1 juillet 2015

Pappochelys rosinae: l'ancêtre des tortues

Fascinante découverte:

It’s a primitive turtle, but it looks nothing like today’s dome-shelled reptiles. Resembling a broad-bodied, short-snouted lizard, the 240-million-year-old creature—dubbed Pappochelys rosinae—appears to be a missing link between prototurtles and their modern relatives, according to a new study. If so, the find could fill in a number of pieces about turtle evolution.

(...) About two dozen or so fossils of the creature have been recovered, all of them from 240-million-year-old rocks deposited as sediment on the floor of a shallow, 5-kilometer-long lake in what is now southern Germany. 

(...) P. rosinae adults likely measured about 20 centimeters long, with half of that being a long, whiplike tail. (The species name is a combination of the Greek words for “grandfather turtle” and the person who helped clean rock from the fossils to prepare them for analysis.) Its peglike teeth suggest the animal fed on worms and other soft-bodied prey, Sues says. 

(...) Unlike lizards, but much like the earliest known relative of turtles (Eunotosaurus, which lived in what is now South Africa about 20 million years earlier), Pappochelys’s ribs are broad, dense, and have a T-shaped cross section. In later, full-shelled species of turtles, those ribs are even wider and have fused with each other and certain bones in the shoulder girdle to form a carapace, or upper shell. But unlike the earlier Eunotosaurus, Pappochelys has gastralia, or belly ribs. These free-floating bones developed within the tissue of the underbelly, Sues says; in more evolved species of turtles, these gastralia broaden and fuse to form a plastron, or lower shell.

Because the fossils were originally entombed in lake floor sediments, the researchers suggest that Pappochelys spent a lot of its time in the water and around the lakeshore—a lifestyle similar to that of today’s marine iguanas, Sues says. So having broad, dense bones and gastralia would have acted like a diver’s weight belt, helping Pappochelys fight buoyancy and forage on the lake’s bottom. But these bones would also have had a beneficial side effect: They would have offered some degree of protection from predators, such as large amphibians or fish living in the lake, by deflecting or blunting their bites.

“In the water, predators can get you from all angles,” Sues notes. Over millions of years, evolution sculpted the bones to create the full set of body armor seen in modern-day turtles. The first full-shelled turtles show up in the fossil record about 205 million years ago.



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