Odontochelys semitestacea (Li et al. 2008) Lower Carnian, Late Triassic ~225 mya, 40cm in length, was originally considered the most primitive and oldest turtle, surpassing the previous oldest turtle, Proganochelys. Here, however, Odontochelys was derived from a sister taxon to Milleretta RC70 and left no known descendants.

Real turtles and their kin have an anterior naris, a wider rounder gape, a shorter postorbital, a longer supratemporal a higher maxilla, medial internal nares, longer pedal phalanges and many other traits not found in Odontochelys.

Moving Odontochelys next to Proganochelys adds 18 steps. Moving Milleretta AND Odontochelys next to Proganochelys adds only six steps.

Distinct from Milleretta (RC70), the skull of Odontochelys had a longer post orbital areas and a shorter rostrum. The nares were within a naris length of the orbit. 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.

Left: Stephanospondylus. Center: Odontochelys. Right: Proganochelys.

Lyson et al. (2010) attempted to phylogenetically nest turtles by adding Eunotosaurus to a tree created by Li et al. (2009) in their study of Odontochelys. They employed 30 taxa. Note how many of the Bremer scores are under 70. Here members of the two major branches recovered by the large reptile study (270+) taxa are interspersed in this tree, blue for the new Lepidosauromorpha and yellow for the new Archosauromorpha. Unfortunately they specified "Diadectomorpha" as the outgroup, along with Seymouria. That was a bad decision because Diadectes and Limnoscelis are reptiles nesting deep within the Lepidosauromorpha clade, as determined by the large study. See below.

Click to enlarge. On the left are the taxa employed by Lyson et al. (2010). On the right are two columns representing the same taxa split into the two major clades recovered by the large study. Of course, dozens of transitional taxa employed by the large study are not included above, but are listed here.

Li C, Wu X-C, Rieppel O, Wang L-T and Zhao L-J 2008. An ancestral turtle from the Late Triassic of southwestern China. Nature 456: 497-501.

Layson TR, Bever GS, Bhullar B-AS, Joyce WG and Gauthier JA 2010. Transitional fossils and the origin of turtles. Biology Letters June 9 2010. doi: 10.1098/rsbl.2010.0371

wiki/Odontochelys