Early-diverging plesiosaurs from the Pliensbachian (Lower Jurassic) of northwestern Germany

Geological and stratigraphic setting

The studied specimens (GZG.V.000092 and Namu ES/jL-3868) originate from successions preserved within the Herford Syncline (Herforder Liasmulde, e.g., Schubert, 2007) and its southwestern boundary, the Osning Fault Zone, a multi-phasic normal/reverse/strike-slip fault system (Fig. 2). Both structural elements attained their current configuration during the Late Cretaceous and underwent exhumation since the late Paleogene (e.g., Drozdzewski & Dölling, 2018). During the Early Jurassic the area was part of the Central European Archipelago, an array of islands separated by marine straits and local basins (e.g., Ziegler, 1982). Within the area of the Herford Syncline and its surroundings, marine sedimentation during the Sinemurian and Pliensbachian mostly comprised claystones and marlstones. The discontinuity surface at the base of the Pliensbachian is a notable exception from this rather monotonous succession. Caused by a local regression, it is overlain by a highly condensed section (Rottorf Formation, Mönnig, 2023a), representing the lowermost Pliensbachian (Uptonia jamesoni Zone). These marginal facies passed basin-wards and upwards into the claystones and marlstones of the Capricornumergel Formation, representing the continuous distal facies of the lower Pliensbachian (U. jamesoni to Tragophylloceras ibex to Prodactylioceras davoei Zones, Mönnig, 2023b). The fine-grained, clayey-marly lithofacies continues upward into the overlying Amaltheenton Formation (Amaltheus margaritatus Zone, upper Pliensbachian; Fig. 3).

The material from the former Spilker clay-pit at Werther lacks precise stratigraphic information. However, the lithology and fossil content of the matrix attached to the skeletal remains allow determining their stratigraphic position within the succession that was once exposed at this outcrop. According to Büchner, Hoffmann & Jordan (1986), the clay-pit exposed the Osning Fault Zone, resulting in steep to nearly vertical dip of the beds. By reverse faulting, lower Middle Triassic sandstones (Röt Formation) in the NE were upfaulted against the Lower Jurassic succession in the SW. The latter comprised an upper Sinemurian section of unspecified thickness, overlain by 1.5 m of the condensed Rottorf Formation (lower Pliensbachian, U. jamesoni Zone), which is, in turn, overlain by 115 m of dark, partially pyritiferous claystones with calcareous concretions of the Capricornumergel Formation (lower Pliensbachian, T. ibex and P. davoei zones; Meyer, 1907; Büchner, Hoffmann & Jordan, 1986).

The Rottorf Formation consists of bioclastic clay- and marlstones with high amounts of dispersed Fe-hydroxide (“iron stones”), causing a distinct pink to reddish color on the weathered surface, and in many layers contain Fe-ooids or interbedded Fe-oolites. According to Büchner, Hoffmann & Jordan (1986), there are also abundant calcareous pebbles that are often altered by bioerosion (borings) and wood fragments.

The plesiosaur remains from the Spilker clay-pit (GZG.V.000092) are associated with pinkish, bioclastic, particle-bearing claystones and well-rounded calcareous pebbles, with a Fe-hydroxide coating, adhering to the skeletal fragments. Accompanying invertebrate fossils include shell debris (identifiable are the bivalves Oxytoma sp. and Eopecten? sp., and a pleurotomariid gastropod), indeterminate belemnites, as well as the ammonites Phricodoceras cf. taylori (Sowerby, 1829), Platypleuroceras cf. caprarium (Quenstedt, 1856), and Platypleuroceras sp.; consequently, the plesiosaur material can stratigraphically be unambiguously referred to the Rottorf Formation. The accompanying ammonites indicate the taylori Subzone of the jamesoni Zone (Hoffmann, 1982). It is worth noting that Büchner, Hoffmann & Jordan (1986) have previously reported the brevispina Subzone as the lowermost section of the Rottorf Formation. If the identification of the taylori Subzone is correct, the 1.5 m thick Rottorf Formation at the Spilker clay-pit represents the condensed section of three subzones (taylori, polymorphus, and brevispina) of the jamesoni Zone.

