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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.
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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.”
Literature
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