang et al., 2019; Dzik, 2020). Specimens from Shandong (here regarded as belonging to S. bullatum (Chen & Qi in Chen et al., 1979a) and S. sinense (Chen & Qi in Chen et al., 1979a)) and from Liaoning (interpreted as S. wanwanense (Kobayashi, 1933)) show very similar trajectories (p-values for regression coefficient of RCL = 0.07 and ER_h = 0.66 between S. bullatum and S. wanwanense), but approximately contemporaneous plectronoceratids from Zhejiang, South China (here attributed to S. endogastrum (Li, 1984)), have a much lower expansion rate and more rapid ontogenetic reduction in RCL. In this regard, the single specimen described from Inner Mongolia as S. magicum Chen in Lu, Zhou & Zhou, 1984, is closer to the Zhejiang plectronoceratids than to those from Liaoning and Shandong. The Laurentian plectronoceratids (here all assigned to Palaeoceras mutabile Flower, 1954) differ from either of the two previous groups, as they show the lowest expansion rate and steepest ontogenetic trajectory of RCL of all plectronoceratids investigated here. The single species known from the Ust-Kut Formation of Siberia, S. sibiriense (Balashov, 1959), displays similarities to the plectronoceratids of Liaoning and Shandong, although its expansion rate remains lower throughout ontogeny.

In summary, although the range of variation across all specimens is considerable, they do not fall into distinct groups, making species diagnoses based on small differences in those ratios very doubtful. Importantly, there is a general trend of reduction in RCL and ER, which means that ontogenetic changes must be considered when assessing their diagnostic potential. Differences between regional or temporal populations are usually larger than differences within a single population.

Body size

Comparing body size reveals temporal and spatial differences (Fig. 11). The largest specimens come from the Wanwankou Member of Shandong (mean = 20.8 mm, n = 20), followed by those from contemporaneous beds of Liaoning (mean = 15.5 mm, n = 36). In both provinces, many specimens are documented with preserved body chambers (Shandong mean body chamber diameter = 23.1 mm, n = 7; Liaoning mean body chamber diameter = 18.3 mm, n = 16) and thus, these size distributions are representative for their time of death, even if they may include juvenile specimens. The single known plectronoceratid from the Ust-Kut Formation of Siberia falls within the upper half of the Wanwankou size spectrum but does not have its body chamber preserved (maximum diameter = 24 mm). Plectronoceratids from the Fengshan Formation of northern Anhui are distinctly smaller, reaching average diameters of only 6.5 mm, with specimens from the lower Quadraticephalus Zone (6.7 mm, n = 5; mean body chamber diameter = 7.2 mm, n = 4) and upper Quadraticephalus Zone (7.3 mm, n = 3; all with body chamber) of the middle Jiagou Member reaching slightly larger sizes than those from the upper Jiagou Member (Acaroceras-Eburoceras Zone, approximately equivalent to the Wanwankou Member, 6.0 mm, n = 13; mean body chamber diameter = 5.8 mm, n = 8). Comparable sizes are reached by the few specimens known from the Siyangshan Formation of Zhejiang (mean = 8.4 mm, n = 2) and the Sinoeremoceras Zone of Inner Mongolia (10 mm, n = 1), both of which have been correlated with the Wanwankou Member, although the Siyangshan plectronoceratids are likely slightly older, corresponding to the Lotagnostus americanus Zone (Peng et al., 2012). The plectronoceratids from Texas are similarly small (6.8 mm, n = 12; mean body chamber diameter = 7.3 mm, n = 6), although the known size range is narrower (all body chambers with diameters 6.5–8.0 mm). Specimens of Plectronoceras are the smallest of all plectronoceratids, reaching average diameters of only 3.8 mm (n = 5; all except one with body chamber). They are restricted to the Ptychaspis-Tsinania Zone of North China, but their size does not vary between regions; specimens from Anhui, Liaoning and Shandong are of almost identical size. The Queensland material is intermediate between the small Anhui plectronoceratids and the large Wanwankou plectronoceratids, with an average conch diameter of 14.1 mm. However, in contrast to other regions, specimens with preserved body chambers are rare, and thus the average adult size is probably slightly larger. It is not clear whether the scarcity of body chambers in the Black Mountain material is a taphonomic effect or represents a collection bias, although the sheer amount of material collected by Mary Wade and colleagues indicates that they did not discriminate based on preservation status. In any case, the few preserved body chambers in the material at hand suggest that our body size sample does not represent a gross underestimation. Furthermore, the trajectories of RCL are steeper than in the material from Liaoning and Shandong, but shallower than in specimens from Anhui, suggesting that this parameter approaches similar values in late ontogenetic stages and thus may serve as a tentative indication of adulthood in plectronoceratids.

Systematic palaeontology

Synonymy lists are organised in three separate parts: 1) a list of all synonyms referring to their original description, 2) non-original uses of synonyms and 3) erroneous referral to the taxon. Synonymy lists of taxa above species-level consist only of the first part.

Phylum MOLLUSCA Linnaeus, 1758

Class CEPHALOPODA Cuvier, 1797

STEM CEPHALOPODA

Order PLECTRONOCERATIDA Flower, 1964

Plectronoceratina Flower, 1964: 28.

Protactinocerida Chen & Qi in Chen et al., 1979a: 11.

Plectronocerida [nom. transl.]—Chen et al., 1979a: 6.

Plectronoceratida [nom. corr.]—King & Evans, 2019: 76.

Protactinoceratida [nom. corr.]—King & Evans, 2019: 76.

Included family

Plectronoceratidae Kobayashi, 1935.

Emended diagnosis

Relatively small (up to 30 mm in diameter) orthoconic to slightly cyrtoconic conchs, with extremely short chambers and marginal siphuncle on ventral side. If cyrtoconic, the curvature is endogastric. Orthoconic forms may grade into exogastric curvature, but the difference is very subtle. Siphuncle bilaterally symmetrical, expanded ventrally and laterally, mostly with cyrtochoanitic septal necks, while the dorsal side of the neck is tongue-like elongated, forming a holochoanitic septal neck (septal flap). Siphuncular segments form two bulges dorsally that reach behind the septal flap in late ontogenetic stages, without touching each other. Complex diaphragms usually present adapically.

Remarks

We consider the Protactinoceratida Chen & Qi in Chen et al., 1979a a junior synonym of the Plectronoceratida, because both show a strong bilateral symmetry in their siphuncular segments and septal necks. The segments are expanded laterally and straight dorsally, while representatives of the Ellesmeroceratida can be distinguished by their more or less radially symmetrical siphuncle with straight or concave segments and the usually slightly longer chambers. Many previously established genera have been based on longitudinal sections, which do not show the three-dimensional outline of the siphuncle. For example, a total of seven genera have been established within the Protactinoceratida. The three-dimensional siphuncular structure seen in the Queensland specimens shows that these genera can be synonymized, as they are based on sections that are not in the median plane. The different sections, classified as “genera”, can be reproduced by sectioning specimens in various planes that differ to some extent from the median plane. Ontogenetic changes from cyrtochoanitic to ortho- to hemi- and holochoanitic septal necks and strongly expanded segments with siphuncular bulbs indicate skewed longitudinal sections. As these characters occur in both orders, it is impossible to assign taxa to one or the other order (see also Wade & Stait, 1998; Mutvei, Zhang & Dunca, 2007; Mutvei, 2020). The detailed three-dimensional structure of the siphuncle is not known in many taxa; thus, some variation from the three-dimensional pattern described here may exist.

Our revised taxonomy presents a significant reduction in the number of taxa. This opens the question whether ranking the Plectronoceratida as an order carries any benefit, or whether its only family should be placed within the Ellesmeroceratida, especially as Palaeoceras appears to be transitional between the two groups. As this is mostly a matter of preference, we keep the current classification (see also King & Evans, 2019; Pohle et al., 2022) but call for further discussions on this topic. Note that we follow the proposal to return to the classical “-ceratida” endings for the revised edition of the Treatise on Invertebrate Palaeontology, Part K, but maintain the “-ceratoidea” endings for subclasses, because this matter is more complicated and requires further discussion to achieve congruence across major groups of cephalopods (King & Evans, 2019; see also Hoffmann et al., 2022).

Geographic and stratigraphic occurrences

Anhui, Inner Mongolia, Liaoning, Shandong and Shanxi Provinces of North China, Zhejiang Province of South China, Texas, North America, Siberia, Russia and Queensland, Australia; Jiangshanian to Stage 10, Furongian, late Cambrian.

Family PLECTRONOCERATIDAE Kobayashi, 1935

Plectronoceratidae Kobayashi, 1935: 20.

Balkoceratidae Flower, 1964: 33.

Protactinoceratidae Chen & Qi in Chen et al., 1979a: 11.

Type genus

Plectronoceras Ulrich & Foerste, 1933.

Included genera

Plectronoceras Ulrich & Foerste, 1933; Palaeoceras Flower, 1954; Sinoeremoceras Kobayashi, 1933.

Emended diagnosis

As for the order.

Remarks

As outlined above, the supposed differences between protactinoceratids and plectronoceratids result mainly from the orientation of the plane of section relative to the growth axis. It is therefore impossible to distinguish between the two taxa on familial level. In addition to the Plectronoceratidae, the order was also proposed to contain the Balkoceratidae Flower (1964), which was distinguished by a slight exogastric curvature. However, the presumed balkoceratid Theskeloceras Chen & Teichert, 1983, is likely not exogastric and rather a synonym of Sinoeremoceras of the Plectronoceratidae. Teichert (1967) was initially reluctant to accept the family, but later changed his opinion (Chen & Teichert, 1983). In contrast to the Australian and Chinese plectronoceratids, the type specimens of Balkoceras appear to be slightly exogastric, which led to its separation into the Balkoceratidae Flower (1964). However, as we regard Balkoceras as a synonym of Palaeoceras, we consider the Balkoceratidae as a subjective junior synonym of the Plectronoceratidae as well.

Genus Sinoeremoceras Kobayashi, 1933

Sinoeremoceras Kobayashi, 1933: 272.

Wanwanoceras Kobayashi, 1933: 271.

Multicameroceras Kobayashi, 1933: 273.

Paraplectronoceras Chen, Qi & Chen in Chen et al., 1979a: 7.

Jiagouceras Chen & Zou in Chen et al., 1979a: 8.

Lunanoceras Chen & Qi in Chen et al., 1979a: 9.

Eodiaphragmoceras Chen & Qi in Chen et al., 1979a: 10.

? Rectseptoceras Zou & Chen in Chen et al., 1979a: 11.

Protactinoceras Chen & Qi in Chen et al., 1979a: 12.

Physalactinoceras Chen & Qi in Chen et al., 1979a: 13.

Theskeloceras Chen & Teichert, 1983: 52.

? Hunyuanoceras Chen & Teichert, 1983: 59.

Benxioceras Chen & Teichert, 1983: 89.

Mastoceras Chen & Teichert, 1983: 92.

Parapalaeoceras Li, 1984: 229.

Type species

Eremoceras wanwanense Kobayashi, 1931

Included species

Sinoeremoceras wanwanense (Kobayashi, 1931); Sinoeremoceras bullatum (Chen & Qi in Chen et al., 1979a) comb. nov.; Sinoeremoceras endogastrum (Li, 1984) comb. nov.; Sinoeremoceras inflatum (Chen & Zou in Chen et al., 1979a) comb. nov.; Sinoeremoceras magicum Chen in Lu, Zhou & Zhou, 1984; Sinoeremoceras (?) shanxiense (Chen & Teichert, 1983) comb. nov. Sinoeremoceras sibiriense (Balashov, 1959) comb. nov., Sinoeremoceras sinense (Chen & Qi in Chen et al., 1979a) comb. nov.; Sinoeremoceras marywadeae sp. nov.

