Thalattosuchia

Young, Mark T., Dufeau, David, Bowman, Charlotte, Cowgill, Thomas, Schwab, Julia A., Witmer, Lawrence M., Herrera, Yanina, Katsamenis, Orestis L., Steel, Lorna, Rigby, Martin & Brusatte, Stephen L., 2024, Thalattosuchian crocodylomorphs from the Sinemurian (Early Jurassic) of the UK, Zoological Journal of the Linnean Society 201, pp. 1-35 : 12-16

Thalattosuchia indet.

( Fig. 9 )

Specimen: NHMUK PV R 36 711 * dorsal vertebra from the caudal end of the dorsal series* and the neural arch of another dorsal vertebra* both held together by matrix .

Locality: Conesby Quarry * Scunthorpe * Lincolnshire * England * United Kingdom.

Horizon: Charmouth Mudstone Formation* Lias Group.

Age: Late Sinemurian or Early Pliensbachian.

Description: The specimen was donated to the NHMUK in the late 1980s by one of us (M.R.). The specimen was collected from the Scunthorpe Mudstone Formation at the Conesby Quarry. It was discovered in a clay layer above the Frodingham Ironstone Member of the Scunthorpe Mudstone Formation* thus being from the Charmouth Mudstone Formation ( Simms et al. 2004). This makes the specimen either Late Sinemurian or Early Pliensbachian in age ( Simms et al. 2004). The preserved dorsal vertebra is incomplete ( Fig. 9 )* with most of the neural spine missing* and the transverse processes broken. The centrum is approximately 26 mm long craniocaudally* while the centrum face not adhered to matrix is 15 mm tall and 13 mm wide (although the centrum surface is slightly broken). The neurocentral suture is still visible* suggesting that this specimen was not morphologically mature* which is consistent with its small size. Overall* the centrum resembles those of thalattosuchians* with an hourglass shape in ventral view (e.g. see: Fraas 1902 * Arthaber 1906 * Andrews 1913 * Pierce and Benton 2006* Herrera et al. 2013c * Parrilla-Bel and Canudo 2015). The cranial and caudal articular surfaces of the centrum appear to be flat* rather than poorly concave. However* there is still some matrix adhering to both surfaces* which could be covering the depressions. Based on what is preserved of the transverse processes we can identify the vertebra as part of the caudal dorsal series* this is due to the transverse processes having no clear separation between the parapopohysis and the diapophysis (which we would expect for cranial or middle thalattosuchian dorsal vertebrae; see: Andrews 1913). In cranial view* the transverse process projects out from the neural arch in an almost horizontal manner* although the dorsal surface of the transverse process is visible* suggesting that there was an inclination to the process in life. In lateral view* the preserved transverse process lacks any noticeable cranial or caudal curvature* again suggesting that the vertebra is not from the cranial dorsal series (see: Andrews 1913).

The prezygapophyses are large and are angled broadly* with an angle less than 45°. The ventral margins of the prezygapophyses are continuous with the lateral margin of the transverse process* due to the wide angle of the prezygapophyses* which gives the impression that they lie on top of the transverse processes ( Fig. 9E ). This morphology is also seen in early diverging metriorhynchoids (e.g. Pelagosaurus typus NHMUK PV OR 32 598) and metriorhynchids (e.g. the ‘ Metriorhynchus ’ cultridens Andrews* 1913 holotype NHMUK PV R 3804). The prezygapophyses are more clearly separate from the transverse processes in Early and Middle Jurassic teleosauroids* such as Macrospondylus bollensis ( von Jäger 1828) * Charitomenosuchus leedsi ( Andrews 1909) and Neosteneosaurus edwardsi ( Eudes-Deslongchamps 1868) (e.g. Andrews 1913 * Westphal 1962)* perhaps due to the sub-vertical orientation of the prezygapophyses. Given how frequently broken the neural arches are of thalattosuchian fossils* the distribution of this character is unknown; however* it does support the referral of NHMUK PV R 36 711 to Thalattosuchia.