In any case, the plesiosaur specimens from the jamesoni zone of the Spilker clay-pit represent the stratigraphically oldest of the specimens under consideration herein, followed by the material (Namu ES/jL-3868) from the Bielefeld-Sudbrack locality (P. davoei Zone, Capricornumergel Formation, lower Pliensbachian), and the type horizon of Arminisaurus schuberti (Amaltheus subnodosus Subzone, Amaltheus margaritatus Zone, Amaltheenton Formation, upper Pliensbachian; Sachs & Kear, 2018).

Phylogenetic analyses

The Werther plesiosaur (GZG.V.000092) is geographically and stratigraphically proximate to the late Pliensbachian pliosaurid Arminisaurus schuberti from the Beukenhorst II clay-pit that is located in the Bielefeld district of Jöllenbeck, with which it also shares the majority of character states (see Discussion for detailed information). In order to assess whether these similarities have phylogenetic significance, we supplement our descriptions and comparisons through exploration of the placement of GZG.V.000092 among plesiosaurs using the dataset of Sachs, Eggmaier & Madzia (2024), which represents a substantially modified version of the matrix originally assembled by Benson & Druckenmiller (2014), and includes first-hand scores of Arminisaurus schuberti obtained from Sachs et al. (2023). The final version of the matrix, which differs from that of Sachs, Eggmaier & Madzia (2024) only in the addition of the Werther plesiosaur, includes 131 operational taxonomic units (OTUs) and 270 characters; 67 of which were set as ‘additive’ (= ‘ordered’) following Madzia, Sachs & Lindgren (2019).

Our analyses were performed using maximum parsimony as the optimality criterion and through TNT 1.6 (Goloboff & Morales, 2023). We have conducted four runs. The first run wase based on equal weights; the other three runs used the implied weighting function, with the concavity parameter (K) set to 6, 9, and 12. In all our analyses, we used Neusticosaurus pusillus as the outgroup. For each of the phylogenetic analyses, we fixed the maximum number of most parsimonious trees to 200,000 (command “hold 200000”). Then, we ran the ‘New Technology’ (NT) search which involved 500 addition sequences and default settings for sectorial searches, ratchet, drift, and tree fusing (all activated). Following the NT search, we performed a ‘Traditional Search’ with tree bisection-reconnection (TBR) branch-swapping on trees saved to RAM. For the phylogenetic analysis using equal weights, nodal support was assessed through the Bremer support values (with TBR and retaining sub-optimal trees incorporating up to 3 additional steps). Nodal support for the parsimony analyses with implied weighting was assessed through Symmetric Resampling, using a ‘Traditional Search’, 1,000 replicates, a default change probability (set at 33), and output expressed as frequency differences (GC).

See Supplemental Informations 1 and 2 for the character list and a TNT-executable code, respectively.

Systematic paleontology

Material. GZG.V.000092, three cervical vertebrae and three indeterminable vertebrae (Figs. 4 and 5). The fragments are catalogued under a single number, owing to the assumption that they belong to a single individual. The separate fragments are distinguished by suffix letters ‘a’ through ‘f’, as indicated in Figs. 4 and 5.

Locality and horizon. Former Spilker clay-pit, Werther (Westfalen), Gütersloh district, North Rhine-Westphalia, Germany; Rottorf Formation, lower Pliensbachian (Uptonia jamesoni Zone), Lower Jurassic.

Remarks. GZG.V.000092 lacks detailed documentation, including information regarding its discovery.

Description and comparisons

Six incomplete vertebrae are preserved; three of them can be identified as cervicals, shown by the lateroventrally placed rib facets (Fig. 4). The remaining three are not preserved well enough to be identified with certainty (Fig. 5). One of the cervicals (Figs. 4K–4O) is considerably larger and shows more prominent rib facets. Consequently, this vertebra likely derives from the posterior section of the neck. The size of the two other cervicals (Figs. 4A–4J) indicates that they likely originate from the anterior or middle section of the neck.

