They suggest a 260-million-year-old reptile from South Africa, Eunotosaurus africanus, is the earliest known version of a turtle, in part because of its distinctive T-shaped ribs -- which represent an early step in the evolutionary development of the carapace, the hard, upper part of the shell of today's turtles.
"We are the first to thoroughly describe the whole skeleton and we found that Eunotosaurus uniquely shares numerous features with [modern] turtles, including development and muscle anatomy," Gabriel Bever, an anatomy professor at New York Institute of Technology, said.
"Our data support Eunotosaurus as an important link in that evolutionary chain that eventually produced modern turtles," he said. "This is an earlier version of the turtle."
Previously the oldest known turtle was a 220-million-year-old reptile from China. Confirming Eunotosaurus as part of the turtle lineage pushes the age of the evolutionary story back 40 million years, researchers said.
They acknowledge that because Eunotosaurus lacks many iconic turtle features -- such as the portion of the hard shell covering its belly, or plastron -- it will remain a controversial species.
However, the distinctive ribs are strong evidence Eunotosaurus fills in an early chapter in the turtle story, researcher Tyler Lyson of Yale University said.
"This helps fill the morphological gap between a lizard body plan and the highly modified body form found in turtles today," he said.