Proganochelys

Proganochelys was once considered to be the oldest known stem-turtle until the description of Odontochelys  Proganochelys is the oldest stem-turtle species with a complete shell discovered to date, known from fossils found in Germany, Greenland, and Thailand in strata from the late Triassic Proganochelys is considered to have lived in the giant continent Laurasia during the Triassic period. The fossil records show that Proganochelys might have lived anywhere in between Thailand and Germany. During the Triassic period, Laurasia was primarily dry and warm, especially in arid areas.

Proganochelys possess a few chelonian synapomorphies including: a bony shell containing fused ribs, neural bones with fused thoracic segments, and a carapace and plastron that enclose the pelvic and shoulder girdle.[1] Proganochelys was also known for its autapomorphy features which included a tail club and a tubercle on the basioccipital.[1] The tail of Proganochelys was noticeably long and is hypothesized to have been used as a club for protection against predators. Although evolution of the shell has been clearly defined, the mechanisms behind the movement of the neck has been a subject of debate for Proganochelys. It has been hypothesized that Proganochelys were able to retract their necks by tucking in their skull under the front of their shell when needed. Proganochelys belongs to the group of tetrapods with a shell known as Testudinata and is the oldest primitive stem turtle. The group does not include Odontochelys.

The cladogram below follows an analysis by Jérémy Anquetin (2012).

Psammochelys, Stegochelys, and Triassochelys are junior synonyms of Proganochelys.

On the other hand, it is noted that some believe Proganochelys were solely terrestrial. Shell bone histology of extant turtles revealed congruence with terrestrial turtles for the earliest basal turtles, including Proganochelys Proganochelys lived in small water bodies such as ponds, but it was mainly earthbound The broadened ribs on Proganochelys show “metaplastic ossification of the dermis”.[7] The enlarged ribs suggest that the endochondral rib ossifications were joined by a second ossification instead of having expanded ribs.[7] The 220-million-year-old stem-turtle Odontochelys only has a partially formed shell.[8] Odontochelys is believed to only possess the underside element of a shell known as a plastron. The 5-million-year difference that distinguish Odontochelys from Proganochelys tell us that the evolution of the shell occurred relatively quickly in time.[7] Proganochelys possess both a carapace, the upper formation of the shell, and the plastron, the lower. The shell is believed to be used for protection an enhanced feature for survival. Proganochelys fits well into the order as a turtle, as the shell of Proganochelys is in agreement with the evolution of other stem-turtles.