The articular faces of the centra are exposed in all three cervicals and in one of the indeterminable vertebrae (Fig. 5B). In the other specimens they are either damaged or obscured by matrix.

All cervical centra are wider and higher than long, a condition found in several early-diverging plesiosaurs, including Rhaeticosaurus mertensi and the pliosaurids Arminisaurus schuberti and Cryonectes neustriacus (Vincent, Bardet & Mattioli, 2013; Wintrich et al., 2017; Sachs & Kear, 2018).

As visible, the articular facets are amphicoelous and surrounded by a thickened rim. Anteroventrally, a prominent lip is present in two vertebrae (Figs. 4C, 4D and 4H). A similar prominent lip is found in the pliosaurid Arminisaurus schuberti (Sachs & Kear, 2018). The posterior articular face is preserved in one of the vertebrae (Fig. 5E) and here a ventral lip is absent.

The lateral sides of the cervical centra are anteroposteriorly concave. In the best preserved cervical (Fig. 4D), there is a circular lateral depression present dorsal to the rib facet on both sides. Dorsal to this concavity, there is a structure resembling a lateral ridge. However, neither of these structures is present in the other cervicals. Therefore, it is unclear if they are a taphonomic artefact. A similar circular depression can otherwise be found in some cervicals of Cryonectes neustriacus (Vincent, Bardet & Mattioli, 2013, fig. 9) and Brancasaurus brancai (S Sachs, pers. obs., 2013). In one of the cervicals, remnants of the cervical ribs are still fused to the centrum (Figs. 4A–4E). The rib facets in all cervicals are placed lateroventrally and slightly more posterior to the mid-length of the centrum. In all cervicals, the morphology of the rib facets is somewhat obscured by either the cervical ribs or by matrix. However, a slight depression in the posterior margin of the cervical ribs indicates that two co-joined rib facets were formed (Fig. 4E). This condition is found in most pliosaurids and rhomaleosaurids, but also in some Early Jurassic plesiosauroids, such as microcleidids or Westphaliasaurus simonsensii (Benson & Druckenmiller, 2014: Appendix 2, character 160).

The dorsolaterally-placed neural arches are fused to the centra but a semicircular neurocentral suture is still indicated in the cervical vertebrae (Figs. 4A). A circular neural canal is visible in two cervicals (Figs. 4A and 4F) and one of the indeterminate vertebrae (Fig. 5B). The zygapophyses are hard to assess; they are either largely broken off or otherwise damaged. One right postzygapophysis is reasonably well preserved (Figs. 4B, 4D and 4E). It exceeds the level of the centrum posteriorly with most of its length. Laterally, the postzygapophyses are subequal to the width of the centrum having articular faces that are planar. All zygapophyses face dorsoventrally. The neural spines are broken off in all vertebrae, preserving only their base in one anterior cervical (Fig. 4B).

In ventral view, the anterior and posterior edges of the articular surface rims are transversely widened and form a triangular-shaped bulge. In the anterior articular face this bulge extends further ventrally and forms the lip-like protrusion described above. Both, the anterior and posterior articular surface bulges extend towards the mid-length of the centrum where they merge with a thickened and rounded midline keel (Figs. 4C and 4H). This condition is also present in the geographically and stratigraphically proximal Pliensbachian pliosaurid Arminisaurus schuberti. A similar rounded ventral midline keel is also present in the cervicals of later-diverging pliosaurids; e.g., Peloneustes philarchus and Eardasaurus powelli (Linder, 1913; Ketchum & Benson, 2022). There are two subcentral foramina.

Plesiosauria indet.

Material. Namu ES/jL-3868, two associated pectoral or anterior dorsal vertebrae (Fig. 6)

Locality and horizon. Former Klarhorst clay-pit, Sudbrackgebiet, Bielefeld, North Rhine-Westphalia, Germany; Capricornumergel Formation, lower Pliensbachian (Prodactylioceras davoei Zone), Lower Jurassic.

Remarks. Namu ES/jL-3868 was found in the early 1930s in association with skeletal remains of a giant specimen of the ichthyosaur Temnodontosaurus. Hungerbühler & Sachs (1996) assigned them to the ichthyosaur specimen.