Emended diagnosis

Endogastrically curved conch with moderate to high expansion rate in early stages, decreasing during ontogeny. Cross-section compressed, dorsum or venter may be more narrowly rounded. Shell surface poorly known, but apparently smooth or with fine, directly transverse lirae. Cameral length very short, usually <1 mm; RCL decreasing during ontogeny from about 0.15 to 0.05 or less. Siphuncle on ventral side of the conch, laterally expanding between chambers, but straight mid-dorsally, where the otherwise cyrtochoanitic septal necks are elongated and form a triangular, adapically directed septal flap that overlaps the adapically adjacent septal neck. Siphuncular bulges dorsal to septal flap, not joining middorsally. Siphuncular diaphragms present in early stages, with same spacing as septa, with base extending from the septal foramen and adapically convex, parallel to septum, except for central ridge that extends more horizontally towards the shell wall.

Remarks

Kobayashi (1933) erected Sinoeremoceras, Wanwanoceras and Multicameroceras in the same publication, and despite Wanwanoceras having page priority, we, as first revisers under ICZN Article 24(a), give preference to Sinoeremoceras, because the type shows the typical siphuncular outline, and Multicameroceras was already considered as a synonym by Chen & Teichert (1983). In addition to the “protactinoceratids”, most genera previously assigned to the Plectronoceratidae are synonymized with Sinoeremoceras. Plectronoceras Ulrich & Foerste, 1933 differs from Sinoeremoceras mainly by its small adult size. Palaeoceras Flower, 1954 has a lower expansion rate and is essentially straight or only very slightly cyrtoconic; it also apparently lacks cyrtochoanitic septal necks.

Multicameroceras Kobayashi, 1933 (type species: Ellesmeroceras? multicameratum Kobayashi, 1931, original designation) was established alongside Sinoeremoceras Kobayashi, 1933, citing a “cylindrical to conical elongate” conch compared to a “somewhat fusiform” conch as the major difference between the two (Kobayashi, 1933, p. 273). However, besides the very challenging task of drawing an objective boundary between these two states, we find specimens, which may be considered either one of these two and any transitional forms in between, among the Australian material and we attribute these differences to intraspecific or ontogenetic variation. We thus agree with Chen & Teichert (1983), who synonymised the two genera.

Wanwanoceras Kobayashi, 1933 (type species: W. peculiare Kobayashi, 1933, original designation) was provided without differential diagnosis. Kobayashi (1933) regarded it as distinct from Sinoeremoceras and Multicameroceras based on the shape of the septal necks, which seem to be more strongly curved (cyrtochoanitic) in early ontogenetic stages. Chen & Teichert (1983, p. 89) noted that Sinoeremoceras differed in that “the septal necks gradually are shortened adorally”, but this contradicts their own diagnosis of Sinoeremoceras, where they stated that the septal necks are “much longer in adoral part of siphuncle” (Chen & Teichert, 1983, p. 86). There is no difference between Wanwanoceras and Sinoeremoceras even if those definitions are followed. As in Sinoeremoceras and Multicameroceras, the apparent ontogenetic changes in Wanwanoceras can be better explained by a slight y-rotation of the plane of section, potentially in combination with a small x-translation.

Protactinoceras Chen & Qi in Chen et al., 1979a (type species: P. magnitubulum Chen & Qi in Chen et al., 1979a, original designation), is perhaps the most extreme example of a misaligned section. Although Mutvei, Zhang & Dunca (2007) recognised the three-dimensional structure of the plectronoceratid siphuncle and synonymy of the Protactinoceratida with the Plectronoceratida, they did not mention Protactinoceras specifically, and in fact, Mutvei (2020) later regarded it as distinct from plectronoceratids. Sections of Protactinoceras may appear very different from sections of Sinoeremoceras. However, a first hint on the suboptimal alignment of the plane of section can be seen in the holotype of the type species (Chen et al., 1979a, pl. 3, fig. 12-13; text-fig. 10), in which the siphuncle contracts rapidly adaperturally and eventually even disappears. Aperturally, the septal necks are longer than apically and tilted inwards, providing evidence of the dorsal septal flap. A very similar outline can be produced in S. marywadeae sp. nov. by polishing a specimen directly from the venter (Fig. 7L). Thus, Protactinoceras is a section that very strongly deviates from the median plane by being rotated nearly 90° along the z-axis, probably with some additional x-translation (in order to go through the siphuncle) and y-rotation (in order to be more or less parallel to the siphuncle). Because of the very strong misalignment, it is almost impossible to compare these specimens with species of Sinoeremoceras, although they fit within the concept of regional species employed here.

Paraplectronoceras Chen, Qi & Chen in Chen et al., 1979a (type species: P. pyriforme Chen, Qi & Chen in Chen et al., 1979a, original designation) was originally described without differential diagnosis, but Chen & Teichert (1983, p. 41) mentioned that it differs from Plectronoceras by its slightly larger size and an ontogenetic change from orthochoanitic towards cyrtochoanitic septal necks, while the segments are less expanded. As before, this “ontogenetic” change is interpreted as indication for a septal flap. Thus, Paraplectronoceras represents a slight y-rotation of the plane of section, possibly with additional x-translation or z-rotation so that the segments appear less strongly expanded.

Jiagouceras Chen & Zou in Chen et al., 1979a (type species: J. cordatum Chen & Zou in Chen et al., 1979a, original designation) shows obvious signs of a misaligned section in the very flat appearance of the septa. While flat septa cannot be excluded per se, in the absence of any indication of alignment of the plane of section, this strong deviation from the usually concave septa is better interpreted as a misaligned section, especially as the siphuncle appears removed from the ventral wall by some distance, which is unknown in any cross-section of a Cambrian cephalopod. The suborthochoanitic septal necks and expanded siphuncular segments prevent its interpretation as an ellesmeroceratid. By contrast, Jiagouceras fits perfectly in the interpretation of a Sinoeremoceras with a plane of section moderately rotated in the z-axis (though not as strong as in “Protactinoceras”), probably combined with a slight x-translation, which would explain the less strongly expanded siphuncle.

Lunanoceras Chen & Qi in Chen et al., 1979a (type species: L. precordium Chen & Qi in Chen et al., 1979a, original designation) was considered very similar to Eodiaphragmoceras except for the shape of its septal necks and diaphragms. Five species have been ascribed to the genus, namely the type, L. changshanense Chen & Qi in Chen et al., 1979a; L. compressum Chen & Qi in Chen et al., 1979a; L. densum Chen & Qi, 1982 and L. longatum Chen & Qi, 1982 (also erroneously spelled L. elongatum in the same publication). Chen & Teichert (1983) considered Lunanoceras to be very similar to Sinoeremoceras (but classified the two genera in different families) though differing mainly in the shape of the diaphragms. We do not consider diaphragm shape (at the current state of knowledge) to be a generic discriminator and assign Lunanoceras as another synonym of Sinoeremoceras.

Eodiaphragmoceras Chen & Qi in Chen et al., 1979a (type species: E. sinense Chen & Qi in Chen et al., 1979a, original designation) is a monospecific genus with a laterally expanded siphuncle and partly holochoanitic and cyrtochoanitic septal necks on the dorsal side, strongly suggesting a septal flap as in Sinoeremoceras. The shape of the diaphragms and the sometimes slightly S-shaped septal necks are difficult to explain by the plane of section alone, but we think that these differences do not justify generic separation, because variation in the shape of the septal flap and the diaphragms is insufficiently known. We consider Eodiaphragmoceras a synonym of Sinoeremoceras with a relatively centrally aligned plane of section but regard Sinoeremoceras sinense (Chen & Qi in Chen et al., 1979a) comb. nov. as a valid species.

Physalactinoceras Chen & Qi in Chen et al., 1979a (type species: P. bullatum Chen & Qi in Chen et al., 1979a, original designation) is the genus with the highest number of originally attributed species and was differentiated from Sinoeremoceras mainly by the more strongly expanded and peculiarly shaped siphuncular segments. The species are separated on the various extents of ontogenetic changes in the septal necks (e.g., shortening in P. changshanense Chen & Qi in Chen et al., 1979a or lengthening in P. planoconvexum Chen & Teichert, 1983) or segment shape (e.g., circular in P. globosum Chen & Qi in Chen et al., 1979a; oval in P. bullatum Chen & Qi in Chen et al., 1979a; pear-shaped in P. breviconicum Chen & Qi in Chen et al., 1979a; balloon-shaped in P. papilla Chen & Teichert, 1983 or narrowly expanded in P. confusum Chen & Teichert, 1983). The frequent changes in ontogeny and seemingly large differences can be explained by planes of section with various slight deviations from the median plane, but in all species the plane of section is aligned through the septal flap, at least to some extent. The different shapes of the segments are likely caused by different positions of the section relative to the ends of the bulges, which end relatively abruptly dorsal to the septal flap. Physalactinoceras has a plane of section with slight x-translation (so that the dorsal ends of the siphuncular bulges are visible) and/or slight y-rotation (for ontogenetic changes) with the possibility of slight z-rotation as well.

Rectseptoceras Zou & Chen in Chen et al., 1979a (type species: Rectseptoceras eccentricum Zou & Chen in Chen et al., 1979a, original designation) is perhaps not even a plectronoceratid, but instead an ellesmeroceratid, as the septal necks appear to be only slightly curved. However, the connection ring is not preserved (thus making it unclear whether the segments are really expanded or not) and the plane of section is clearly misaligned as indicated by the flat apical septa, the ontogenetic change in siphuncle size (including its apical “disappearance”) and position and the erratic expansion rate that seemingly decreases during the earliest stages (which is likely caused by conch curvature). Because of its short septal spacing and slightly suborthochoanitic septal necks, we only tentatively place this genus in synonymy with Sinoeremoceras.

Benxioceras Chen & Teichert, 1983 (type species: B. rapidum Chen & Teichert, 1983, original designation) was another supposedly monospecific genus, differentiated from Physalactinoceras by the less expanded segments that do not protrude into the chambers on the dorsal side. However, cross-sections of B. rapidum (Chen & Teichert, 1983, pl. 17, fig. 8, pl. 19, fig. 5, text-fig. 23, 24) show the same outline of the siphuncle as specimens attributed to Physalactinoceras (e.g., P. qiushugouense Chen & Teichert, 1983, text-fig. 20) and Sinoeremoceras marywadeae sp. nov. (Figs. 7G, 7J), demonstrating the septal flap. Thus, Benxioceras represents a section of Sinoeremoceras that is close to the median plane, although probably with a slight rotation of the z-axis, which would explain the short distance of the septal necks from the ventral wall (while being contiguous in cross-section), the relatively small siphuncle and rapid expansion rate. In terms of cameral length, siphuncle size and conch compression the species does not differ markedly from other species and cannot be separated from Sinoeremoceras. We therefore consider B. rapidum as a synonym of S. wanwanense.

Mastoceras Chen & Teichert, 1983 (type species: M. qiushugouense Chen & Teichert, 1983, original designation) demonstrates some of the difficulties in applying the original definitions to differentiating between Plectronoceratida and Protactinoceratida when sections are assumed to be correctly placed in the median plane. The only genus to which Mastoceras was compared was the plectronoceratid Theskeloceras, from which it was said to differ by the straight, more rapidly expanding conch and shape of the siphuncular segments. The latter was also the reason why Mastoceras was placed in the Protactinoceratida. The inferred changes in ontogeny of the dorsal septal necks from holochoanitic to hemichoanitic can again be explained by a slight y-rotation of the plane of section. The holotype of the type species represents one of the rare cases, where longitudinal and cross-section of the same specimen are available, indicating that the section is misaligned with a slight z-rotation. Moreover, this cross-section demonstrates that the siphuncle is expanded laterally with cyrtochoanitic septal necks (Chen & Teichert, 1983, pl. 9, fig. 4, text-fig. 25) while the adapertural necks of the longitudinal section of the same specimen are hemichoanitic (Chen & Teichert, 1983, pl. 9, fig. 1-2, 7, text-fig. 25), implying a septal flap.

