Historical significance and taxonomic status of Ischyrodon meriani (Pliosauridae) from the Middle Jurassic of Switzerland

Regional geology and the geographic origin

‘Wölflinswyl’, now spelled Wölflinswil, is located in the Swiss canton Aargau near the villages of Frick and Herznach (Fig. 1) and, on a broader scale, between the towns of Basel and Zürich. In this area, Triassic and Jurassic sediments crop out. Here, we focus on the Jurassic since predominantly Jurassic sediments are exposed in this area. In this part of Switzerland, the Jura mountains, the Mesozoic strata are strongly folded and faulted tectonically in the south (‘Faltenjura’—folded Jura), whereas the area of interest lies in the ‘Tafeljura’ (table Jura), which was much less affected by the Alpidic orogeny (Herzog, 1956).

In the region between Frick (Gipf-Oberfrick), Hornussen, Wölflinswil and Herznach, Hettangian to Oxfordian sedimentary rocks reach the surface. While the strata of the Lower Jurassic Staffelegg-Formation (Reisdorf et al., 2011) are predominantly clayey with some marls and a few limestone beds, the Middle Jurassic sequence bears a great variety of marine sedimentary rocks including marls, oolites, crinoidal limestones, etc. (e.g., Gonzalez & Wetzel, 1996).

Both the tooth and a rock sample contained in the same box show that the matrix is extremely rich in iron. This suggests that the tooth comes from the iron ore that was mined from around 1,200 until 1967 in various quarries and underground mines (Hüsser, 1996; Hüsser, 2002). Some of these former mines are indicated in the map (Fig. 1). Since the labels accompanying the specimen provide Wöflinswil as the locality, we must assume a locality that is closer to that village than to the neighboring village of Herznach, where parts of the biggest and most recent mine is still accessible. The former mines and quarries of Hinter-Raibach and Dachseln lie closest to Wöflinswil. According to Hüsser (1996), mines on the Chornberg—a small mountain between Herznach and Wöflinswil—were documented already in 1772. This fits reasonably well with the possible times of mining around 1800 at Hinter-Raibach or Dachseln. Nevertheless, we cannot rule out that the tooth was found outside of the mines in the scree.

Stratigraphic position

Detailed accounts of the stratigraphic position of the iron ores in the Frick region have been published repeatedly (Jeannet, 1951; Jeannet, 1954; Fehlmann & Rickenbach, 1962; Gygi, 1981; Stössel, 2002; Stössel, Pika-Biolzi & Ungricht, 2010; Bitterli-Dreher, 2021). The original labels indicate ‘Rogeneisenstein’. Together with the sedimentary facies of the rock still attached to the fossil, the host rock is an iron oolite. In that region, these red iron oolites are known from the Callovian exclusively (Jeannet, 1951; Stössel, Pika-Biolzi & Ungricht, 2010; Bitterli-Dreher, 2021). The miners referred to the ‘Untere Erzbank’ (lower ore bed) and the ‘Oberes Erzlager’ (upper ore bed). These overlie the Hauptrogenstein, which is Bajocian to Bathonian in age. The Bajocian and Bathonian of Switzerland occasionally yields remains of marine reptiles such as plesiosaurs (Sachs, Klug & Kear, 2019) and large ichthyosaurs (CK, ongoing research), but usually, these sediments are grey and rich in echinoderms.

Around Herznach and Wölflinswil, the two ore layers contain abundant and well preserved internal moulds of a diverse ammonite fauna (Jeannet, 1954; Gygi & Marchand, 1982). In particular, large specimens of Erymnoceras, Kamptocephalites, Macrocephalites, and phylloceratids have been found (Jeannet, 1951; Bitterli-Dreher, 2021), indicating a Callovian age (Fig. 2). The biostratigraphy of the Ifenthal Formation was portrayed by Bitterli-Dreher (2012), but the last comprehensive revision dates back to Jeannet (1951). In his dissertation, Hostettler (2014) revised the Middle Jurassic ammonites. Younger sedimentary rocks of Oxfordian age may also contain ooids and may be fairly rich in iron, but they tend to have a more greyish to greenish color.

Multivariate analyses

In order to explore the morphospace occupation of NMB L.D.37 among thalassophonean pliosaurids, and assess previously mentioned similarities of the type of Ischyrodon to Pliosaurus (Lydekker, 1889) and Liopleurodon (Tarlo, 1960), we performed cluster and principal coordinates analyses. We used the dataset of Bastiaans et al. (2021), which is a recent version of the one first published by Zverkov et al. (2018), and added NMB L.D.37. No further modifications have been provided. See Supplemental Information I for the matrix.

