Samplings: February 2009

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How did this turtle (a modern red-bellied short-necked turtle, Emydura subglobosa) get its shell?

Photo © Anita Salzberg

Newly discovered fossils from New Mexico and China are providing contradictory clues to the origin of the turtle’s shell. Two hypotheses have long competed as the evolutionary explanation. One proposes that ancestral turtles grew bony plates on their skin that eventually fused with one another and with underlying ribs. The other holds that the ribs expanded and became embedded in the skin, which ossified to form the shell.

Walter G. Joyce, now at the University of Tübingen in Germany, and four colleagues report that the shell of the New Mexican fossil—about 210 million years old—is very thin and doesn’t quite connect to the ribs. Bony spines on the neck show that ancient turtles’ skin could readily ossify. Points go to the “from the skin in” hypothesis.

Chun Li, of the Chinese Academy of Sciences in Beijing, and four colleagues describe three Chinese fossils. At 220 million years old, the fossils represent the most primitive turtle species known. The species has a plastron—the ventral part of the shell—but, intriguingly, only a poorly developed carapace—the dorsal part. The dorsal ribs are markedly expanded, scoring points for the “from the ribs out” scenario.

To confound matters even more, the Chinese turtle was aquatic, the American turtle terrestrial. Perhaps the turtle shell evolved in one environment and adapted to suit the other. For now, it seems, the shell hides its origins just as well as it does its owner. (Proceedings of the Royal Society B, Nature)