Theskeloceras Chen & Teichert, 1983 (type species: T. benxiense Chen & Teichert, 1983, original designation) was described as exogastric and therefore assigned to the Balkoceratidae Flower (1964), but the only specimen (Chen & Teichert, 1983, pl. 4, fig. 1) has a weathered dorsal side, which was interpreted as originally concave by the authors based on the convex ventral outline. However, this outline can also result from sectioning an endogastric shell with y-rotation, the convex outline rather reflecting the rounded cross-section. The other species, T. subrectum Chen & Teichert, 1983 (pl. 3, fig. 1) is essentially straight, with the ventral side slightly more convex. Having examined the original specimens, we conclude that there is no evidence for a marked exogastric curvature of Theskeloceras and the genus is more similar to Sinoeremoceras than to Balkoceras (or rather Palaeoceras gracile) because of the typical “protactinoceratid” outline of the siphuncle, the more rapid expansion rate and the short cameral length. Slight variations in the ventral and dorsal conch outline (e.g., between straight or slightly convex or concave) may also be a result of the misalignment of the plane of section and are only reliable when it is perfectly aligned (which is usually difficult to tell) or in three-dimensionally preserved specimens. The plane of section appears to exhibit slight y-rotation (due to ontogenetic changes) and possibly slight z-rotation and x-translation.

Parapalaeoceras Li, 1984 (type species: P. endogastrum Li, 1984, original designation) has suborthochoanitic septal necks and expanded siphuncular segments throughout, indicating that the section does not pass through the septal flap, but is positioned close to it. It thus represents a section parallel to the median plane with slight x-translation, though it is possible that it is accompanied by some z-rotation. This genus is the only report of a Cambrian cephalopod with cyrtochoanitic septal necks from Zhejiang, which makes it interesting from a palaeobiogeographical perspective. Although there is no unambiguous evidence of a septal flap in the type species, the very close similarity of these specimens to some smaller species of Sinoeremoceras and the overlapping morphospace occupation supports its assignment to this genus.

Hunyuanoceras Chen & Teichert, 1983 (type species: H. shanxiense Chen & Teichert, 1983, original designation) was described as one of the oldest ellesmeroceratids, differing from Plectronoceras in its tubular siphuncle and the development of diaphragms. The holotype of the type species is poorly preserved and based on a misaligned section, but according to the drawing by Chen & Teichert (1983, text-fig. 13), the specimen contains both cyrtochoanitic and orthochoanitic septal necks, thus providing evidence of the presence of a septal flap. Correspondingly, the species more closely resembles a plectronoceratid than an ellesmeroceratid. The lack of diaphragms in Plectronoceras may not be primary and their presence possibly represents the ancestral state in cephalopods (Flower, 1964), so they cannot serve as a distinguishing character. As explained further below, the distinction between Plectronoceras and Sinoeremoceras mainly relies on stratigraphic distribution and body size, and since “Hunyuanoceras” corresponds to Sinoeremoceras in both regards, we tentatively assign its only species to this genus.

Geographic and stratigraphic occurrences

Anhui, Inner Mongolia, Liaoning, Shandong and Shanxi provinces of North China; Zhejiang province of South China; Krasnoyarsk, Siberia, Russia; Queensland, Australia; Jiangshanian/Stage 10, Furongian, Cambrian.

Sinoeremoceras marywadeae sp. nov.

urn:lsid:zoobank.org:act:581A8E45-9064-4209-B256-5BCACB199967

Figure 4–9, 15; Tables 4, 5

Protactinoceras—Wade, 1988: 17.

Protactinoceras—Grégoire, 1988: 76.

Protactinoceras—Wade & Stait, 1998: 489; fig. 12.8 A, B, E.

Sinoeremoceras sp.—Pohle et al., 2022: 6; fig. 2; Supplemental Material.

Diagnosis

Slightly curved endogastric conch, tending to become straighter in adult stages but some intraspecific variation of curvature is evident across ontogenetic stages. Cross-section compressed but constant throughout ontogeny, with CWI ≅ 0.8. Early ontogenetic stages with more narrowly rounded venter, with a tendency towards equally rounded venter and dorsum in later stages. Expansion rate variable, higher dorsoventrally than laterally but generally decreasing during ontogeny, ER_h ≅ 23° and ER_w ≅ 20° at a conch height of 5 mm and ER_h ≅ 9° and ER_w ≅ 6° at a conch height of 20 mm. Adult body chamber diameter at least 22 mm. Chambers very closely spaced, between 0.3–0.9 mm with a tendency to increase during ontogeny while RCL decreases from around 0.07 in juvenile stages of 5 mm conch height to 0.02 at 20 mm conch height. No abrupt septal crowding near (assumed) maturity but nearly linear decrease in RCL. Sutures with distinct lateral lobes and ventral and dorsal saddles somewhat variable in shape but correlated to conch curvature, i.e., more strongly curved specimens or earlier ontogenetic stages usually have higher dorsal than ventral saddles, while the saddles of nearly straight or mature specimens are roughly equal in height. Siphuncle consisting of strongly oblique segments but siphuncle cross-section (perpendicular to growth axis) roughly circular. Relative size of the siphuncle variable with an apparent tendency to decrease during ontogeny, RSD approximately 0.15–0.25 (mean RSD = 0.2). Septal necks cyrtochoanitic laterally and ventrally, folding adapically and becoming holochoanitic on the dorsum, forming a triangular septal flap that becomes longer during ontogeny until it slightly overlaps the flap of the preceding segment. Connecting ring strongly expanded, laterally attached to septal flap and bulging dorsally, directed towards the median plane but without touching each other, with a kidney-shaped cross-section. Diaphragms present, with central dorsoventral ridge that is adapically convex and crosses the siphuncle nearly transversely, while lateral parts of the diaphragm slope ventraperturally.

Etymology

We name the species for Mary Wade, in honour of her pioneering work in collecting, preparing, describing, and interpreting the cephalopods from Black Mountain, with most of her prior work relating to this species.

Holotype

QMF 39529

Paratypes

QMF 13332, QMF 13992, QMF 39533, QMF 39542, QMF 40855–40856, QMF 61277, QMF 61634

Other material

200 specimens from the type locality and horizon (QMF 13314, QMF 13334, QMF 13337, QMF 13340, QMF 13988–13991, QMF 13993–14003, QMF 14005–14006, QMF 39522–39528, QMF 39530–39532, QMF 39534–39541, QMF 39543–39547, QMF 39549, QMF 40852, QMF 40853, QMF 40857–40859, QMF 40861–40862, QMF 40865, QMF 61276, QMF 61278–61281, QMF 61284, QMF 61288–61294, QMF 61315–61316, QMF 61320, QMF 61327–61340, QMF 61343, QMF 61346–61347, QMF 61349, QMF 61351, QMF 61355, QMF 61358–61360, QMF 61362–61363, QMF 61368–61370, QMF 61372, QMF 61375, QMF 61378–61380, QMF 61382–61384, QMF 61386–61388, QMF 61393–61397, QMF 61468–61519, QMF 61523–61540, QMF 61565–61569, QMF 61622–61626, QMF 61632–61633, QMF 61635–61638).

Type locality and horizon

Black Mountain, near Boulia, western Queensland, Australia; Assemblage Zone 3 (= Hirsutodontus appressus Zone), Unbunmaroo Member (BMT 1), lower Ninmaroo Formation, Stage 10, Furongian, late Cambrian.

Description

The holotype, QMF 39529 (Figs. 5A–5D, 7A) is a slightly endogastrically curved phragmocone fragment 6.3 mm long with a compressed cross-section, the dorsum slightly narrower than the venter and expanding from a width of 12.4 mm and a height of 15.3 mm (CWI = 0.81) to 13.5 and 17.6 mm (CWI = 0.77), respectively (i.e., ER_w = 9.7° and ER_h = 20.8°). Septa are extremely closely spaced, < 1mm throughout the specimen, leading to RCL < 0.05. Sutures are incompletely visible in the holotype, apparently slightly inclined dorsaperturally and consisting of slight lateral lobes and pronounced dorsal and ventral saddles. The siphuncle has a diameter between 3.1 mm at the apical end and 3.8 mm at the apertural end (RSD = 0.2). Adaperturally, the siphuncle is preserved as a partial external mould, revealing the strongly elongated septal necks on the dorsal side of the siphuncle (septal flap). In the holotype, five consecutive septal flaps are preserved, the adapertural ones are broken but the adapical two septal flaps exceed 1 mm in length, almost twice the lengths of the corresponding chambers, although the siphuncular segments are steeply inclined and thus, the septal flaps only slightly overlap the preceding segment. The septal flaps are very narrow, about 1.5 mm at their widest (i.e., adaperturalmost), but quickly decrease in width to about 0.5 mm adapically. Laterally, the septal foramina indicate cyrtochoanitic septal necks by their rounded outline and the smooth transition to the septal flap indicates that the flap’s lateral margins represent essentially 90° tilted cyrtochoanitic septal necks as well.

One of the paratypes, QMF 40856 (Figs. 6H, 6I), had been designated by Mary Wade, in her working notes, as potential holotype. However, the specimen is partially embedded in matrix, making exact measurements of width, height, and siphuncle at different ontogenetic positions difficult. The specimen is 30.9 mm long and has an adapertural diameter of 14.1 and 18.3 mm (CWI = 0.77) with closely spaced chambers at the adapertural end (RCL = 0.03) and a siphuncle diameter of 3.2 mm (RSD = 0.18). The siphuncle is exposed adaperturally as an external mould, but the septal flaps are only partly preserved. At the apical end, the siphuncle is preserved as an internal mould, revealing the laterally and ventrally expanded, strongly oblique siphuncular segments. The shell surface is recrystallized, but it is apparently smooth.

Another paratype, QMF 13332 (Fig. 5I), was deliberately polished by Mary Wade with approximately 90° of z-rotation from the ventral side, exposing a “Protactinoceras”-like outline of the siphuncle (Fig. 7L). However, as the polished surface passes towards the dorsal margin of the siphuncle, the septal necks become longer and straight, thus also exposing the septal flap. The adapical surface of the specimen shows that it belongs to the same species as the other types, because the septal flap is visible as a rounded dorsal inflexion of the siphuncle (Fig. 7I). The adapical end of the siphuncle is sealed, and its symmetrical structure likely represents a diaphragm. The diaphragm has a broad surface that slopes ventraperturally and has a more directly transverse, dorsoventral central ridge about the same width as the septal flap. The central ridge does not appear to touch the ventral shell wall, but this may be caused by preservation. Two diaphragms are visible in the polished part of the section, revealing the same structure with a central ridge.

An earlier ontogenetic stage is represented by the paratype QMF 39533 (Figs. 5E–5H). This specimen is more strongly curved endogastrically, has a relatively rapid expansion rate and a more narrowly rounded dorsum. Because transitional forms between this morphology and larger specimens exist, these differences are attributed to ontogeny. The sutures of the specimens are distinct in the dorsal half, having a narrow dorsal saddle and slight lateral lobes. The siphuncle is only exposed adapically, but no details of its structure are visible.