We used the same protocol as Zverkov et al. (2018) and Bastiaans et al. (2021). The analyses were carried out in the R statistical environment (RStudio Version 1.2.5033 R Studio Team, 2019). We applied a 50% completeness threshold to remove the influence of taxa which are based upon insufficiently complete or preserved specimens, scaled the data to equal variance and a mean of zero through subtraction of the mean value for each character and divided it by the standard deviation. Using the package cluster v2.1.0 we applied the Gower metric (Gower, 1971) to create a distance matrix. A cluster dendrogram was produced from the resulting matrix with the stats package, using the Ward.D2 method.

Then, we took the same matrix and used ape v5.3 (Paradis, Claude & Strimmer, 2004) to explore the dental morphospace occupation of I. meriani among the thalassophonean pliosaurids through a principal coordinates analysis. We applied the Gower metric and the Cailliez correction for negative eigenvalues. See Supplemental Information II and Supplemental Information III for the R code and extended results, respectively.

Terminology of tooth orientation and morphology

We follow the terminology of tooth orientation of Smith & Dodson (2003): apical, toward the apices of the tooth crown or the tooth root; basal, toward the cervix dentis; distal, away from the tip of the snout; labial, toward the lips; lingual, toward the tongue; mesial, toward the tip of the snout.

The description of the characters of the outer enamel surface is provided using the nomenclature advocated by Zverkov et al. (2018) and followed in Páramo-Fonseca, Benavides-Cabra & Gutiérrez (2018), Madzia, Sachs & Lindgren (2019), Lukeneder & Zverkov (2020), Bastiaans et al. (2021), Madzia, Szczygielski & Wolniewicz (2021): apicobasal ridges, longitudinally running enamel ridges of variable apicobasal extent that can be developed around the entire crown circumference and are usually semicircular or triangular in cross-section; ridglets, subtle apicobasally-oriented enamel structures often developed between adjacent apicobasal ridges or on an unridged enamel surface; the appearance of ridglets varies from being very smooth to producing a distinct vermicular pattern (see Madzia, 2016: Fig. 7).

Systematic paleontology

Type specimen. NMB L.D.37, an isolated tooth crown (Fig. 3).

Locality and age. Wölflinswil, near the villages of Frick and Herznach, Canton of Aargau, Switzerland; most likely Callovian, Middle Jurassic.

Description and comparisons. The size, stoutness, and character of the linguodistal curvature of the tooth crown indicate that it originates from the anterior half of the right maxilla or the left dentary. The preserved part of the crown of NMB L.D.37 is approximately 107.8 mm high (total crown height is estimated to reach ∼110 mm) and its maximum cross-sectional diameter is ∼50 mm. The basal cross-section is sub-circular, similar to the condition observed in Acostasaurus pavachoquensis (Gómez-Pérez & Noè, 2017), Brachauchenius lucasi (Liggett et al., 2005; Albright, Gillette & Titus, 2007), Monquirasaurus boyacensis (Hampe, 1992; Noè & Gómez-Pérez, 2021), Kronosaurus queenslandicus (McHenry, 2009), Liopleurodon ferox (Noè, 2001), Marmornectes candrewi (Ketchum & Benson, 2011a), Megacephalosaurus eulerti (Madzia, Sachs & Lindgren, 2019), Pachycostasaurus dawni (Cruickshank, Martill & Noè, 1996; Noè, 2001), Peloneustes philarchus (Ketchum & Benson, 2011b), ‘Pliosaurus’ andrewsi (Tarlo, 1960; Noè, 2001; Zverkov et al., 2018), ‘Polyptychodon’ hudsoni (D. Madzia, personal observation, 2018), Sachicasaurus vitae (Páramo-Fonseca, Benavides-Cabra & Gutiérrez, 2018), Simolestes vorax (Tarlo, 1960; Noè, 2001; Zverkov et al., 2018), the assemblage historically assigned to ‘Polyptychodon interruptus’ (Madzia, 2016), and other isolated pliosaurid teeth with indeterminate taxonomic status and phylogenetic placement (e.g., Kear et al., 2014; Zverkov, 2015; Madzia & Machalski, 2017; Zverkov et al., 2018; Lukeneder & Zverkov, 2020; Solonin, Vodorezov & Kear, 2021; Bastiaans et al., 2021). It differs from Gallardosaurus itturraldei (Gasparini, 2009), Luskhan itilensis (Fischer et al., 2017), Makhaira rossica (Fischer et al., 2015), Pliosaurus (e.g., Andrews, 1913; Knutsen, 2012; Benson et al., 2013), Stenorhynchosaurus munozi (Páramo-Fonseca et al., 2016), and some other tooth crowns (e.g., Zverkov, 2015) that possess alternatively sub-trihedral (G. itturraldei, L. itilensis, P. kevani, St. munozi) to trihedral (Mak. rossica, Pliosaurus spp.), and trihedral-to-trapezoidal (Zverkov, 2015) crown cross-sections. The specimen NMB L.D.37 lacks carinae, unlike numerous Late Jurassic and Early Cretaceous pliosaurids that show one (G. itturraldei, L. itilensis, P. kevani, S. munozi), two (most species of Pliosaurus), or three (Mak. rossica) carinae (see Gasparini, 2009; Benson et al., 2013; Fischer et al., 2015; Zverkov, 2015; Páramo-Fonseca et al., 2016; Fischer et al., 2017; Zverkov et al., 2018).