The second vertebra is also incomplete* missing the centrum* neural spine* and most of the transverse processes (i.e. being composed only of the neural arch). Little can be discerned; however* the transverse processes are much more ‘rod-like’ than the more complete vertebra ( Fig. 9F )* suggesting it comes from further back in the series.

Systematic identification

As there is only a single well-preserved vertebra* we can only make an identification of Thalattosuchia indeterminabilis. Dorsal vertebrae with a low angle of the prezygapophyses and their ventral margins being continuous with the transverse processes is consistent with some members of Metriorhynchoidea (e.g. Pelagosaurus typus and ‘ Metriorhynchus ’ cultridens)* although some metriorhynchids* such as Gracilineustes leedsi ( Andrews 1913) (NHMUK PV R 3014 and R 3015) and Ŋalatosuchus superciliosus (NHMUK PV R 2032 and R 2054)* had more vertically oriented prezygapophyses (e.g. Andrews 1913 * Krebs 1962 * Westphal 1962 * Wellnhofer 1978 * Hua 2003 * Molnar et al. 2015). Many teleosauroids had dorsal vertebrae with more vertically oriented prezygapophyses* and the ventral margins of the prezygapophyseal processes remaining distinct from the transverse processes (e.g. Andrews 1913 * Krebs 1962 * Westphal 1962 * Wellnhofer 1978 * Hua 2003 * Molnar et al. 2015). Unfortunately* variation in vertebral morphology along the column* and between species* is still poorly understood in Thalattosuchia. As such* we cannot conclusively refer NHMUK PV R 36 711 to any specific subclade* but it possibly belongs to a metriorhynchoid.

Phylogenetic analyses

Equal weights’ analysis: The shortest length (4166) was hit 20 times in the driven search* recovering 182 most parsimonious cladograms (MPCs). The TBR search on the saved cladograms filled the tree buffer (hold limit of 5000) but did not find any topologies with a shorter length. The descriptive statistics of the shortest MPCs are length = 4166* CI (ensemble consistency index) = 0.302* RI (ensemble retention index) = 0.831* RC (ensemble rescaled consistency index) = 0.251* and the HI (ensemble homoplasy index) = 0.698.

The strict consensus topology of the equal weights’ analysis is identical to that recovered by Young et al. (2024) * except for the addition of the ‘Sinemurian snout’ (NHMUK PV R 36 710) ( Fig. 10 ; Supporting Information* Appendix S2: Fig. S3 ). The ‘Sinemurian snout’ was recovered in a polytomy with Turnersuchus hingleyae and the unnamed Plagiophthalmosuchus + Neothalattosuchia subclade.

The iterative PCR (Pol and Escapa 2009) function of the TNT Analyses.run script found that the resolution of the strict consensus could be improved by removal of the following 10 OTUs: the pholidosaurid Pholidosaurus purbeckensis ( Mansel-Pleydell 1888)* the dyrosaurid Atlantosuchus coupatezi Buffetaut * 1979b* the extant crocodylid Crocodylus siamensis ( Schneider 1801) * the Moroccan teleosauroid* the machimosaurid Proexochokefalos cf. bouchardi * the metriorhynchine metriorhynchid Metriorhynchus breoirostris ( Holl 1829) * the rhacheosaurin metriorhynchids Cricosaurus elegans ( Wagner 1852) * and Cric. puelchorum Herrera et al. * 2021* and the geosaurine metriorhynchids Torooneustes sp. ( England) and Plesiosuchina indet. ( France).

Extended implied weights’ (k = 15) analysis: The best score (111.563) was hit 20 times in the driven search* recovering 160 MPCs. The TBR search on the saved cladograms did not find any topologies with a better score but found 4500 MPCs. The descriptive statistics of the shortest MPCs are length = 4180* fit = 111.563* CI = 0.301* RI = 0.830* RC = 0.250* and the HI = 0.699.