Description and comparisons

Two associated centra with attached neural arch pedicles are preserved. The lateral apophyses are broken off, but they have been placed dorsally which indicates that these vertebrae are either pectoral vertebrae (sensu Sachs, Kear & Everhart, 2013) or anterior dorsal vertebrae (compare e.g., Smith & Benson, 2014: pl. 13, Fig. 2). Both centra are wider than long and high and higher than long. The articular faces of the centra are mostly obscured but they appear only slightly amphicoelous (Fig. 6), being surrounded by flattened articular surface rims. Laterally, both centra are largely occupied by the broken rib facets/transverse processes, which are placed dorsally, adjacent to the neural canal. Transverse processes of dorsal vertebrae being placed adjacent to the neural canal is a common condition in plesiosaurs (Benson & Druckenmiller, 2014: Appendix 2, character 181). This differs, however, from the dorsal vertebrae of Arminisaurus schuberti where the transverse processes are placed dorsal to the neural canal (Sachs & Kear, 2018: Fig. 5A). Ventral to the remnants of the rib facets/transverse processes, a transverse buttress is indicated. This buttress resembles the condition observable in R. thorntoni (see Smith & Benson, 2014, pl. 13, fig. 2). Adjacent to the buttress, a subdiapophyseal fossa (sensu Hampe, 2013) is formed on each side (Fig. 6B). In A. schuberti, the ventral sides of the transverse processes of the dorsal vertebrae show a transverse buttress and an associated anterior fossa (Sachs & Kear, 2018).

A circular neural canal is visible in the anterior of the two vertebrae (Fig. 6A). The prezygapophyses are preserved in the posterior vertebra (Fig. 6). They are lobate in lateral view and extend over the level of the centrum with about half of their length. The postzygapophyses of the anterior-more vertebra articulate with the prezygapophyses of the second vertebrae and are thus only partly visible. They extend over the level of the centrum with most of their length. The postzygapophyses of the posterior-more vertebra are largely damaged. The articular faces are not visible in any of the zygopophyses. The neural spines are broken off in both vertebrae. The ventrolateral and ventral sides of the centra are concave and bear foramina subcentralia (Fig. 6).

Results of phylogenetic analyses

See Table 1 for the numerical results of our phylogenetic analyses, Fig. 7 for reduced tree topologies focusing on the pliosaurid segments of the trees that are relevant for the assessment of the phylogenetic placement of the Werther plesiosaur, and Supplemental Information 3 for full tree topologies and nodal support values.

The parsimony analysis using equal weights has produced a very poorly resolved strict consensus tree, failing to reconstruct a monophyletic Pliosauridae. However, inspection of the most parsimonious trees (MPTs) has shown that the basal topological instability was mainly caused by the nesting of Thalassiodracon hawkinsii that was alternatively inferred as either, an early-diverging pliosaurid or an early-diverging plesiosauroid, and Rhaeticosaurus mertensi that was found either as an early-diverging pliosaurid or to lie outside the basal branching of Pliosauridae, Rhomaleosauridae, and Plesiosauroidea. The Werther plesiosaur (GZG.V.000092) although scored for only 15 characters out of 270 (5.55%), has been consistently inferred as a pliosaurid in all MPTs though its precise placement could not be found due to the fragmentary nature of the material. In contrast, all analyses using the implied weighting inferred a monophyletic Pliosauridae (including Thalassiodracon that was found at the base of the clade but excluding Rhaeticosaurus that lay outside the basal branching of the three major plesiosaur clades). All these runs also reconstructed the Werther plesiosaur among pliosaurids though, again, the runs did not find a stable position for the OTU. As such, examination of the inferred trees and character state mapping does not enable to identify a certain character state combination in GZG.V.000092 that would be common for all results placing the specimen on the pliosaurid lineage, although, for example, the transformation of character 165 (the appearance of median ventral surface) from 0 (approximately flat or convex surface) to 1 (presence of a round midline keel) was among those that nested GZG.V.000092 close to some Late Jurassic thalassophoneans. It also occasionally connected the OTU with Anguanax zignoi.