Developments in Brief : Missing Link in Evolution of Turtles Filled In With Northeast Arizona Find

Scientists in northeastern Arizona have discovered the oldest known member of one of the two branches of the turtle family tree, filling in an important missing link in turtle evolution.

Eight fairly complete shells, two skulls, and pieces of several other skulls were found in a rock formation that has been the scene of paleontological exploration for many years. The fossils date to about 180 million to 135 million years ago, said Peter Meylan, a fellow in paleontology at the American Museum of Natural History in New York and an author of a report on the discovery in the current Science magazine.

“It’s a pretty small turtle, less than a foot long,” he said. Despite its age, “you would recognize it as a turtle right away. There have been changes in the shell and changes in the skull, but the basic structures are all there. It’s not the oldest turtle; it’s the oldest of one of the two major groups of turtles.”

The newly discovered turtle belongs to the branch called cryptodire. The other main branch is the pleurodires. The oldest known turtle dates to about 200 million to 210 million years, Meylan said. Called proganochelys, it apparently belongs to neither group. A pleurodire has also been found from that era. Both were discovered in West Germany.

The cryptodires and pleurodires had different ways of folding their necks into their shells and had different mechanisms for closing their lower jaws, Meylan said.