In many specimens, the septal flap is not preserved, leaving only the imprint of the two siphuncular bulges on the dorsal side of the siphuncle, which results in an hourglass shaped elevation mid-dorsally. This structure is best seen in paratype QMF 39542 (Fig. 5J). The bulges themselves, or rather the internal mould of the siphuncle, are visible in paratype QMF 40855, where the bulges almost touch each other but leave a small gap adaperturally. This gap causes the hourglass-shaped ridge in its negative and represents the position of the septal flap. The latter is hidden behind (ventral to) the bulges. That the specimens exposing imprints of siphuncular bulges are conspecific with those exposing septal flaps is shown by paratype QMF 61277, which combines both preservation types. The adapical part of the specimen preserves the imprints of the bulges, while adaperturally, the (somewhat corroded) septal flaps are evident. The septal flaps are very slightly displaced towards the venter compared to the imprints of the bulges, indicating that the bulges reach behind (dorsal) the flap.

The rest of the material consists of fragments of phragmocones and siphuncles of variable preservation. The siphuncle morphology was already described in detail in the Result section. Conch height 4–23 mm, measured in 77 specimens at 212 individual ontogenetic stages. Conch is compressed regardless of ontogenetic stage, with CWI ≅ 0.78. Cameral length is always very short, below 1 mm, and although there is a significant ontogenetic decrease with cl = 0.45 + 0.00094 * ch, this includes considerable variation, and as R² = 0.054, ontogeny only explains a negligible part of this variation. There is a stronger relationship with relative septal spacing, as RCL = 0.092–0.0035 * ch and R² = 0.51. The size of the siphuncle linearly increases during ontogeny, but its relative size decreases, with RSD = 0.24–0.0037 * ch, corresponding to RSD ≅ 0.22 at a conch height of 5 mm and RSD ≅ 0.17 at a conch height of 20 mm. It is possible that this decrease is at least partially caused by taphonomic effects, as larger specimens tend to have more poorly preserved siphuncles due to weathering, which makes exact measurements challenging. The decrease is relatively minor, and the linear regression of RSD results in R² = 0.18 and σ = 0.025, meaning that there is considerable variation in RSD that is not explained by ontogeny. Expansion rate decreases during ontogeny, with ER_h = 28.2°–0.96° * ch and ER_w = 24.1°–0.89° * ch, corresponding to ER_h ≅ 23.4° and ER_w ≅ 19.7° at a conch height of 5 mm and ER_h ≅ 9.0° and ER_w ≅ 6.3° at a conch height of 20 mm. However, there is considerable variation in expansion rate at all ontogenetic stages, as illustrated by R² = 0.27 and σ = 4.7° for ER_h and R² = 0.24 and σ = 5.1° for ER_w.

Remarks

Comparison with other species from China and Siberia is hampered by the fact that the latter are almost exclusively known from longitudinal sections, thus the three-dimensional outline of the conch and particularly the siphuncle is unknown in all other species of Sinoeremoceras. The diagnoses of many proposed plectronoceratid and “protactinoceratid” species are doubtful because they are based on misaligned sections not aligned with the median section, which potentially has a large effect on diagnostic characters such as expansion rate and relative size of the siphuncle. Importantly, this also applies to the length and shape of the septal necks and the shape of the siphuncular segments because of the peculiar plectronoceratid siphuncle morphology. Another character that has been frequently used to differentiate between species, is the seemingly large ontogenetic change within the siphuncle, which is shown above to be a result of the misalignment of the plane of section. Without new material from the original Chinese localities, it is impossible to evaluate the number and morphology of the involved species. Therefore, the specimens described here cannot be referred to any other species. The material—and also any future material—can only be referred to other species of Sinoeremoceras if new material is collected from the original localities and 3D-reconstructions become available. The only described species of plectronoceratids that are known from the three-dimensional outline of the conch are S. wanwanense (Kobayashi, 1931); Multicameroceras multicameratum (Kobayashi, 1931); M. cylindricum Kobayashi, 1933 and Wanwanoceras peculiare Kobayashi, 1933, all of which are here synonymised with the type species. S. marywadeae sp. nov. differs from S. wanwanense by its comparatively deeper lateral lobes. Measurements of Physalactinoceras qiushugouense Chen & Teichert, 1983 (NIGP 73801, pl. 14, fig. 1, 3) and Sinoeremoceras foliosum Chen & Qi in Chen et al., 1979a (NIGP 46128, pl. 1, fig. 3, 4) suggest that they may be distinguished by their slightly higher expansion rate, although this is probably strongly influenced by alignment of the plane of section and thus somewhat questionable. Otherwise, in terms of siphuncle size, cameral length and curvature, the species seem to be close.

Geographic and stratigraphic occurrences

Type locality and horizon only.

Sinoeremoceras wanwanense (Kobayashi, 1931)

Figure 9, 16, 17; Table 6

Eremoceras wanwanense Kobayashi, 1931: 164; pl. 16, fig. 4a, b.

Ellesmeroceras (?) multicameratum Kobayashi, 1931: 163; pl. 16, fig. 7; pl. 19, fig. 2a, b, 3.

Wanwanoceras peculiare Kobayashi, 1933: 271; pl. 1, fig. 6, 10; pl. 2, fig. 12; pl. 4, fig. 9.

Multicameroceras cylindricum Kobayashi, 1933: 274; pl. 2, fig. 14; pl. 4, fig. 5.

Theskeloceras benxiense Chen & Teichert, 1983: 53; pl. 4, fig. 1, 2, 4.

Theskeloceras subrectum Chen & Teichert, 1983: 53; pl. 3, fig. 1, 3; pl. 10, fig. 3, 4; pl. 12, fig. 3; text-fig. 12.

Physalactinoceras benxiense Chen & Teichert, 1983: 76; pl. 5, fig. 4, 5.

Physalactinoceras confusum Chen & Teichert, 1983: 78; pl. 17, fig. 12; pl. 19, fig. 1.

Physalactinoceras cornutum Chen & Teichert, 1983: 79; pl. 14, fig. 5.

Physalactinoceras niuxintaiense Chen & Teichert, 1983: 81; pl. 1, fig. 8; pl. 19, fig. 11.

Physalactinoceras papilla Chen & Teichert, 1983: 81; pl. 1, fig. 2, 7; pl. 7, fig. 5, 8; pl. 17, fig. 7, 11; text-fig. 19.

Physalactinoceras planoconvexum Chen & Teichert, 1983: 83; pl. 7, fig. 6, 7; pl. 9, fig. 5; pl. 16, fig. 1, 3.

Physalactinoceras qiushugouense Chen & Teichert, 1983: 83; pl. 14, fig. 1, 3; text-fig. 20.

Physalactinoceras rarum Chen & Teichert, 1983: 85; pl. 3, fig. 5.

Physalactinoceras speciosum Chen & Teichert, 1983: 85; pl. 14, fig. 2; pl. 18, fig. 5.

Sinoeremoceras magnum Chen & Teichert, 1983: 87; pl. 12, fig. 1, 2, 5.

Sinoeremoceras taiziheense Chen & Teichert, 1983: 87; pl. 2, fig. 1, 4.

Wanwanoceras peculiare curtum Chen & Teichert, 1983: 89; pl. 10, fig. 1.

Benxioceras rapidum Chen & Teichert, 1983: 89; pl. 1, fig. 6; pl. 8, fig. 1–3; pl. 17, fig. 2, 8; pl. 19, fig. 5; text-fig. 21–24.

Mastoceras qiushugouense Chen & Teichert, 1983: 92; pl. 9, fig. 1–4, 7; pl. 13, fig. 1, 3; text-fig. 25, 26.

Mastoceras? obliquum Chen & Teichert, 1983: 94; pl. 2, fig. 2, 3.

Multicameroceras multicameratum—Kobayashi, 1933: 274; pl. 2, fig. 8; pl. 3, fig. 1, 3; pl. 4, fig. 1.

Sinoeremoceras wanwanense—Kobayashi, 1933: 305; pl. 2, fig. 6, 7, 9, 10; pl. 3, fig. 2, 5.

Multicameroceras multicameratum—Flower, 1954: 17; fig. 4A-D.

Sinoeremoceras wanwanense—Flower, 1954: 19; fig. 4E-H.

Multicameroceras multicameratum—Furnish & Glenister, 1964: K146; fig. 82,1a–c.

Sinoeremoceras wanwanense—Furnish & Glenister, 1964: K147; fig. 82,2a, b

Wanwanoceras peculiare—Furnish & Glenister, 1964: K147; fig. 82,3.

Protactinoceras magnitubulatum [sic.]—Chen et al., 1979a: 23; pl. 1, fig. 6.

Protactinoceras magnitubulum—Chen & Teichert, 1983: 75; pl. 7, fig. 9

Protactinoceras magnitubulatum [sic.]—Chen & Teichert, 1983: 98; pl. 3, fig. 4.

Physalactinoceras breviconicum—Chen & Teichert, 1983: 76; pl. 13, fig. 6; pl. 19, fig. 6.

Physalactinoceras changshanense—Chen & Teichert, 1983: 77; pl. 12, fig. 6.

Physalactinoceras globosum—Chen & Teichert, 1983: 79; pl. 5, fig. 3; pl. 12, fig. 4, 7; pl. 15, fig. 4, 5; pl. 19, fig. 9, 10.

Sinoeremoceras wanwanense—Kobayashi, 1989: 372; text-fig. 5.

Theskeloceras subrectum—Mutvei, Zhang & Dunca, 2007: 1328; text-fig. 1B.

Theskeloceras benxiense—Mutvei, Zhang & Dunca, 2007: 1328; text-fig. 1C, 2B.

Physalactinoceras speciosum—Mutvei, Zhang & Dunca, 2007: 1328; text-fig. 4A–D.

Physalactinoceras globosum—Mutvei, Zhang & Dunca, 2007: 1328.

Physalactinoceras cf. globosum—Mutvei, Zhang & Dunca, 2007: 1328; text-fig. 3.

Sinoeremoceras taiziheense—Mutvei, Zhang & Dunca, 2007: 1328.

Physalactinoceras globosum—Mutvei, 2020: 119; fig. 3B.

Theskeloceras benxiense—Mutvei, 2020: 119: fig. 3C, 4, 5.

Physalactinoceras cf. globosum—Mutvei, 2020: 122; fig. 6A, B.

non Sinoeremoceras taiziheense—Chen & Teichert, 1983: 88; pl. 13, fig. 4.

Emended diagnosis

Large species with adult conch height of more than 20 mm. Conch distinctly curved endogastrically during early ontogenetic stages, almost orthoconic when approaching maturity, conch cross-section compressed (CWI ≅ 0.8). Expansion rate variable (6°–41°), but with tendency to decrease during ontogeny, with ER_h ≅ 29.5° at 5 mm conch height and ER_h ≅ 17.5° at 20 mm conch height. Septal spacing displays a similar decreasing, but less variable trend during ontogeny with RCL = 0.10 – 0.004 * ch (n = 32, p < 0.001), corresponding to RCL = 0.08 at 5 mm conch height and RCL = 0.02 at 20 mm conch height. The ontogenetic increase in siphuncle size is not statistically significant (RSD = 0.16 + 0.002 * ch, n = 33), and may be caused by the placement of the plane of section. The size of the siphuncle varies between RSD = 0.11 and RSD = 0.25, with mean RSD = 0.19. Long septal flap, reaching holochoanitic condition mid-dorsally. Siphuncular segments with pronounced dorsal siphuncular bulge, moderately expanded laterally. Diaphragms generally sloping ventraperturally.

