Thalattosaurs: Wet, Wild and Largely Ignored.

Today’s Post Introduces the Thalattosauria.
Rarely, if ever have the thalattosauria been portrayed or illustrated as a clade. Thalattosaurs are not the most popular extinct reptiles. They’re often overlooked in cladistic analyses. Some thalattosaurs (I’m thinking of Vancleavea and Helveticosaurus at the moment) have been wrongly considered members of other clades. Wumengosaurus  was considered an aberrant sauropterygian. Here I’ll attempt to remedy that situation with a single image of all the thalattosaurs (and their ancestors) to scale currently listed in reptileevolution.com and a short description of each taxon.

 

Figure 2. The Thalattosauria nesting within the Enaliosauria

Figure 2. The Thalattosauria nesting within the Enaliosauria

How Thalattosaurs Nest
Here (Fig. 2) thalattosaurs nested as sisters to ichthyosaurs and mesosaurs. Specifically Stereosternum and Wumengosaurus were their common ancestors.

Xinpusaurus
Despite its many unique traits, Xinpusaurus nested at the base of the Thalattosauria, close to the Ichythosauria. With fins rather than feet and a long, essentially toothless, sword-like rostrum, Xinpusaurus is really off in a clade all by itself. However to move it into the Ichthyosauria takes at least 14 extra steps.

Askeptosaurus
More on the main line of thalattosaur evolution (without transforming its feet into flippers), Askeptosaurus was a larger version of Wumengosaurus with a longer temporal region, more gracile dorsal ribs and relatively shorter limbs.  Two clades arose from this taxon.

Figure 1. The Thalattosauria to scale.

Figure 1. The Thalattosauria to scale.

The Short-Faced Thalattosaurs
Miodentosaurus, EusaurosphargisHelveticosaurus and Vancleavea were the short-faced thalattosaurs. They encompass a wide gamut of morphologies with distinct tooth patterns, vertebral counts and overall sizes. Miodentosaurus lost most of its teeth and became the largest thalattosaur by enlarging the post-crania without enlarging the skull. Eusaurophargis had an upturned jawline, short teeth, a wide, low body and fewer but longer dorsal vertebrae. Helveticosaurus had huge teeth, some so long and closely packed that they must have strained seawater. Vancleavea had a carnivorous dentition, was armored with bony scutes and had a deep tail ideal for sculling.

The Long(er)-Faced Thalattosaurs
Endennasaurus, Clarazia and Thalattosaurus were the long-faced thalattosaurs. Endennasaurus was much smaller than Askeptosaurus and lost its teeth. Clarazia had a shorter rostrum that extended beyond the mandibles; short, blunt teeth and a shorter neck. Thalattosaurus had sharp teeth and blunt teeth and a smaller postorbital region.

