Chinlechelys

''Chinlechelys'' ( meaning ''Chinle turtle'') is an extinct genus of stem-turtle belonging to Testudinata. It lived in the Norian age of the Late Triassic and is the oldest turtle known from North America. Among turtles it is unique, mostly because of its very thin shell. The type and only species, ''C. tenertesta'', was named and described with the genus by Walter G. Joyce ''et al.'' in 2009. It was probably terrestrial, and was found by Joyce ''et al.'' to be closely related to '' Proganochelys'', another terrestrial testudinatan.

Discovery and naming

''Chinlechelys'' is known from only one assigned specimen, the holotype NMMNH P-16697 (New Mexico Museum of Natural History and Science) recovered from the Norian Bull Canyon Formation (New Mexico, United States). The skeleton consists of the middle of the carapace, the left hypoplastron, rear ribs, a neck spine, isolated osteoderms, and portions of the bridge. The materials were collected separately over several years, but exhibit the same preservation, features, and are all from the same area, so they were assigned to the same individual.

Etymology

''Chinlechelys tenertesta'' was named by Walter G. Joyce ''et al.'' in 2009. The generic name is derived from ''chinle'', after the Triassic Chinle Group the holotype was found in, and ''chelys'', Greek for "turtle". ''Tener'' and ''testa'' and based on the Latin words for "delicate" and "shell", respectively, based on the thinness of its shell.

Description

''Chinlechelys'' is a unique turtle, and was preserved to have an extremely thin shell. Even though the shell has an estimated length of , the thickest section, above the keel, is only thick, with the average shell thickness being . The shell thickness makes ''Chinlechelys'' the thinnest-shelled turtle known from a fully grown shell. The other well-discussed materials of ''Chinlechelys'' are dermal osteoderms. Two prominent spines, from the neck region, are well-preserved. They were decidedly not from the edge of the shell, because large amounts of the region of the carapace they would be from were preserved. Also, the pair of osteoderms were unlikely to be from the tail, because of a comparison with '' Proganochelys''. Some spines from the shell are known from ''Chinlechelys'', but it was considered likely that they were extended fully around the shell because of their presence on ''Proganochelys''.

Distinguishing characteristics

''Chinlechelys'' can be distinguished from both derived amniotes and primitive turtles based on the below features, noticed by Joyce ''et al.'': the presence of a plastron, carapace and multi-element neck and tail armour; dorsal centra hourglass-shaped, platycoelous, and with a distinct ventral keel; dorsal ribs in contact two dorsal vertebrae; the appearance of compressed dorsal ribs that are oriented vertically and only lightly associated with overlying dermal armour; a double contact between the dorsal vertebrae and dorsal ribs only incipient; the carapace and most of plastron laminar in thickness; a carapace with distinct medial ridge that widens towards the end; a plastron with a sloping inguinal notch; and neck armour prongs forming an angular cone.

Classification

''Chinlechelys'' was determined to be a primitive genus, assigned as a relative of ''Proganochelys'' in Testudinata. Its armouring shows the evolution of the turtle carapace, and although some basal features like thin ribs suggest a more primitive placement. The cladogram below shows the suspected relationships of ''Chinlechelys'', along with ''Pronganochelys'', '' Kayentachelys'' and other basal turtles:

References

{{Taxonbar, from=Q16826606

Chinle fauna
Prehistoric turtle genera
Testudinata