Holotype

UMUT PM 00115

Type locality and horizon

Near Qiushugou Village, Taizihe Valley, Liaoning Province, North China; Sinoeremoceras Zone (= upper Proconodontus muelleri Zone to lower Eoconodontus notchpeakensis Subzone), upper Wanwankou Member, Fengshan Formation, late Furongian, late Cambrian.

Remarks

Together with Multicameroceras multicameratum (Kobayashi, 1931), this was the first description of the typical “protactinoceratid” siphuncle. Although details of the structure of the siphuncle cannot be recognised in the original publication, they are clear in a paratype described by Kobayashi (1933). Sinoeremoceras was distinguished from Multicameroceras by the general outline of the conch (Kobayashi, 1933) but we regard the variation to be intraspecific or ontogenetic, especially considering the similarly wide and continuous variation in the Australian material of S. marywadeae sp. nov. The types of S. wanwanense and M. multicameratum differ in preservation, which may have misled Kobayashi (1931, 1933). Chen & Teichert (1983) regarded the two type species as congeneric. We cannot separate them at species level either and consider M. multicameroceratum and M. cylindricum Kobayashi, 1931 (originally separated on its lower expansion rate) to be synonyms of S. wanwanense. Many species established by Chen et al. (1979a, 1979b), Chen & Qi (1982), Chen & Teichert (1983) were based on characters that can be explained by different alignments of the plane of section; since most specimens are only available as thin sections, no indication of their relation to the median plane is available. For example, S. magnum Chen & Teichert, 1983 was distinguished from S. multicameratum solely by its larger siphuncle, but this difference may easily be caused by x-translation and/or z-rotation. Although the observed RSD varies considerably between the holotypes of those two species (0.16 vs. 0.25), it falls well within the limits of intraspecific variation seen in S. marywadeae sp. nov. and there are other specimens (which were again designated as separate species) with intermediate siphuncle sizes, e.g., S. taiziheense Chen & Teichert, 1983 (RSD = 0.23); Physalactinoceras niuxintaiense Chen & Teichert, 1983 (RSD = 0.2); S. wanwanense (RSD = 0.19 in holotype) or P. qiushugouense (RSD = 0.17). As explained above, we rely mostly on ontogenetic trajectories of conch parameters and adult size under consideration of their stratigraphic and geographic distribution for species assignment. Because intra-population variability is lower than between different populations, we group all species previously described from the Wanwankou Member of the Fengshan Formation of Liaoning under this species, although there is a strong overlap with S. bullatum from equivalent horizons of Shandong. Within Liaoning, Sinoeremoceras has been documented from three different localities within the Taizihe Valley (Wangouli and Qiushukou, both Niuxintai area and Yingzi, Huolianzhai area) and at Fuzhouwan, Liaodong Peninsula (Chen & Teichert, 1983). In summary, we consider 20 previously established species and one subspecies as subjective synonyms of S. wanwanense (Kobayashi, 1931). Specimens described from the Wanwankou Member of Liaoning reach sizes of more than 20 mm in diameter, which is larger than any of the known specimens from Anhui. We therefore consider adult size as another distinguishing character between S. wanwanense and other species of Sinoeremoceras. The diaphragms of S. wanwanense are steeply sloping adaperturally towards the venter, which may distinguish it from S. bullatum (Chen & Qi in Chen et al., 1979a) with regularly concave transverse diaphragms and S. sinense (Chen & Qi in Chen et al., 1979a) with almost straight diaphragms that slope in the opposite direction. However, the slight ω-shape of the diaphragms in the co-occurring “Physalactinoceras compressum” Chen & Qi in Chen et al., 1979a (here referred to Sinoeremoceras bullatum), and supposed species of Protactinoceras Chen & Qi in Chen et al., 1979a suggests that the diaphragms may not simply traverse the siphuncle but show a more complex three-dimensional shape, as also shown in the Australian material (Figs. 7H, 7I). There appears to be a central siphuncular ridge that may be more steeply sloping, while the diaphragms are more or less transversely concave laterally. The strong bilateral symmetry would thus also extend to the diaphragms. Consequently, distinguishing species within Sinoeremoceras based on longitudinal sections of diaphragms is also questionable. Another likely synonym is Benxioceras rapidum Chen & Teichert, 1983, which was distinguished by its large expansion rate and the shape of the septal necks. As shown above, the latter cannot serve as a diagnostic character. The expansion rate is extremely high in the holotype; however, after examination of the specimen we conclude that the expansion rate was probably overestimated, since the dorsum of the specimen appears to be strongly weathered and the plane of section was misaligned. The other specimens fit well into the variation of other Chinese Sinoeremoceras species and since all specimens are rather small, it fits into the ontogenetic trajectory of the decreasing apical angle. Unfortunately, no diaphragms were described from B. rapidum, and the diaphragms indicated by Chen & Teichert (1983) in their text-fig. 21 are difficult to recognise in the original specimen. The conch is more strongly curved than in most other specimens of S. wanwanense because it is an early ontogenetic stage, which show higher curvatures in S. marywadeae sp. nov. as well. Mastoceras qiushugouense Chen & Teichert, 1983 is interpreted as another synonym of S. wanwanense. It was established based on apparently conical diaphragms; however, they already indicated in their description that the diaphragm itself is not preserved and only indicated by the conical boundary between the calcite spar filling and the matrix. From direct observation, we conclude that this conical shape is diagenetic in origin and cannot be used in taxonomy. The supposed slight exogastric curvature of M. qiushugouense is barely recognisable and we consider the conch to be more or less straight, which is also in line with the variation and ontogenetic decrease in curvature seen in other Sinoeremoceras representatives. Similar minor differences exist between other synonymised species that can be explained by misalignment of the plane of section. Variation in conch parameters in specimens from the Wanwankou Member of Liaoning shows a continuous variation, thus not allowing the separation of multiple species (Fig. 17).

Sinoeremoceras wanwanense differs from S. inflatum and S. (?) shanxiense in its larger adult size and less strongly curved conch particularly during late ontogeny. Similarly, S. wanwanense is larger than S. magicum and S. endogastrum, but those species are characterised by a very low expansion rate. In comparison with these smaller species, the siphuncular segments appear to be more strongly expanded and the septal flaps longer in S. wanwanense, although the possible misalignment of the plane of section and the ontogenetic trajectory of the siphuncle need further studies. S. bullatum is close to S. wanwanense, but it differs by its larger conch size. This size difference also applies to the large species S. sibiriense, which is distinguished by its nearly orthoconic conch, and S. sinense that differs by its peculiarly shaped septal necks. The similarly sized S. marywadeae sp. nov. can be distinguished by its deeper lateral lobes and lower expansion rate.

Geographic and stratigraphic occurrences

Liaoning Province, North China; close to Jiangshanian/Stage 10 boundary, Furongian, late Cambrian.

Sinoeremoceras inflatum (Chen & Zou in Chen et al., 1979a) comb. nov.

Figures 18, 19; Table 7

Paraplectronoceras inflatum Chen & Zou in Chen et al., 1979a: 8; pl. 3, fig. 6.

Paraplectronoceras pyriforme Chen, Qi & Chen in Chen et al., 1979a: 7; pl. 4, fig. 12.

Paraplectronoceras suxianense Chen, Qi & Chen in Chen et al., 1979a: 7; pl. 1, fig. 5; pl. 3, fig. 14.

Paraplectronoceras impromptum Chen & Zou in Chen et al., 1979a: 7; pl. 3, fig. 2

Jiagouceras cordatum Chen & Zou in Chen et al., 1979a: 8; pl. 4, fig. 17; text-fig. 5.

Lunanoceras compressum Chen & Qi in Chen et al., 1979a: 10; pl. 3, fig. 3, 4; pl. 4, fig. 4, 5.

? Rectseptoceras eccentricum Zou & Chen in Chen et al., 1979a: 11; pl. 3, fig. 1; text-fig. 9.

Sinoeremoceras anhuiense Zou & Chen in Chen et al., 1979b: 118; pl. 3, fig. 5, 6.

Wanwanoceras multiseptum Zou & Chen in Chen et al., 1979b: 118; pl. 1, fig. 14, 15; pl. 2, fig. 13.

Paraplectronoceras curvatum Chen & Qi in Chen et al., 1980: 167; pl. 1, fig. 12.

Paraplectronoceras abruptum Chen & Qi in Chen et al., 1980: 168; pl. 1, fig. 15.

Paraplectronoceras vescum Chen & Qi in Chen et al., 1980: 168; pl. 1, fig. 20.

Parapectronoceras parvum Chen & Qi in Chen et al., 1980: 168; pl. 2, fig. 3.

Sinoeremoceras pisinum Chen, Zou & Qi in Chen et al., 1980: 179; pl. 1, fig. 18.

Paraplectronoceras pandum Chen & Qi, 1982: 396; pl. 2, fig. 11, text-fig. 4.

Paraplectronoceras longicollum Chen & Qi, 1982: 396; pl. 1, fig. 6; pl. 2, fig. 12.

Lunanoceras longatum Chen & Qi, 1982: 397; pl. 1, fig. 15.

Lunanoceras densum Chen & Qi, 1982: 397; pl. 2, fig. 10.

Acaroceras primordium Chen & Qi, 1982: 399; pl. 1, fig. 11; text-fig. 8.

Wanwanoceras exiguum Chen & Qi, 1982: 400; pl. 1, fig. 1, 2; text-fig. 11.

Paraplectronoceras pyriforme—Chen et al., 1980: 167; pl. 3, fig. 12, 13; text-fig. 2.

Paraplectronoceras suxianense—Chen et al., 1980: 168; pl. 1, fig. 9, 10.

Plectronoceras cf. huaibeiense—Chen & Qi, 1982: 396; pl. 1, fig. 3.

? Paraplectronoceras sp.—Chen & Qi, 1982: 397; pl. 1, fig. 10.

Emended diagnosis

Small conch with a maximum height of 13 mm, with compressed cross-section (CWI = 0.69, n = 6; potentially biased), slightly cyrtoconic and endogastric, expansion rate variable between 4° and 35° (mean = 13.6°, n = 20; potentially biased), without significant ontogenetic trend (ER_h = 8.1 + 0.85 * ch; p = 0.16). Septal spacing decreasing during ontogeny with RCL = 0.15 – 0.011 * ch (n = 19, p = 0.0016), corresponding to RCL = 0.1 at 5 mm conch height and RCL = 0.04 at 10 mm conch height. Siphuncle size varies strongly (likely due to misaligned planes of section) with RSD between 0.07 and 0.29 (mean = 0.16, n = 18; potentially biased). Expansion of the siphuncular segments apparently weaker than in larger species. Diaphragms poorly known, but apparently transverse, concave.

Holotype

NIGP 46198

Type locality and horizon

Hanjia, near Jiagou Town, Suxian County, Anhui Provine, North China; Acaroceras-Eburoceras Zone, upper Jiagou Member, Fengshan Formation.