Learn more about thalattosaurs and check out the references at reptileevolution.com.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Cheng L 2003. A new species of Triassic Thalattosauria from Guanling, Guizhou. Geological Bulletin of China 22:274–277.
Cheng YN, Wu XC, Li C, Sato T 2007. A new thalattosaurian (Reptilia: Diapsida) from the Upper Triassic of Guizhou, China. Vertebrata PalAsiatica 45: 246–260.
Jiang D-A, Maisch MW, Sun S-L, Matzke AT and Hao WC 2004. A new species of Xinpusaurus (Thalattosauria) from the Upper Triassic of China. Journal of Vertebrate Paleontology 24:80–88. BioOne
Jiang D-Y, Rieppel O, Motani R, Hao W-C, Sun Y-I, Schmitz L and Sun Z-Y. 2008. A new middle Triassic eosauropterygian (Reptilia, Sauropterygia) from southwestern China. Journal of Vertebrate Paleontology 28:1055–1062.
Maisch MW 2010. Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art. Palaeodiversity 3: 151-214.
Merriam JC 1904. A new marine reptile from the Triassic of California. University of California Publications, Bulletin of the Department of Geology, 3, 419–421.
Merriam JC 1905. The Thalattosauria, a group of marine reptiles from the Triassic of California. Memoirs of the California Academy of Sciences, 5, 52 pp.
Nicholls EL 1999. A reexamination of Thalattosaurus and Nectosaurus and the relationships of the Thalattosauria (Reptilia, Diapsida). Paleobios 19:1–29.
Müller J, Renesto S and Evans SE 2005. The marine diapsid reptile Endennasaurus(Reptilia: Thalattosauriformes) from the Late Triassic of Italy. Palaeontology 48:15-30.
Nesbitt SJ, Stocker MR, Small BJ and Downs A 2009. The osteology and relationships of Vancleavea campi (Reptilia: Archosauriformes). Zoological Journal of the Linnean Society 157 (4): 814–864. doi:10.1111/j.1096-3642.2009.00530.x.
Nopcsa F 1925. Askeptosaurus, ein neues reptil der Trias von Besano: Centralblatt für Mineralogie, Geologie und Paläontologie, p. 265-267.
Nosotti S and Rieppel O 2003. Eusaurosphargis dalsassoi n.gen. n.sp., a new, unusual diapsid reptile from the Middle Triassic of Besano (Lombardy, N Italy). Memories of the Italian Society of Natural Science and the Museum of Natural History in Milan, XXXI (II).
Parker WG and Barton B 2008. New information on the Upper Triassic archosauriform Vancleavea campi based on new material from the Chinle Formation of Arizona. Palaeontologia Electronica 11 (3): 20p.
Peyer B 1936. Die Triasfauna der Tessiner Kalkalpen. X. Clarazia schinzi nov. gen. nov. spec. Abhandlungen der Schweizerischen Pala¨ontologischen Gesellschaft, 57, 1–61.
Peyer B 1955. Die Triasfauna der Tessiner Kalkalpen. XVIII. Helveticosaurus zollingeri, n.g. n.sp. Schweizerische Paläontologische Abhandlungen 72:3-50.
Renesto S 1992. The anatomy and relationships of Endennasaurus acutirostris (Reptilia: Neodiapsida) from the Norian (Late Triassic) of Lombardy. Rivista Italiana di Paleontologia e Stratigrafia, 97:409-430
Rieppel O 1987. Clarazia and Hescheleria; a reinvestigation of two problematic reptiles from the Middle Triassic of Monte San Giorgio (Switzerland). Palaeontographica, A, 195, 101–129.
Rieppel O 1989. Helveticosaurus zollingeri Peyer (Reptilia, Diapsida): skeletal paedomorphosis; functional anatomy and systematic affinities. Palaeontographica A 208:123-152.
Wu XC, Cheng YN, Sato T, Shan HY 2009. Miodentosaurus brevis Cheng et al., 2007 (Diapsida: Thalattosauria): its postcranial skeleton and phylogenetic relationships. Vertebrata PalAsiatica 47: 1–20.
Wu X-C, Cheng Y-N, Li C, Zhao L-J and Sato T 2011. New Information on Wumengosaurus delicatomandibularis Jiang et al., 2008, (Diapsida: Sauropterygia), with a Revision of the Osteology and Phylogeny of the Taxon. Journal of Vertebrate Paleontology 31(1):70–83.
Yin G-Z, Zhuo X-G, Cao Z-T, Yu Y-Y and Luo Y-M 2000. A preliminary study on the early Late Triassic marine reptiles from Gunanling, Guizhou, China. Geology, Geochemistry 28(3):1–22.
Zhao LJ, Sato T, Liu T, Li JC, Wu XC. 2010. A new skeleton of Miodentosaurus brevis (Diapsida:Thalattosauria) with a further study of the taxon. Vertebrata Palasiatica 48: 1–10.

wiki/Askeptosaurus
wiki/Endennasaurus
wiki/Eusaurosphargis
wiki/Helveticosaurus
wiki/Miodentosaurus
wiki/Wumengosaurus
wiki/Thalattosaur
wiki/Vancleavea

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