Odontochelys

Odontochelys semitestacea was first described from three 220-million-year-old specimens excavated in Triassic deposits in Guizhou, China. The locale of its discovery at one time was the Nanpanjiang Trough basin, a shallow marine environment surrounded on three sides by land. These deposits preserve an ecosystem known as the Guanling biota, which was dominated by marine reptiles. ==Description==

Odontochelys differed grossly from modern turtles. Modern turtles have a horny beak without teeth in their mouth. In contrast, Odontochelys fossils were found to have had teeth embedded in their upper and lower jaws. 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 is 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. Based on this interpretation, the development of the carapace was likely driven in a land animal. Reisz and Head (2008), however, have a different interpretation on the same specimen. Instead, they suggest that the carapace on O. semitestacea was in fact present; it just lacked ossification of some of its dermal components. With this interpretation, the authors suggest that either turtle shells originally evolved in aquatic environments, or this fossil represents the earliest turtle transferring from terrestrial environments to marine habitats. ==Paleoecology==

Even though the Odontochelys specimens were found in marine deposits, there is uncertainty over whether it was a primarily aquatic reptile, in transition from aquatic to terrestrial habitats, or fully terrestrial. Li et al. (2008) argued that Odontochelys lived in coastal or freshwater environments based on its hand proportions, which were similar to those of modern turtle species which lived in small, slow-moving bodies of water. Evidence from δ13C, δ18O, and δ34S measurements derived from its bones indicates that Odontochelys lived in nearshore marine habitats and rules out that it was a strictly terrestrial animal. This isotopic evidence also suggests that it was a herbivore that foraged in a similar manner to the modern marine iguana. ==References==