Remarks

Seven species that we consider to be synonymous were described simultaneously in Chen et al. (1979a), which was published 2 months earlier than Chen et al. (1979b), containing two additional new species. We choose Paraplectronoceras inflatum Chen & Zou in Chen et al., 1979a as senior synonym, despite its holotype displaying strong signals of a misaligned section with y-rotation. The reason for this decision is that due to the misaligned section (y-rotation), the septal flap and the expanded part of the segment are clearly visible (Fig. 18A), while the specimen is otherwise average for the species in its revised scope. The holotype comes from the Acaroceras-Eburoceras Zone of the upper Jiagou Member of the Fengshan Formation of northern Anhui, which is equivalent to the Sinoeremoceras Zone (Wanwankou Member) in Shangdong and Liaoning (Chen & Teichert, 1983). Further specimens considered by us to be conspecific have been reported from the lower and upper Quadraticephalus Zone of the middle Jiagou Member (Chen et al., 1979a, 1980; Chen & Qi, 1982). S. inflatum thus has potentially the longest stratigraphic range within the genus. Lunanoceras densum Chen & Qi, 1982 and L. longatum Chen & Qi, 1982 (which was also referred to as L. elongatum Chen & Qi, 1982 in the same publication) from the lower Quadraticephalus Zone at Huaibei do not differ significantly from the slightly younger S. inflatum in any character that cannot be attributed to misalignment in the plane of section. Chen et al. (1979b) reported described and illustrated Sinoeremoceras anhuiense Zou & Chen in Chen et al., 1979b and Wanwanoceras multiseptum Zou & Chen in Chen et al., 1979b from the Acaroceras-Eburoceras Zone, which are further relatively small sized, rapidly expanding Sinoeremoceras, their siphuncles including the typical expanded segments and we see no reason to separate these species. We regard L. compressum Chen & Qi in Chen et al., 1979a as another subjective synonym of S. inflatum, because it is similar in size despite its lower expansion rate (which might result from sectioning). Small species such as S. exiguum (Chen & Qi, 1982) may be regarded as a juvenile of S. inflatum with a more slender conch. We also synonymise all previously described species of Paraplectronoceras with S. inflatum, as all of them come from the Jiagou Member of Anhui and correspond well in terms of their size and conch parameters, while supposed differences are attributable either to intraspecific or ontogenetic variation or to variation in alignment in the plane of section (Fig. 19). This includes nine additional species, i.e., P. abruptum Chen & Qi in Chen et al., 1980; P. curvatum Chen & Qi in Chen et al., 1980; P. impromptum Chen & Zou in Chen et al., 1979a; P. longicollum Chen & Qi, 1982; P. pandum Chen & Qi, 1982; P. parvum Chen & Qi in Chen et al., 1980; P. pyriforme Chen & Qi in Chen et al., 1979a; P. suxianense Chen & Qi in Chen et al., 1979a and P. vescum Chen & Qi in Chen et al., 1980. We include in S. inflatum one species previously described as an ellesmeroceratid, Acaroceras primordium Chen & Qi, 1982, due to its narrow septal spacing and slightly expanded segments with the section showing at least suborthochoanitic septal necks. This is significant insofar as all ellesmeroceratids older than the upper Quadraticephalus Zone are now re-identified as plectronoceratids, pushing the origin of the Ellesmeroceratida to slightly younger strata (see also S. (?) shanxiense (Chen & Teichert, 1983) below). The fact that no Sinoeremoceras from the Quadraticephalus or Acaroceras-Eburoceras zones of Anhui is known with diameters above 13 mm (with several species known from body chambers), supports the retention of S. inflatum as a valid separate species.

S. inflatum differs from S. marywadeae, S. wanwanense, S.bullatum, S. sibiriense and S. sinense by its small conch size. It differs from S. magicum and S. endogastrum by its more rapid expansion rate and from the latter also by its more strongly curved conch. S. (?) shanxiense is generally poorly known but appears to be slightly smaller and might be distinguished by the more compressed cross-section and a smaller siphuncle.

Geographic and stratigraphic occurrences

Northern Anhui, North China; Quadraticephalus to Acaroceras-Eburoceras Zone (= Proconodontus posterocostatus Zone to lower Eoconodontus notchpeakensis Subzone), middle to upper Jiagou Member, Fengshan Formation, late Jiangshanian to early Stage 10, Furongian, late Cambrian.

Sinoeremoceras bullatum (Chen & Qi in Chen et al., 1979a) comb. nov.

Figure 9, 20A–20C, 21; Table 8

Physalactinoceras bullatum Chen & Qi in Chen et al., 1979a: 13; pl. 3, fig. 10, 11; text-fig. 11.

Lunanoceras precordium Chen & Qi in Chen et al., 1979a: 9; pl. 2, fig. 12, 13; pl. 4, fig. 16; text-fig. 6.

Lunanoceras changshanense Chen & Qi in Chen et al., 1979a: 9; pl. 3, fig. 5; text-fig. 7.

? Protactinoceras magnitubulum Chen & Qi in Chen et al., 1979a: 12; pl. 2, fig. 5; pl. 3, fig. 12, 13; text-fig. 10.

? Protactinoceras lunanense Chen & Qi in Chen et al., 1979a: 12; pl. 2, fig. 8, 9; pl. 4, fig. 1, 2.

Physalactinoceras globosum Chen & Qi in Chen et al., 1979a: 14; pl. 2, fig. 6, 7; text-fig. 12.

Physalactinoceras changshanense Chen & Qi in Chen et al., 1979a: 14; pl. 3, fig. 7, 8.

Physalactinoceras breviconicum Chen & Qi in Chen et al., 1979a: 14; pl. 1, fig. 7; text-fig. 13.

Physalactinoceras subcirculum Chen & Qi in Chen et al., 1979a: 15; pl. 1, fig. 12, 13.

Sinoeremoceras foliosum Chen & Qi in Chen et al., 1979a: 15; pl. 1, fig. 3, 4; pl. 2, fig. 10; pl 4, fig. 13; text-fig. 14.

Sinoeremoceras zaozhuangense Chen & Qi in Chen et al., 1979a: 16; pl. 2, fig. 1, 2; text-fig. 15.

Wanwanoceras lunanense Chen & Qi in Chen et al., 1979a: 16; pl. 4, fig. 3; text-fig. 16.

Physalactinoceras compressum Chen & Teichert, 1983: 77; pl. 16, fig. 2, 4; text-fig. 18.

Physalactinoceras longiconum Chen & Teichert, 1983: 80; pl. 10, fig. 5.

Lunanoceras precordium—Chen & Teichert, 1983: 52; pl. 1, fig. 1.

Physalactinoceras cf. globosum—Chen & Teichert, 1983: 80; pl. 15, fig. 2, 3.

Sinoeremoceras foliosum—Chen & Teichert, 1983: 86; pl. 4, fig. 3.

Sinoeremoceras taiziheense—Chen & Teichert, 1983: 88; pl. 13, fig. 4.

Sinoeremoceras zaozhuangense—Chen & Teichert, 1983: 88; pl. 15, fig. 1; pl. 17, fig. 6; pl. 19, fig. 2.

Mastoceras qiushugouense—Mutvei, Zhang & Dunca, 2007: 1328; text-fig. 1A, 2A.

Mastoceras qiushugouese [sic.]—Mutvei, 2020: 119; fig. 3A.

Physalactinoceras bullatum—Pohle et al., 2022: 6; fig. 2; Supplementary Material.

non Protactinoceras magnitubulum—Chen et al., 1979a: 12; pl. 1, fig. 6.

non Physalactinoceras breviconicum—Chen & Teichert, 1983: 76; pl. 13, fig. 6; pl. 19, fig. 6.

non Physalactinoceras changshanense—Chen & Teichert, 1983: 77; pl. 12, fig. 6.

non Physalactinoceras globosum—Chen & Teichert, 1983: 79; pl. 5, fig. 3; pl. 12, fig. 4, 7; pl. 15, fig. 4, 5; pl. 19, fig. 9, 10.

Emended diagnosis

This species has a maximum known conch height of 32 mm and is thus the largest known plectronoceratid. It has a compressed cross-section with CWI = 0.75 (n = 7; potentially biased). Conch shape is slightly cyrtoconic and endogastric, with initially rapid expansion rate that decreases during ontogeny with ER = 47.2 – 1.2 * ch. (n = 7, p = 0.07; potentially biased). This results approximately in ER = 41.2° at 5 mm conch height, 29.2° at 15 mm conch height and 11.2° at 30 mm conch height. Septal spacing decreases slowly during ontogeny with RCL = 0.12–0.0035 * ch (n = 7, p = 0.002), corresponding to RCL = 0.1 at 5 mm conch height, RCL = 0.07 at 15 mm conch height and RCL = 0.02 at 30 mm conch height. Siphuncle size has been reported between RSD = 0.1 and RSD = 0.24 (mean = 0.16, n = 9; potentially biased). Siphuncular segments bulging dorsal to septal flap. Diaphragms directly transverse, concave.

Holotype

NIGP 46150

Type locality and horizon

Near Changshan Village, Taozhuang Town, Zaozhuang area, Shandong Province, North China; Sinoeremoceras Zone, Upper Middle Algal Limestone (= Wanwankou Member, upper Procondodontus muelleri Zone and lower Eoconodontus notchpeakensis Subzone), Fengshan Formation, close to Jiangshanian/Stage 10 boundary, Furongian, late Cambrian.

Remarks

A few plectronoceratid species from Shandong are characterised by concave, transverse diaphragms, such as Sinoeremoceras bullatum (Chen & Qi in Chen et al., 1979a) comb. nov., which shows the shape of the diaphragms and the transition of the septal flap and the siphuncular bulges quite well. Although seemingly different inclinations of the diaphragms may result from differently aligned sections of the siphuncle with regards to the central ridge of the diaphragm, but it is a conspicuous pattern of directly transverse diaphragms that is not known in the otherwise very similar S. wanwanense from Liaoning. Additional indications that these species are separate are given by the larger size of the specimens from Shandong and by the tendency for more strongly endogastrically curved conchs in S. bullatum, in contrast to the nearly orthoconic conch of most specimens from Liaoning. Conch parameters display continuous variation among specimens here assigned to S. bullatum. According to Chen et al. (1979a), the difference between Lunanoceras precordium and L. changshanense Chen & Qi in Chen et al., 1979a are more sharply bent septal necks and a rounder cross-section, both of which can easily be explained by a misaligned plane of section or slight intraspecific variation. Physalactinoceras compressum Chen & Teichert, 1983 has transverse diaphragms, which are somewhat ω-shaped. We interpret this as further evidence for more complex diaphragm morphology than might be anticipated (compare diaphragms in S. marywadeae sp. nov.) but treat the species as a synonym of S. bullatum until they are better understood. Using the siphuncle of S. marywadeae sp. nov. as a reference, we cannot find indications for distinct species within the Wanwankou Member of Shandong, with the possible exception of S. sinense, which has peculiarly S-shaped septal necks that can at the moment not be explained by misalignment of the plane of section. Thus, we keep both S. bullatum and S. sinense as the only sympatric species pair but note that S. sinense falls otherwise within the range of variation in S. bullatum. Several species from Shandong are only known from sections outside the septal flap; thus, synonymisation with S. bullatum is mainly for convenience and to avoid declaring nomina dubia. These synonyms are marked as tentative above.

S. bullatum differs from S. wanwanense in its more strongly curved conch and its more regularly concave and transverse diaphragms. S. bullatum is larger than most other species of Sinoeremoceras except for S. sibiriense, which differs by having a straight conch.

Geographic and stratigraphic occurrences

Type locality and horizon only.

Sinoeremoceras endogastrum (Li, 1984) comb. nov.

Figures 22, 23C, 24; Table 9

Parapalaeoceras endogastrum Li, 1984: 230; pl. 7, fig. 12, 13; text-fig. 18.

Parapalaeoceras sinense Li, 1984: 232; pl. 7, fig. 14, 15; text-fig. 19.

Emended diagnosis

Relatively small, up to 9 mm phragmocone diameter; body chamber unknown. Conch nearly orthoconic, with low expansion rate, mean ER_h = 6.5° (n = 2) and apparently relatively broad cross-section (CWI = 0.87) and more narrowly rounded ventral side (single specimen with probably misaligned plane of section). Septal spacing relatively wide, RCL = 0.09 at a conch height of approximately 8 mm, though wider adapically. Siphuncle with mean RSD = 0.16 (n = 2). The expansion of the segments is slight.

