More on the Origin of Turtles – Lyson et al. 2010

Lyson et al.  (2010 – available online) put together their hypothesis on the origin of turtles. In their abstract, they wrote, “We reanalysed a recent dataset that allied turtles with the lizard–tuatara clade and found that the inclusion of the stem turtle Proganochelys quenstedti  and the ‘parareptile’ Eunotosaurus africanus  results in a single overriding morphological signal, with turtles outside Diapsida.”

Milleretta (RC14 specimen) and the Lyson et al. 2010 tree on the origin of turtles.

Figure 1. Milleretta (RC14 specimen) and the Lyson et al. 2010 tree on the origin of turtles. Note the broad ribs already developing in Milleretta, a sister to Acleistorhinus and Eunotosaurus. On its face this seems like a slam dunk for Eunotosaurus and turtles. However, according to the large reptile tree the origin of turtles parallleled the origin of Eunotosaurus. Missing from the Lyson et al. 2010 analysis is Romeria primus and Stephanospondylus, which are closer to the lineage of turtles. A sister to Romeria primus is the last common ancestor of Eunotosaurus and turtles.

Unfortunately,
Lyson et al. (2010) did not include Romeria primusOrobates (Fig. 2) and Stephanospondylus, three taxa found to be closer to the origin of turtles than Eunotosaurus, a terminal taxon with only one known sister, Acleistorhinus. Unfortunately we have no post-crania for Romeria primus (other than slender manual digits) or Acleistorhinus. That lack of data makes it less obvious how they are related to other taxa, but still the large reptile tree nested them in that fully resolved tree. Stephanospondylus was also the sister to the pareiasaurs, a derived clade previously and correctly associated with turtles, but only at the bases of both clades.

Click to enlarge. These skulls are arranged phylogenetically according to the results recovered from the large reptile tree.

Figure 2. Click to enlarge. These skulls are arranged phylogenetically according to the results recovered from the large reptile tree. This was first published a few days ago.

Would be nice to find the common ancestor of both pareiasaurs and turtles, something a little less turtle-like than Stephanspondylus. For now, Orobates(in yellow, Fig. 2) is the best candidate, and prior to that, Romeria primus (in pink). Orobates and Stephanospondylus are Early Permian. The two turtles are Late Triassic. That gives 60-70 million years to evolve a carapace and plastron, plenty of time for transitional taxa to be discovered in. 

Eunotosaurus

Figure 3. Eunotosaurus, a milleretid not related to turtles, but converged with them in several ways. Actually Eunotosaurus is closer to Acleistorhinus and the Caseasauria, which makes sense if put these two together, like Clark Kent and Superman.

Lyson et al. 2012 did find turtle genes closer to lizard genes, while others did not.

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
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, Bever GS, Bhullar B-AS, Joyce WG and Gauthier JA. 2010. Transitional fossils and the origin of turtles. Biology Letters 2010 6, 830-833 first published online 9 June 2010. doi: 10.1098/rsbl.2010.0371
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

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