Origin of Turtles Update

Earlier we looked at Stephanospondylus (Fig. 1) as the ancestor of turtles like Proganochelys, more primitive than Odontochelys, which represents a splinter off the main line. Unfortunately I never put Stephanospondylus together as a basic silhouette reconstruction, common to all other taxa at reptileevolution.com. Here it is (Fig. 1) along with sister taxa.

Based on the wide ribs, Stephanospondylus had a low, wide torso, some of which were broadened with costal plates, derived from those in Milleretta and destined to form the carapace in turtles. Here (Fig. 1) you can see the evolution of the pectoral girdle, humerus and femur.

Diadectes (Fig. 2) had similar large costal ribs, but these were smooth, not highly textured as in Stephanospondylus. We don’t know where the costal ribs were located in Stephanospondylus. In Proganochelys and Odontochelys the anterior ribs are narrow, not provided with large costal plates (Fig. 1). Stephanospondylus also had a broad, textured interclavicle, the starting point for the plastron.

Note that Stephanospondylus did not have a central neural spine as did Diadectes and Milleretta. That spine, as you know, anchors back muscles. Not having that spine indicates that back muscles were on the way out for Stephanospondylus, one more sign that it was developing a carapace, as in turtles, which also, obviously, lack back muscles.

References
Broom R 1924. On the classification of the reptiles. Bulletin of the American Museum of Natural History 51:39-45.
Geinitz HB and Deichmüller JV 1882. Die Saurier der unteren Dyas von Sachsen. Paleontographica, N. F. 9:1-46.
Gregory WK 1946. Pareiasaurs versus placodonts as near ancestors to turtles. Bulletin of the American Museum of Natural History 86:275-326
Kissel R 2010. Morphology, Phylogeny, and Evolution of Diadectidae (Cotylosauria: Diadectomorpha). Toronto: University of Toronto Press. pp. 185. online pdf
Li C, Wu X-C, Rieppel O, Wang L-T, Zhao L-J 2008. An ancestral turtle from the Late Triassic of southwestern China. Nature 456: 497-501.
Lyson TR, Sperling EA, Heimberg AM, GauthierJA, King BL, and Peterson KJ 2011.MicroRNAs support a turtle + lizard clade. Biol Lett 2011 : rsbl.2011.0477v1-rsbl20110477. abstract - online news story
Reisz RR and Head JJ 2008. Turtle origins out to sea. Nature 456, 450–451.
Rieppel O and deBraga M 1996. Turtles as diapsid reptiles. Nature 384:453-454.
Rieppel O and Reisz RR 1999. The Origin and Early Evolution of Turtles. Annual Review of Ecology and Systematics 30: 1-22.
Romer AS 1925. Permian amphibian and reptilian remains described as Stephanospondylus. Journal of Geololgy 33: 447-463.
Stappenbeck R 1905. Uber Stephanospondylus n. g. und Phanerosaurus H. v. Meyer: Zeitschrift der Deutschen Geologischen Gesellschaft, v. 57, p. 380-437.
Williston SW 1917. The phylogeny and classification of Reptilies. Journal of Geology 28: 41-421.
wiki/Stephanospondylus