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 long. The total length of
Proganochelys was about . 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.
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.
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.
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. ==Paleoecology==