The enamel of NMB L.D.37 shows prominent apicobasal ridges that are approximately triangular in cross-section, similar to the condition observed, for example, in A. pavachoquensis, B. lucasi, Mo. boyacensis, L. ferox, Mak. rossica, Mar. candrewi, Pa. dawni, Pe. philarchus, Si. vorax, St. munozi, some species of Pliosaurus, and some tooth crowns from the assemblage historically assigned to ‘Polyptychodon’ (Madzia, 2016; Zverkov et al., 2018). No ridge has been observed to be branching, as in most pliosaurids, but unlike in Pa. dawni, the Western Interior brachauchenines (Madzia, Sachs & Lindgren, 2019), and the ‘Maryevka pliosaurid’ (Zverkov et al., 2018). Enamel surface shows clear ridglets, similar to those in L. ferox, some species of Pliosaurus, some isolated pliosaurid tooth crowns from the Jurassic/Cretaceous boundary interval of Russia (Zverkov et al., 2018), some tooth crowns from the assemblage historically assigned to ‘Polyptychodon’ (Madzia, 2016; Zverkov et al., 2018), and Sa. vitae and the ‘Venezuelan pliosaurid’ (Bastiaans et al., 2021).

Finally, NMB L.D.37 shows a pattern of apicobasal ridges that is characterized by presence of three mesiolabially positioned ridges running through the entire apicobasal height of the tooth crown. Such pattern has been previously described for some tooth crowns from the mid-Cretaceous ‘Polyptychodon’ assemblage of East and South East England (Madzia, 2016: Fig. 5).

Results of multivariate analyses

The results of multivariate analyses are broadly congruent with those published by Zverkov et al. (2018) and Bastiaans et al. (2021) and we refer to these two studies for more detailed account of the results and interpretations thereof. The principal coordinates analysis (PCoA) placed NMB L.D.37 on the positive side of the first axis and the negative side of the second axis, in a close proximity to ‘Polyptychodon’ type 3 and Liopleurodon ferox (Fig. 4A). The cluster analysis placed NMB L.D.37 within the ‘conical’ morphogroup of the cluster dendrogram and grouped it with Liopleurodon ferox (Fig. 4B).

When was the name Ischyrodon meriani formally established?

Owing to the fact that the publication, in which the name I. meriani was used for the first time, did not include detailed description of the type tooth, it has not been clear whether von Meyer’s (1838) contribution actually formally established the taxon name. For example, Tarlo (1960) stated that I. meriani remained a nomen nudum until von Meyer (1856a).

Article 12 of the ICZN (International Commission on Zoological Nomenclature, 1999) specifies that “[t]o be available, every new name published before 1931 must satisfy the provisions of Article 11 and must be accompanied by a description or a definition of the taxon that it denotes, or by an indication” (Art. 12.1.) and that a vernacular name, locality, geological horizon, host, label, or specimen do not in itself constitute a description, definition, or indication (Art. 12.3.).

In the original study, von Meyer (1838) noted the provenance of the tooth crown and mentioned that he would like to name it “I. Meriani” (also providing the name Ischyrodon in the same sentence). The description provided by von Meyer (1838) is limited to a statement that the type tooth surely belonged to a giant reptile (“Riesen-Saurus”) and that only Mastodonsaurus had similarly massive teeth; further noting that the teeth of the two taxa are very different (see von Meyer, 1838: 414, 415). Albeit brief and vague, this wording may qualify as “a description or a definition of the taxon that it denotes”. Owing to the absence of a universally followed set of rules that would govern the zoological nomenclature in the 1830s and the rather frequent use of the name I. meriani between 1838 and von Meyer’s (1856a) much more detailed description (von Meyer, 1841; Hoffmann, 1845; Geinitz, 1846; Giebel, 1847; Pictet, 1853), indicating that the authors mentioning the name treated it as an established taxon name, we are inclined to consider von Meyer’s (1838) contribution as the reference that established the name I. meriani as an available name in accordance with Article 12 of the ICZN. Such an interpretation renders I. meriani to be the historically earliest established taxon name of Pliosauridae.

Is Ischyrodon meriani conspecific with Liopleurodon ferox?

