Odontochelys semitestacea (Li et al. 2008) Lower Carnian, Late Triassic ~225 mya, 40cm in length, is the most primitive and oldest turtle. Derived from a sister taxon to Stephanospondylus, Odontochelys phylogenetically preceded Proganochelys. Despite the antiquity of Odontochelys, the differences between it and Proganochelys as compared to Stephanospondylus indicate that turtles had already evolved much earlier within the 65 million years that separated them. Thus the lack of a carapace (top shell) in Odontochelys could be a reversal.
Distinct from Stephanospondylus, the skull of Odontochelys was lower and narrower. The nares opened dorsolaterally and were elongated posterioroly. The postfrontal fused to the postorbital and the postorbital portion of the skull was elongated. The supratemporal/opisthotic ledge was larger and more distinct. The nasals were shorter. The quadratojugal was straight. In dorsal view the jawline was medially pinched. The pineal opening was absent. Teeth were retained. No other turtle has teeth.
The scapula was a narrow strut and leaned anteriorly. The limbs were longer, especially the forelimbs.
Distinct from Anthodon, the cervicals of Odontochelys were longer and smaller. Only nine dorsal vertebrae were present as in the unrelated Eunotosaurus. The ribs were broad with intercostal plates. A plastron and hypoischium were present.