Sunday, November 14, 2010

What Are Turtles?

Odontochelys semitestacea
Students are often very surprised to look at a turtle shell and discover that it has ribs fused to the shell. And, more than one have asked, “is this why turtles can crawl out of their shells, like they do in cartoons?” Turtle shells are novel structures, and turtles, themselves are novel vertebrates that have created controversies and interesting speculations as to what they are related to. Three hypotheses  are available: (1) turtles are the sister to the crocodilians – turtles and crocs have long been called shield reptiles, because both have substantial dermal armor; (2) turtles are the sister to lizards and tuataras; and (3) turtles are the sister to the diapsid reptiles [Araeoscelidia, Avicephala, Hupehsuchia, Thalattosauria, Younginiformes, Ichthyopterygia (ichthyosaurs); Lepidosauromorpha; and the Archosauromorpha] in other words animals most people consider living reptiles, plus many extinct forms of reptiles including the dinosaurs and birds.

Fossil turtles are known from the Triassic, and perhaps the most spectacular find was in 2008. The oldest turtle lived about 220 million years ago in what is now southwestern China. It had a mouth full of small, peg-like teeth, and it had it only the bottom half a shell. The remains were described and named Odontochelys semitestacea by Chun Li, of the Chinese Academy of Sciences. Odontochelys was a small about 35 cm, and its shell consisted of only a plastron. Li' and colleagues suggest that Odontochelys' incomplete shell represents an intermediate step along the evolutionary path to living turtles.
Artist reconstruction of Odontochelys semitestacea. 
This month, Tyler Lyson of Yale University and colleagues, have published the results of a re-analysis of a morphological data set that suggested turtles are the sister to the lizard-tuatara clade. They added two extinct species (Proganochelys and Eunotosaurus) that have been long suspected to be close turtle relatives, and did not make any other changes to the data set. They point out that various anatomy supports all three of the hypotheses. The results of Lyson et al. place turtles outside of Diapsida, a finding that is contrary to most recent molecular work. They note that molecular studies suggest a turtle + archosaur relationship, but that there is little morphological evidence to support this clade. Another study that combined morphological and molecular data also concluded that turtles are outside Diapsida, agreeing with the Lyson results.
The very old fossils, and the unique morphology suggest turtles are in fact quite distinct from any living animals currently considered reptiles. Thus, do turtles belong in their own class of vertebrates? 

The original caption from Lyson et al.  “The position of turtles based on molecular (1: e.g. Hugall et al. 2007) and morphological datasets (2: e.g. deBraga & Rieppel 1997; 3: Gauthier et al. 1988). The addition of key fossils eliminates the apparent disagreement among morphological datasets in support of turtles outside Diapsida (3). The Permian ‘parareptile’ Eunotosaurus shares uniquely derived features with turtles that help fill important gaps in the evolutionary origin of the turtle shell. Bootstrap (top) and Bremer (bottom) support values are provided for the Eunotosaurus-turtle clade. Star indicates complete shell.”

Chun Li, Xiao-Chun Wu, Olivier Rieppel, Li-Ting Wang, Li-Jun Zhao. 2008. An ancestral turtle from the Late Triassic of southwestern China. Nature, 456: 497-501 DOI: 10.1038/nature07533

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