The morphology and enamel character distribution of NMB L.D.37, as assessed through comparisons to teeth of other thalassophonean pliosaurids and by means of our multivariate analyses, show close similarities with Liopleurodon ferox. Such results reopen the question of the potential conspecificity of the specimens assigned to the two taxa. Tarlo (1960) was the first to note that the teeth of I. meriani and L. ferox are very similar but added that “[i]t is not certain from Meyer’s figure whether this assignment is altogether justified” (Tarlo, 1960: 166). Interestingly, Tarlo (1960) suggested that it was the name I. meriani that could be synonymized with L. ferox, rather than treating L. ferox as the potential junior synonym of I. meriani. Even though the material that is currently assigned to L. ferox is much better researched, much more complete, and taxonomically informative (see especially Andrews, 1913; Tarlo, 1960; Noè, 2001), it is I. meriani that would have nomenclatural priority (providing that I. meriani is declared diagnostic).

Still, despite that both these taxa originate from strata that are comparable in age, the fragmentary nature of the type tooth of I. meriani, combined with the observed similarities between the specimen and teeth of other thalassophoneans, even including some of the latest-diverging members of the clade (Madzia, 2016: Fig. 5), would make such taxonomic decisions unsubstantiated at present. We agree with Tarlo (1960), and some subsequent authors, such as Noè (2001: 27, 31), that there are good reasons to consider I. meriani and L. ferox to likely represent the same thalassophonean pliosaurid. However, the synonymization of the two names would have to be preceded with a morphometric and morphological assessment of the teeth of particular individuals assigned to L. ferox that would explore the dental variability within and between their jaws and reveal whether the tooth of I. meriani falls within that variability.

Is Ischyrodon meriani a diagnostic taxon?

Despite its fragmentary nature, the type tooth of Ischyrodon meriani shows enough morphological characters to make a detailed comparison to teeth of other thalassophonean pliosaurid taxa feasible. At present, it appears impossible to list features, either autapomorphies or a unique combination of characters, that would enable the taxon to be diagnosed. Therefore, I. meriani is currently a nomen dubium, and the type tooth is best treated as Thalassophonea indet.

Is Liopleurodon ferox a diagnostic taxon?

Sauvage (1873) established Liopleurodon ferox based on a single isolated tooth crown (BHN 3R 197) originating from the upper Callovian (uppermost Middle Jurassic) strata of Le Wast, France. The description of the morphology of the holotype has been generally considered adequate to treat L. ferox as a diagnostic pliosaurid taxon and numerous additional specimens have been assigned to it or associated with it since its establishment (see especially Noè (2001) for detailed discussion of the history of L. ferox, specimens referred to the taxon, and their stratigraphic range and geographic distribution). Some of these specimens have originated from distant localities (e.g., Zverkov, Shmakov & Arkhangelsky, 2017) or are a few million years younger than the type tooth (e.g., Madzia, Březina & Calábková, 2018).

Owing to the rather broad geographic and stratigraphic distribution of the specimens assigned to or associated with L. ferox, it remains unclear whether these are in fact representatives of a single taxon (or clade), or whether the Liopleurodon dental morphology does not simply reflect a ‘stage’ in pliosaurid evolutionary history.

Additionally, it remains unclear whether the Liopleurodon lineage comprises one taxon, L. ferox, or two taxa, L. ferox and L. pachydeirus. Tarlo (1960) was convinced that L. pachydeirus, established as Pliosaurus pachydeirus by Seeley (1869), represents a diagnostic taxon that can be distinguished from L. ferox based on the morphology (coarseness) of the apicobasal ridges of the tooth crowns, and the proportions of cervical vertebrae and femora, citing two specimens, the type CAMSM J.46912 and NHMUK PV R 2446, as representatives of the taxon. In the most recent revision of Liopleurodon by Noè (2001), however, L. ferox and L. pachydeirus are considered synonyms. According to Noè (2001: 172), “[t]he cervical vertebral characteristics utilised to distinguish Liopleurodon pachydeirus from Liopleurodon ferox (Tarlo, 1960) are non-diagnostic (based on Brown, 1981), and those of the teeth are here considered to represent individual variation”, though Noè (2001: 172) also notes that “[t]he holotype material of the two species cannot be directly compared, as that of Liopleurodon ferox is a single tooth, and that of Liopleurodon pachydeirus is a string of 17 cervical vertebrae including an atlas-axis complex”. Unfortunately, Noè’s (2001) PhD dissertation, though commonly referenced in the literature, remains formally unpublished and no detailed revision of Liopleurodon has been provided since the early 2000s.

Even if the differences observed between the specimens assigned to L. ferox and those considered to form L. pachydeirus represent individual variation, which indeed appears plausible, Liopleurodon should be still subjected to detailed reevaluation. As per Noè (2001: 142), the crown ornamentation of Liopleurodon is variable. It may therefore be needed that an ICZN petition is filed to replace BHN 3R 197, the type of L. ferox, with a different specimen, for example NHMUK PV R 3536 that is reasonably complete and has been commonly treated in comparative studies as a ‘typical’ representative of L. ferox.