Turtles can’t just crawl out of their shells as some cartoons have implied. Their bony shells – with the carapace on the back and a plastron covering the belly – are linked by bony bridges on either side and fused to their spine and ribs. Now, a 240-million-year-old primitive turtle reveals a previously unknown stage in the evolution of the turtle body plan. The findings, published in Nature this week, hint at how these amazing creatures and their protective structures came to be.
We know surprisingly little about the origin and evolution of turtles, tortoises, and terrapins. This is mostly because the fossil record lacks intermediate forms between the shelled critters and the more, well, regularly-arranged reptiles. Nearly a decade ago, however, the disarticulated skeleton and incomplete skull of a potentially transitional form was discovered in Schumann quarry in Baden-Württemberg, Germany, along with ancient fish, stem-amphibians, and other reptiles.
After analyzing the Middle Triassic fossils of multiple specimens, Rainer Schoch from the Staatliches Museum fur Naturkunde Stuttgart and Hans-Dieter Sues from the National Museum of Natural History found that they belonged to a previously-unknown primitive turtle species: an intermediate form between turtle ancestors and definitive members of the turtle club.
They named it Pappochelys rosinae, from Greek “pappos” for “grandfather” and “chelys” for “turtle.” And the species name honors a specimen preparer. This new stem-turtle was about 20 centimeters long and had teeth, a short, pointed snout, and a whip-like tail. It either lived along the lakeshore or frequently entered the lake.
240-million-year-old stem-turtle Pappochelys viewed from above (dorsal view) and from the side (lateral view). Rainer Schoch
Structurally and chronologically, it lies between the 260-million-year-old Eunotosaurus from South Africa (the earliest stem-turtle to date) and 220-million-year-old Odontochelys from China. Odontochelys had a partly formed shell, retained marginal teeth, and showed many turtle-like features in its skeleton.
While it didn’t have a shell, Pappochelys did have broad ribs in the trunk section (above, in yellow), which had a T-shaped cross-section – like Odontochelys and Eunotosaurus. But unlike the younger Odontochelys, in the place of a plastron, Pappochelys had a hard wall of paired bones called gastralia along its belly (above, in red). The new species, the authors say, provides evidence that the plastron partly formed through the serial fusion of gastralia.
Furthermore, the skull shape and the two openings on either side of the cranium suggest that turtles may be more closely related to lepidosaurs (a group that includes familiar reptiles such as lizards and snakes) than to archosaurs – the “ruling reptiles,” like crocodiles, pterosaurs, dinosaurs, and birds – who have only one opening on either side.