Holotype

NIGP 79753

Type locality and horizon

Duibian Village, near Jiangshan City, western Zhejiang Province, South China; Cephalopod Bed DD-14 (= KD-60), 1–3 m below the Lotagnostus hedini Zone, in association with L. punctatus (= L. americanus), Acaroceras-Antacaroceras Zone (= L. americanus Zone, corresponding to the Proconodontus posterocostatus Zone in Laurentia), lower Siyangshan Formation (stratotype section of the Jiangshanian Stage, see Peng et al., 2012).

Remarks

This species is closely similar to S. inflatum from the Jiagou Member of the Fengshan Formation of northern Anhui. It differs by the less strongly curved, almost orthoconic conch and lower expansion rate, although the latter is difficult to judge from the few available specimens. The chambers are relatively long compared to other plectronoceratids, though they hardly exceed 1 mm. Since there are only two reported specimens, data on conch parameters and their intraspecific and ontogenetic variation is scarce. The maximum extent of the expanded siphuncular segments and the detailed morphology of the septal flap are unknown. We consider Parapalaeoceras sinense Li, 1984 a subjective junior synonym because the supposed differences, namely the larger expansion rate and siphuncle size hardly go beyond a level that cannot be attributed to intraspecific variation or misaligned planes of section (Fig. 24). S. endogastrum is chosen as the senior synonym because S. sinense Chen & Qi in Chen et al., 1979a is preoccupied. The species is the only plectronoceratid from Zhejiang and South China in general. It is possible that several other species described as ellesmeroceratids or yanheceratids represent further misaligned sections of S. endogastrum, though none shows clearly expanded siphuncular segments. For example, Yanheceras shanbeilingense Li, 1984 shows similarly short septal spacing, but long straight septal necks. Without having access to 3D reconstructions, it is thus impossible to tell, whether these straight septal necks were straight around the entire septal foramen, or whether they represent a section exactly midway through the septal flap.

Geographic and stratigraphic occurrences

Type locality and horizon only.

Sinoeremoceras magicum Chen in Lu, Zhou & Zhou, 1984

Figures 23B, 24; Table 10

Sinoeremoceras magicum Chen in Lu, Zhou & Zhou, 1984: 123; pl. 1, fig. 13, 14.

Emended diagnosis

Medium-sized (body chamber at~10 mm conch height), with endogastric curvature, short septal spacing (RCL = 0.05, n = 1) and low expansion rate (ER_h = 7°, n = 1). Cross-section compressed, elliptical (CWI unknown, no measurements given, and cross-section not figured). Siphuncle ventral marginal, with expanded segments; septal necks cyrtochoanitic, sharply bent outwards. Suture with broad and shallow lateral lobes and narrowly rounded dorsal and ventral saddles.

Holotype

NIGP 65428

Type locality and horizon

Zhusilenghaierhan Hill, Ejin Banner, Alxa League, Inner Mongolia, North China; Sinoeremoceras Zone (bed h13), (corresponding to the late Pre-Payntonian in Australia according to Lu, Zhou & Zhou (1984), i.e., Proconodontus posterocostatus to P. muelleri zones), Jiangshanian/Stage 10, Furongian, late Cambrian.

Remarks

This species was only very briefly described in the figure captions of Lu, Zhou & Zhou (1984); its main characteristics are translated above, adjusted in accordance with our revision of Sinoeremoceras and with the addition of the corresponding conch parameters. The cross-section was described as elliptical, and it appears probable that CWI would fall within a similar range as other species. This species is one of the few described with visible sutures, making comparisons with the Australian species possible. In contrast to the latter, S. magicum has very shallow lateral lobes. It is similar in size to S. inflatum and distinctly smaller than S. wanwanense and S. bullatum. However, the expansion rate is relatively slow, similar to S. endogastrum, from which it differs in its cyrtoconic conch. Another argument for the provisional retention of this species is its isolated geographic position, being the only report of a plectronoceratid from Inner Mongolia. Because only a single specimen is known, nothing can be said about its intraspecific variation or detailed siphuncular structure.

Geographic and stratigraphic occurrences

Type locality and horizon only.

Sinoeremoceras (?) shanxiense (Chen & Teichert, 1983) comb. nov.

Figures 23D, 24, 25; Table 11

Hunyuanoceras shanxiense Chen & Teichert, 1983: 59; pl. 5, fig. 1; Text-fig. 13.

Emended diagnosis

Adult body chamber height slightly below 10 mm; endogastric; septal spacing short (RCL = 0.05 at conch height of 8.7 mm); expansion rate moderate (ER_h = 19°). Cross-section apparently very strongly compressed (CWI = 0.57, possibly biased). Siphuncle marginal ventrally, RSD = 0.14, structure poorly known, but seems to consist of both cyrtochoanitic and orthochoanitic septal necks, the latter of which are tentatively interpreted as evidence for a septal flap. Diaphragms apparently present but poorly preserved.

Holotype

NIGP 73768

Type locality and horizon

Xuankong Temple, Hunyuan County, Datong City, Shanxi Province, North China; lower Quadraticephalus Zone (= Proconodontus posterocostatus Zone), Yenchou Member, Fengshan Formation, Jiangshanian/Stage 10, Furongian, late Cambrian.

Remarks

This species was originally described as an ellesmeroceratid. Although the holotype is poorly preserved, Chen & Teichert (1983, text-fig. 13) illustrated a transition from apparently orthochoanitic to cyrtochoanitic septal necks, which suggests tentative assignment to Sinoeremoceras. However, the septal necks are difficult to differentiate in the holotype itself. The strongly compressed cross-section and correspondingly low CWI is possibly underestimated due to misalignment of the section; it also appears that part of the dorsum is missing in the holotype. The paratype represents a strongly displaced section that does not expose the siphuncle but can be tentatively assigned to the same species based on the very dense septal spacing.

Geographic and stratigraphic occurrences

Type locality and horizon only.

Sinoeremoceras sibiriense (Balashov, 1959) comb. nov.

Figures 23A, 24; Table 12

Multicameroceras sibiriense Balashov, 1959: 37; pl. 5, fig. 12.

Multicameroceras (?) sibiricum [sic.]—Balashov, 1962b: 8; pl. 2, fig. 10a-c.

Multicameroceras siberiense [sic.]—Flower, 1964: 161.

Emended diagnosis

Conch orthoconic, slowly expanding, with smooth shell. Cross-section oval (CWI unknown), slightly compressed laterally. Septal spacing very short, 17 chambers within the same distance as the conch diameter (RCL ≅ 0.06). Septal concavity equals the length of almost two chambers (SCI ≅ 0.12). Suture with weak lateral lobes. Siphuncle ventral, slightly laterally compressed. Septal necks short, slightly bent outwards (= suborthochoanitic or cyrtochoanitic?). Siphuncular segments slightly expanded. Connecting rings poorly visible due to recrystallisation. Diaphragms not known.

Holotype

SPB 13/426 (SPB 880/9135 according to Balashov, 1962b)

Type locality and horizon

Chunya River, Krasnojarsk, Siberia, Russia, specific locality unknown; Chunya Regional Stage.

Remarks

The main characteristics of this species are translated above from Balashov (1959) and adjusted in accordance with the current revision of Sinoeremoceras. This species was first mentioned in English language literature in the addendum to Flower (1964). Dzik (2020) presented rich material from the Ust-Kut Formation, of the Angara River in southern Krasnojarsk, Siberia. The holotype of S. sibiriense comes from the probably coeval or slightly younger Chunya Regional Stage on the Chunya River, which is a few hundred kilometres to the north of Dzik’s (2020) locality, although the exact type locality and horizon of S. sibiriense are not well defined. Nevertheless, Balashov (1962b) reported the same species from the Angara River and the similarity of the ellesmeroceratid specimens from these two localities, reported by Balashov (1962b) and Dzik (2020), suggest that they are roughly of the same age. Furthermore, the cephalopods of the Ust-Kut Formation generally resemble other Cambrian faunas (e.g., that of the Wanwankou Member) and thus, it appears plausible that S. sibiriense is of similar or slightly younger age as other plectronoceratids. Balashov (1959) distinguished the species from M. multicameratum by the presence of diaphragms and absence of connecting rings in the latter species; however, these can hardly serve as diagnostic characters, since diaphragms are restricted to juvenile portions of the conch and both structures may be susceptible to diagenesis. Nevertheless, we retain this species because it is the only report of Sinoeremoceras from Siberia and more material is needed in order to properly assess its similarity to the Australian and Chinese material. The only described specimen is in the upper size range but comparable to Sinoeremoceras species from the Wanwankou Member, although it does not preserve the body chamber. Balashov (1962b) mentioned two additional specimens but did not provide measurements or images. The species may be distinguished from other Sinoeremoceras species by its relatively large size in combination with the very low expansion rate and an essentially orthoconic conch.

Geographic and stratigraphic occurrences

Ust-Kut and Chunya Regional Stages (poorly dated; according to Dzik (2020) not older than Cordylodus proavus Zone, Stage 10, Furongian, but older than Cordylodus angulatus Zone, earliest Tremadocian, Early Ordovician); Chunya and Angara Rivers, Krasnoyarsk, Siberia, Russia.

Sinoeremoceras sinense (Chen & Qi in Chen et al., 1979a) comb. nov.

Figures 20D, 20E, 24; Table 13

Eodiaphragmoceras sinense Chen & Qi in Chen et al., 1979a: 10; pl. 4, fig. 6–10, 15.

Eodiaphragmoceras sinense Chen & Qi, 1979—Chen & Teichert, 1983: 51; pl. 2, fig. 5.

Emended diagnosis

Conch almost orthoconic, only very slightly endogastric, with maximum reported height of 26 mm, though no body chambers are known. Cross-section strongly compressed, CWI = 0.67 (n = 1), though it is possible that this an underestimation, partially caused by an misaligned section. Expansion rate is moderate, with 15°. There is not enough data on the ontogenetic trajectory of septal spacing, but at 26 mm conch height, RCL = 0.03. Sutures partially known, apparently almost straight transverse, with only very slight lateral lobes. The siphuncle has RSD = 0.18. In the known sections, the siphuncle appears to be only slightly expanded and the septal necks somewhat S-shaped. Diaphragms slope towards the dorsum adaperturally.

Holotype

NIGP 46127

Type locality and horizon

Near Changshan Village, Taozhuang Town, Zaozhuang area, Shandong Province, North China; Sinoeremoceras Zone, Upper Middle Algal Limestone (= Wanwankou Member, upper Procondodontus muelleri Zone and lower Eoconodontus notchpeakensis Subzone), Fengshan Formation, close to Jiangshanian/Stage 10 boundary, Furongian, late Cambrian.

Remarks

The structure of the connecting ring of S. sinense, which was originally proposed as the type species of Eodiaphragmoceras Chen & Qi in Chen et al., 1979a is not well known. However, the apparent variation between cyrto-, ortho- and holochoanitic septal necks indicates a septal flap and therefore, Eodiaphragmoceras cannot be separated from Sinoeremoceras. The slight S-shape of the septal necks and the slope direction of the diaphragms are nevertheless difficult to explain by reference to the effect of a misaligned plane of section and they differentiate this species within the genus. Because S. sinense and S. bullatum co-occur, it is difficult to assign some specimens to either species, especially if the plane of section does not pass through the septal flap as in Protactinoceras magnitubulum Chen & Qi in Chen et al., 1979a. Because we only assign specimens to S. sinense if S-shaped septal necks are clearly present, it is possible that some specimens currently assigned to S. bullatum must be transferred to S. sinense in the future.

