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Modern Turtles Much Younger Than Thought?

13 March 2008 (All day)
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(fossil)Juliana Sterli/Biology Letters(turtle)Jorge A.Gonzalez (2008)

Premodern?
The shell of this fossil turtle may predate the ancestor of modern turtles.

As reptiles go, turtles are old--no question. They evolved before snakes and crocodiles and preceded dinosaurs. But establishing when the common ancestor of modern turtles first appeared has recently become controversial. Now a new fossil is backing the idea that modern turtles evolved more recently than previously thought.

Living turtles are divided into two main groups--the Cryptodira and the Pleurodira--based on where on the skull the muscles that close the lower jaw are attached. In the 1970s, paleontologist Eugene Gaffney of the American Museum of Natural History (AMNH) in New York City conducted the first modern analysis of turtle evolution. He proposed that almost all fossil turtles belonged within one or the other of these two modern, or crown, groups. That meant that the common ancestor of these turtles first appeared in the Late Triassic, some 210 million years ago.

Last year, paleontologist Walter Joyce of Yale University outlined a major revision of this classification. After reviewing all of the anatomical features, called characters, of the fossil turtles, he argued in the Bulletin of the Peabody Museum of Natural History that many of the fossil taxa were so different from modern turtles that they don't belong in either the Cryptodira or the Pleurodira groups. The implication is that these two groups only evolved about 150 million years ago. "Joyce's picture of turtle evolution is totally different," says James Parham of the California Academy of Sciences, who is based in Santa Barbara.

The new fossil backs this picture, say Joyce and Parham. It comes from Argentina and was discovered in central Patagonia by a joint expedition of the Museo Paleontológico Egidio Feruglio in Trelew and AMNH. About 35 centimeters long, the fossils of a shell and skull were found in ancient lake rocks, dating to between 160 million and 146 million years old--a period in which turtle fossils are few and far between. Juliana Sterli, a Ph.D. student at the Museo de Historia Natural de San Rafael in Mendoza, Argentina, set about describing and analyzing the fossil, which has been named Condorchelys antiqua. Sterli says her research shows that Condorchelys doesn't belong to the Cryptodira or Pleurodira and fits Joyce's hypothesis that the modern groups are at least 60 million years younger than previously thought.

"It's an important fossil," Gaffney says. "A discovery like this gives an important ... glimpse of early Jurassic turtles" in South America. But Gaffney thinks that the turtle fits within his original classification scheme--as a primitive Cryptodira--and is not evidence for Joyce's reinterpretation of turtle evolution. Sterli disagrees, based on analyses of anatomical details. If Joyce and Sterli are correct, Parham notes, then modern turtles would have taken much less time to evolve.

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