Proganochelys

Proganochelys quenstedtii is the only known species of this genus and is among the oldest known stem-turtle species with a complete shell discovered to date, known from fossils found in Germany and Switzerland in strata from the late Triassic, dating to approximately 210 million years ago. The Thailand species "P." ruchae was later described as a separate genus Thaichelys, and the previously referred Greenland fossil was redescribed as a separate genus of Proganochelyidae, Ypomonetikochelys.

Psammochelys, Stegochelys, and Triassochelys are junior synonyms of Proganochelys. Chelytherium von Meyer, 1863 has been considered a synonym of Proganochelys by some authors, but Joyce (2017) considers it a nomen dubium given the fragmentary nature of the syntype material. Joyce (2017) also considered North American genus Chinlechelys to be a junior synonym of Proganochelys, though the author maintains the type species of the former genus, C. tenertesta, as a distinct species within the genus Proganochelys.

Description and paleobiology

Proganochelys was once considered to be the oldest known stem-turtle until the description of Odontochelys and Eorhynchochelys, two slightly earlier genera that lived in the Carnian stage of the Triassic. Proganochelys had a fully developed shell 60–70 cm (2.0–2.3 ft) long. The total length of Proganochelys was about 91 cm (3 ft). Its overall appearance resembled modern turtles in many respects: it lacked teeth on the upper and lower jaw, likely had a beak and had the characteristic heavily armored shell formed from bony plates and ribs which fused together into a solid cage around the internal organs. Proganochelys had a semi-beak like structure along with denticles fused to its vomer. The plates comprising the carapace and plastron were already in the modern form, although there were additional plates along the margins of the shell that would have served to protect the legs. Also, unlike any modern species of turtle, its long tail had spikes and terminated in a club, its head could not be retracted under the shell, and its neck may have been protected by small spines. While it had no teeth in its jaws, it did have small denticles on the palate. The beak like structure suggests that the Triassic stem-turtles evolved from carnivorous stem-turtles to herbivorous as the loss of teeth and gain of the beak would benefit the crushing of plants in these stem-turtles.

Synapomorphies and autapomorphies

Proganochelys possesses several 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. Proganochelys was also known for its autapomorphies, which included a tail club and a tubercle on the basioccipital. 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 have 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, perhaps in a similar fashion to some modern side-necked turtles.

Shell

The broadened ribs on Proganochelys show "metaplastic ossification of the dermis". The enlarged ribs suggest that the endochondral rib ossifications were joined by a second ossification instead of having expanded ribs. The 220-million-year-old stem-turtle Odontochelys only has a partially formed shell. Odontochelys is believed to only possess 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. Proganochelys possessed both a carapace, and a plastron. The shell is believed to have been used for protection against predators. Proganochelys fits well into the order as a turtle, as the shell of Proganochelys is in agreement with the evolution of other stem-turtles.

Skull

The dermal roofing elements of Proganochelys include a large nasal, a fully roofed skull, a flat squamosal, and an absent pineal foramen. Palatal characteristics include paired vomers, and a dorsal process containing premaxilla. An open interpterygoid vacuity along with a prominent elongated quadrate are notable basicranial elements. Overall, Proganochelys is characterized by having relatively few chelonian features and having a relatively generalized amniote skull. The skull of Proganochelys quenstedtii from Trossingen, West Germany, retains a number of well-known amniote features not found in any other turtle. For instance, the lacrimal bone, supratemporal bone, and lacrimal duct are notable structures that are kept. Furthermore, some traits that are present in modern turtles are not present in Proganochelys and therefore must have come after the evolution of the shell. For instance, jaw differentiation, the fusion of the vomer, and the loss of the lacrimal are clear examples of traits that evolved after the evolution of the shell in Proganochelys.

Discovery

The earliest fossils of Proganochelys were discovered in Germany in the rural towns of Halberstadt, Tübingen, and Trossingen. The fossils were found in an elaborate formation of shales, sandstones, and some limestone piles, with the formation believed to be between 220 and 205 million years old. Consensus among Geologists placed the fossils in the middle of the Norian, around 210 million years ago, although this is largely an estimate. In addition to Proganochelys, the rock formations in Germany have also given fossils for the stem-turtle Proterochersis. Fossils have also been found the Klettgau Formation of Switzerland.