Odontochelys

Odontochelys semitestacea is the oldest known turtle to have been discovered. It is the only known species in the genus Odontochelys and the family Odontochelyidae. The species was first described from three 220 million year-old specimens excavated in Triassic deposits originating from Guizhou, China.

As a primitive turtle, Odontochelys grossly differed from modern turtles in several ways. 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.

Aside from the presence of teeth and the absence of a 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 (behind 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.

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.

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