The strict consensus topology of the extended implied weights’ analysis is identical to that recovered by Young et al. (2024) * except for the addition of the ‘Sinemurian snout’ (NHMUK PV R 36 710) ( Fig. 11 ; Supporting Information* Appendix S2: Fig. S4 ). Just as with the equal weights’ analysis* the ‘Sinemurian snout’ was recovered in a polytomy with Turnersuchus hingleyae and the unnamed Plagiophthalmosuchus + Neothalattosuchia subclade.

The iterative PCR (Pol and Escapa 2009) function of the TNT Analyses.run script found that the resolution of the strict consensus could be improved by removal of the following seven OTUs: the goniopholidid Sunosuchus miaoi Young * 1948* the Moroccan teleosauroid* the metriorhynchine metriorhynchid Metriorhynchus breoirostris * the rhacheosaurin metriorhynchids Cricosaurus elegans and Cric. puelchorum * and the geosaurine metriorhynchids Torooneustes sp. ( England)* and the Druegendorf merged OTU.

Figures

Figure 3. Reconstruction of the internal anatomy of the ‘Sinemurian snout’ (NHMUK PV R 36710). A* dorsal view; B* right lateral view; C* ventral view. Scale bars equal 10 mm. Abbreviations: Alv* alveolus; COS* cancellous osseous structure; DAC* dorsal alveolar canal; NC* nasal cavity; Trig* trigeminal canal.

Figure 4. Reconstruction of the neurovasculature and alveoli of the ‘Sinemurian snout’ (NHMUK PV R 36710). A* dorsal view; B* right lateral view; C* ventral view. Scale bars equal 10 mm. Abbreviations: Alv* alveolus; DAC* dorsal alveolar canal; Trig* trigeminal canal.

Figure 9. Thalattosuchia indeterminabilis (NHMUK PV R 36711). Best preserved dorsal vertebra* in lateral (A)* ventral (B)* caudal (C)* and dorsal (E) views.The second preserved dorsal vertebra* in oblique views (D and F). Scale bar equals 10 mm. Abbreviations: CEN* centrum; POZ* postzygapophysis; PRZ* prezygapophyses; TRP* transverse process.

Figure 10. The strict consensus topology of the equal weights’ analysis.Length = 4166* CI = 0.302* RI = 0.831* RC = 0.251* HI = 0.698* number of most parsimonious cladograms hit the hold limit. Phylogeny pruned to highlight Thalattosuchia (see Supporting Information* Appendix S2: Fig.S3 for the entire topology). A* Thalattosuchia; B* Neothalattosuchia; C* Teleosauridae; D* Machimosauridae; E* Metriorhynchidae.Green shaded area highlights non-neothalattosuchian thalattosuchians* orange shaded area highlights teleosauroid thalattosuchians* and blue shaded area highlights metriorhynchoid thalattosuchians. Images from PhyloPics.org* with the teleosauroid by Scott Hartman* and the metriorhynchid by Dmitry Bogdanov (vectorized by T. Michael Keesey)* no changes were made* and their license is: https://creativecommons.org/licenses/by/3.0/.

Figure 11. The strict consensus topology of the extended implied weighting (k = 15) analysis. Length = 4180* fit = 111.563* CI = 0.301* RI = 0.830* RC = 0.250* HI = 0.699* number of most parsimonious cladograms = 4500. Phylogeny pruned to highlight Thalattosuchia (see Supporting Information* Appendix S2: Fig. S4 for the entire topology). A* Thalattosuchia; B* Neothalattosuchia; C* Teleosauridae; D* Machimosauridae; E* Metriorhynchidae. Green shaded area highlights non-neothalattosuchian thalattosuchians* orange shaded are highlights teleosauroid thalattosuchians* and blue shaded are highlights metriorhynchoid thalattosuchians.Images from PhyloPics.org* with the teleosauroid by Scott Hartman* and the metriorhynchid by Dmitry Bogdanov (vectorized by T.Michael Keesey)* no changes were made* and their license is: https://creativecommons.org/licenses/by/3.0/.