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Lichtig and Lucas 2016 document aspects of turtle origins:
“The Late Triassic turtle Chinlechelys is known from a large number of associated shell fragments. It shows a stage in the development of the turtle shell intermediate between Odontochelys and Proganochelys (1). While the preserved portions of the plastron of Chinlechelys resemble those of Odontochelys (2), the carapace shows a more advanced state. It differs from all other turtles except Dermochelys in that the ribs and overlying osteoderms are not nearly parallel, and instead cross at a 45° angle (3). These ribs are
separated from the overlying osteoderm by compact bone present in both the rib and on the costal sides of the suture. This means that the ribs and overlying osteoderms represent separate ossifications, indicating fusion of the ribs and osteoderms rather than the expansion of ribs alone. Further, the morphology of these costals suggests a terrestrial habit based on the occurrence of raised bosses perpendicular to the midline of the animal as well as the occurrence of near right angles in the costals, both of which would incur a significant hydrodynamic penalty. The carapace of Chinlechelys resembles that of Dermochelys, which has often been referred to as neonate in its morphology. This indicates some of the unique features of Dermochelys are reversals to the primitive state, following the notion that ontogeny mirrors phylogeny. Based on this comparison we speculate that the carapace may not have been absent in Odontochelys but rather made up of a great many separate osteoderms (4), and, as often occurs with modern Dermochelys, they were disarticulated following death. Thus, Odontochelys represents a side branch of turtle evolution–an early sea turtle as some have suggested–rather than the ancestor of later turtles (5). This is primarily based on the derived state of lacking a bony attachment between the pelvis and the vertebral column, unlike all other uncontroversial stem turtles, which have a bony attachment (6). Pappochelys is here disregarded as a stem turtle because the morphology linking it to turtles better indicates it is a basal placodont (7). Based on the presence of turtle footprints in the lower Middle Triassic (8), turtle-like animals must have evolved by that time. These are unlikely to have been made by Odontochelys, given its unique pelvis morphology. We conclude from this that Chinlechelys represents an earlier stage in the evolution of the turtle shell than Proganochelys and other Norian turtles, and this reinforces the conclusion that Odontochelys is not ancestral to other turtles.” (9)
Figure 1. Chinlechelys compared to Proganochelys. The cervical armor is relatively larger, the humerus is relatively smaller based on scaling the vertebrae to the same size.
Yesterday a paper on Sichuanchelys palatodentata (Late Jurassic; Joyce et al. 2016) came out and their cladogram listed Owenetta, Anthodon and Sphenodon as successive outgroup taxa. Unfortunately introducing these taxa indicates that these workers have no idea from whence turtles arose. It is regrettable that the large reptile tree has not been at least tested with regard to turtle origins. That’s what it is here for! What is wrong with testing a few more previously validated taxa? Owenetta is closer to the origin of lepidosaurs. Sphenodon is a lepidosaur. Anthodon is the only valid ancestral taxon there and it’s not as close as several excluded taxa. Missing, of course are Elginia, Sclerosaurus, and other valid turtle ancestors, which in sum would turn the Joyce et al. cladogram inside out.
Joyce WG, Lucas SG, Scheyer TM, Heckert AB and Hunt AP 2009. A thin-shelled reptile from the Late Triassic of North America and the origin of the turtle shell. Proceedings of the Royal Society B Biological Sciences 276: 507-513. PDF
Lichtig AJ and Lucas SG 2016. Chinlechelys: a reexamination of North America’s oldest (Triassic, Revultian, Norian) turtle and its impact on theories of turtle origins. Abstract from the 2016 meeting of the Society of Vertebrate Paleontology.
Joyce WG, Rabi M, Clark JM and Xu X 2016. A toothed turtle from the Late Jurassic of China and the global biogeographic history of turtles. BMC Evolutionary Biology 16:236, 29pp. PDF