Other than septal neck shape and size, this species is distinguished by its sutures, which appear to be almost straight, lacking the lateral lobes of, for example, S. marywadeae sp. nov.

Geographic and stratigraphic occurrences

Type locality and horizon only.

Genus Palaeoceras Flower, 1954

Type species

Palaeoceras mutabile Flower, 1954

Included species

Only the type species.

Emended diagnosis

Conch slender (ER_h ≅ 3–8°), compressed (CWI ≅ 0.7–0.9) orthoconic to very slightly cyrtoconic (exogastric). Septal spacing decreasing strongly during ontogeny, with relatively long chambers compared to other plectronoceratids at the earliest known ontogenetic stages (up to RCL = 0.2), but slightly below RCL = 0.1 in the largest known specimens, i.e., at about 8 mm diameter. Septal necks orthochoanitic or hemichoanitic. Expanded, straight or concave parts of siphuncular segments occur.

Remarks

Palaeoceras Flower, 1954 differs from Sinoeremoceras by its essentially orthoconic conch. Flower (1954, 1964) described marked changes of the siphuncle during ontogeny, from expanded segments with hemichoanitic septal necks to straight segments and macrochoanitic septal necks. These apparent ontogenetic changes are reminiscent of the misinterpretations in other plectronoceratids, where similar ontogenetic changes seemed to be present, but were, in fact, due to misaligned planes of section. From the available specimens, it is impossible to interpret the three-dimensional shape of the siphuncle. Presumably, Flower (1954, 1964)— like other cephalopod researchers at that time—assumed that the siphuncle was more or less radially symmetric (as it is the case in all non-plectronoceratid cephalopods), which led to his interpretation of the siphuncle. Flower (1954, 1964) described the septal necks as longer on the ventral side than on the dorsal side, which together with the lack of cyrtochoanitic septal necks does not correspond to the septal flap seen in Sinoeremoceras marywadeae sp. nov. It is possible that he was misguided by the extremely oblique siphuncular segments, which cause the ventral edge of the septum near the septal foramen to be almost parallel to the siphuncle, making the boundary to the septal necks difficult to discern. Future studies will need to establish, whether the three-dimensional interpretations by Flower (1954, 1964) were accurate, ideally using 3D imaging techniques. At the very least, the published sectioned specimens reveal strongly inclined and expanded siphuncular segments reminiscent of Sinoeremoceras but differ in their lack of cyrtochoanitic septal necks. The shape of the septal flap or if there is one at all is unclear. Accordingly, Palaeoceras is transitional between plectronoceratids and ellesmeroceratids, which have straight to concave siphuncular segments. Because of the shape of these segments, we retain the genus within the Plectronoceratidae.

Balkoceras Flower, 1964 is considered a junior subjective synonym of Palaeoceras Flower, 1954, because its type species Balkoceras gracile Flower, 1964 is essentially identical with Palaeoceras mutabile Flower, 1964, except for the very slight exogastric curvature. Flower (1964, p. 34) wrote “Indeed, I had at first assigned the present species to Palaeoceras, and surely the affinities are close enough that this course would have been eminently justifiable, but so great has the importance of curvature become that such a course would only result in the proposal of a new genus for this form by the next person to give these older cephalopods any attention”. Comparing the proportions of the various specimens of Palaeoceras and Balkoceras described by Flower (1954, 1964) reveals that they are so close to each other that even the separation at species level is untenable, especially as the exogastric curvature is very subtle.

Palaeoceras mutabile Flower, 1954

Figures 26, 27; Table 14

Palaeoceras mutabile Flower, 1954: 10; pl. 1, fig. 5, 9, 10; pl. 2, fig. 11; pl. 3.

Plectronoceras exile Flower, 1964: 30; pl. 4, fig. 13–16; pl. 5, fig. 1.

Palaeoceras undulatum Flower, 1964: 32; pl. 1, fig. 1–10; pl. 2, fig. 8–10.

Balkoceras gracile Flower, 1964: 34; pl. 2, fig. 1–3; pl. 3, fig. 10–15.

Palaeoceras mutabile—Flower, 1964: 31; pl. 2, fig. 4–7, 11–18; pl. 3, fig. 1–9.

Palaeoceras mutabile—Furnish & Glenister, 1964: K146; fig. 83,1a-b.

Emended diagnosis

As for genus, by monotypy.

Holotype

BEG 34757

Type locality and horizon

San Saba Member, 67 feet below the top (= Cambrooistodus minutus Subzone, Miller, Loch & Taylor, 2012, fig. 5), Wilberns Formation; Threadgill Creek section, northern Gillespie County, Texas, USA.

Remarks

The three-dimensional structure of the siphuncle is unknown. Furthermore, the sections are difficult to compare to those of the Asian taxa because those showing variation between hemichoanitic to cyrtochoanitic septal necks were made by polishing the specimens from the ventral side, but the illustrations and descriptions by Flower (1954, 1964) give little indication on the position of the xz-plane of the section relative to the siphuncle. Specimens sectioned longitudinally invariably have long orthochoanitic septal necks. The connecting ring is mostly missing, though the segments appear to be expanded apically (Fig. 26C).

The specimens described as Balkoceras gracile Flower, 1964 are unique among plectronoceratids in their exogastric curvature. For this reason, Flower (1964) established a new family, while noting the close similarity to Palaeoceras mutabile. However, his own measurements show that the dimensions of all specimens described as either P. mutabile Flower, 1954; P. undulatum Flower, 1964; Plectronoceras exile Flower, 1964 (already suggested as referrable to Palaeoceras by Chen & Teichert, 1983) or B. gracile Flower, 1964 fall within a very narrow range, making even a distinction at species level questionable. Thus, curvature is the only character separating P. mutabile from P. gracile, but note that the difference is very slight and we tentatively synonymise those species as they overlap in all other conch parameters (Fig. 27). The combination of an essentially straight conch with low expansion rate, lack of cyrtochoanitic septal necks and small size separate Palaeoceras from species of Sinoeremoceras.

Geographic and stratigraphic occurrences

Type locality and horizon only.

Genus Plectronoceras Ulrich & Foerste, 1933

Type species

Plectronoceras cambria (Walcott, 1905)

Included species

Only the type species.

Emended diagnosis

Small plectronoceratid with adult size below 5 mm, cyrtoconic conch with moderate expansion rate (ER_h ≅ 15°) and compressed cross-section (CWI ≅ 0.8). Chambers short with RCL between about 0.05 and 0.1 with a tendency to decrease during ontogeny.

Remarks

Plectronoceras Ulrich & Foerste, 1933 is the type genus of the Plectronoceratida, and due to its stratigraphic position the most often cited Cambrian cephalopod. Unfortunately, its internal characters are poorly known. However, if the single connecting ring known from Plectronoceras and the variation between orthochoanitic to cyrtochoanitic septal necks is taken as evidence for a septal flap, then there is little that distinguishes this genus from Sinoeremoceras apart from size. Indeed, small species of Sinoeremoceras, such as S. inflatum are so similar that they could also be assigned to Plectronoceras. Nevertheless, this needs to be confirmed, ideally using three-dimensional reconstructions of the siphuncle. We regard the genera as separate but note that a future suppression of Sinoeremoceras in favour of Plectronoceras appears likely to us. As Plectronoceras exile Flower, 1964 is considered by us as a synonym of Palaeoceras mutabile Flower, 1954 (see also Chen & Teichert, 1983), the genus is at present exclusively known from the Ptychaspis-Tsinania Zone of North China.

Plectronoceras cambria (Walcott, 1905)

Figure 28, 29; Table 15

Cyrtoceras cambria Walcott, 1905: 22.

Plectronoceras liaotungense Kobayashi, 1935: 17; pl. 4, fig. 1–3.

Plectronoceras huaibeiense Chen & Qi in Chen et al., 1979a: 6; pl. 3, fig. 9.

Cyrtoceras cambria—Walcott, 1913: 98; pl. 6, fig. 4, 4a-c.

Plectronoceras cambria—Miller, 1943: 99; fig. 1A–D.

Plectronoceras cambria—Ulrich et al., 1944: 133; pl. 68, fig. 4–11.

Plectronoceras cambria—Flower, 1954: 15; fig. 3C–F.

Plectronoceras liaotungense—Flower, 1954: 16; fig. 3A-B.

Plectronoceras cambria—Balashov, 1962a: 73; pl. 5, fig. 6.

Plectronoceras cambria—Furnish & Glenister, 1964: K146; fig. 81a, b.

Plectronoceras liaotungense—Furnish & Glenister, 1964: K146; fig. 81c.

Plectronoceras cambria—Yochelson, Flower & Webers, 1973: 277; fig. 2.

Plectronoceras cf. cambria—Chen et al., 1979a: 6; pl. 4, fig. 11, 14; text-fig. 4.

Plectronoceras cambria—Dzik, 1984: 14; fig. 2.

Plectronoceras liaotungense—Dzik, 1984: 14; fig. 2.

Plectronoceras huaibeiense—Dzik, 1984: 14; fig. 2.

Plectronoceras liaotungense—Kobayashi, 1989: 370; text-fig. 1.

Plectronoceras liaotungense—Webers, Yochelson & Kase, 1991: 347; fig. 1.

Plectronoceras cambria—Kröger, 2003: 43; text-fig. 4, 11.

Plectronoceras cambria—Kröger, Vinther & Fuchs, 2011: 604; fig. 3C, D.

Plectronoceras cambria—Vinther, 2015: 23; fig. 2.

Plectronoceras cf. cambria—Fang et al., 2019: fig. 2.

non Plectronoceras cf. huaibeiense Chen & Qi—Chen & Qi, 1982: 396; pl. 1, fig. 3.

Emended diagnosis

As for genus.

Holotype

USNM 57819

Type locality and horizon

Kaolishan, Tai’ain City, Shandong Province; Ptychaspis-Tsinania Zone (= Proconodontus tenuiserratus Zone, compare Bagnoli et al., 2017), lower Yenchou Member, Fengshan Formation.

Remarks

In terms of proportions and adult size, the three described species of Plectronoceras are indistinguishable. Kobayashi (1935) credited P. cambria Walcott, 1905 with a more slender conch, a more ovate cross-section, longer chambers and a less curved suture to distinguish it from P. liaotungense Kobayashi, 1935. P. huaibeiense Chen & Qi in Chen et al., 1979a was described without differential diagnosis. We find no justification for separation of different Plectronoceras species, as the variability among different specimens assigned to Plectronoceras is generally small (Fig. 29) and we attribute those differences to ontogenetic or intraspecific variation. Variation between different Plectronoceras specimens is smaller than the variation within a single species of Sinoeremoceras. Remarkably, P. cambria appears to have a wider geographic distribution than later plectronoceratids, as contemporaneous specimens from Anhui, Liaoning and Shandong have almost identical proportions, while there are considerable size differences indicating separate populations or species of Sinoeremoceras. The specimen described by Chen & Qi (1982) as Plectronoceras cf. huaibeiense Chen & Qi in Chen et al., 1979a from the lower Quadraticephalus Zone of Suxian, Anhui likely does not belong to Plectronoceras and is better referred to the co-occurring Sinoeremoceras inflatum (Chen & Zou in Chen et al., 1979a).

Geographic and stratigraphic occurrences

Liaoning, Shandong and Anhui Provinces, North China; Ptychaspis-Tsinania Zone (= Proconodontus tenuiserratus Zone), lower Yenchou and lower Jiagou members, respectively, Fengshan Formation, Jiangshanian, Furongian, late Cambrian.