Odontochelys

Odontochelys Temporal range: Late Triassic, 220 Ma PreЄ Є O S D C P T J K Pg N ↓
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Sauropsida
Subclass:	Anapsida
Order:	Testudines
Suborder:	†Proganochelydia
Family:	†Odontochelyidae Li et al., 2008
Genus:	†Odontochelys Li et al., 2008
Species:	†O. semitestacea
Binomial name
†Odontochelys semitestacea Li et al., 2008

Odontochelys semitestacea is an extinct turtle species, the oldest known one. It is the only known species in the genus Odontochelys and the family Odontochelyidae. O. semitestacea was first described from three 220 million year-old specimens excavated in Triassic deposits in Guizhou, China.^[1][2]

Odontochelys differed grossly from modern turtles. Modern turtles possess a horny beak without any teeth in their mouth. In contrast, Odontochelys fossils were found to have had teeth embedded in their upper and lower jaws. One of the most striking features of turtles, both modern and prehistoric alike, are their dorsal shells, forming an armored carapace over the body of the animal. Odontochelys only possessed the bottom portion of a turtle's armor, the plastron. It did not yet have a solid carapace as most other turtles do. Instead of a solid carapace, Odontochelys possessed broadened ribs like those of modern turtle embryos that still have not started developing the ossified plates of a carapace.^[1]

Sketch of Odontochelys fossil.

Aside from the presence of teeth and the absence of a solid carapace, a few other skeletal traits distinguish Odontochelys as basal compared to other turtles, extant and otherwise. The point of articulation between the dorsal ribs and the vertebrae are decidedly different in Odontochelys than in later turtles. In a comparison of skull proportions, the skull of Odontochelys is far more elongated pre-orbitally (in front of the eyes) compared to other turtles. The tail of the prehistoric turtle was longer in proportion to its body than other turtles. In addition, the transverse processes found in the tail are not fused such as in later turtles. Also, the scapulae of the examined specimens were identified to lack acromion processes. Taken together, these anatomical differences have been interpreted by the discoverers to mean that Odontochelys has some of the most primitive features ever seen in a turtle and is somewhat of a transitional fossil.^[1] Some scientists, however, are skeptical of this idea.^[3] Reisz and Head propose that Odontochelys does not represent a turtle with a partly evolved shell, but that it is a descendant of an older, land–living turtle,^[4] and that the shell became reversed. Such shells can be seen on modern turtles, like the leatherback turtles (the genus Dermochelys).

It is likely that Odontochelys was aquatic, as the fossil specimens were found in marine deposits rife with conodonts and ammonites. It is theorized that the primitive turtle frequented shallow marine waters close to shore.^[1]

The species' name, Odontochelys semitestacea literally means "toothed turtle with a half-shell" - an apt description of its most striking physical characteristics.^[1]

References

1. ^ ^{a b c d e} Li, Chun; Xiao-Chun Wu, Olivier Rieppel, Li-Ting Wang & Li-Jun Zhao (2008-11-27). "An ancestral turtle from the Late Triassic of southwestern China". Nature 456 (7221): 497–501. doi:10.1038/nature07533. PMID 19037315. 
2. ^ Reisz, Robert R.; Jason J. Head (2008-11-27). "Palaeontology: Turtle origins out to sea". Nature 456 (7221): 450–451. doi:10.1038/nature07533. PMID 19037315. 
3. ^ ScienceNOW: Sea Change for Turtle Origins?
4. ^ J. Head and R. Reisz, Turtle Origins out to Sea, Nature 456:450-453.

External links

• Figures and Tables from the above-referenced article An ancestral turtle from the Late Triassic of southwestern China in the Journal Nature
• Oldest Turtle Found; May Crack Shell-Evolution Mystery
• First known turtle swam on the half shell

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