By Will Dunham
WASHINGTON (Reuters) - It was a creature that one scientist said resembled "a strange, gluttonous lizard that swallowed a small Frisbee."
But a sophisticated skull analysis showed that this reptile called Eunotosaurus africanus that lived in southern Africa 260 million years ago is actually the earliest-known turtle, even though it had no shell, researchers said on Wednesday.
Eunotosaurus, about a foot (30 cm) long, possessed wide and flat ribs that gave it a distinctly rounded and turtle-like profile. It mixed features of its lizard-like ancestors with emerging turtle-like characteristics that evolved over tens of millions of years into familiar turtle traits, the researchers said.
"Think of your neighborhood box turtle, but much more flattened and with scaly skin and a long tail," said New York Institute of Technology anatomy professor Gaberiel Bever, describing Eunotosaurus. "And teeth, it had a mouthful of them."
Only later did turtles become toothless.
Eunotosaurus lived during the Permian Period 20 million years before Pappochelys, a creature from Germany that in June was identified as the earliest-known turtle, and 30 million years before the first dinosaurs. The earliest-known turtle with a fully formed shell lived around 210 million years ago.
Eunotosaurus fossils were first unearthed in South Africa in the late 19th century. There has been a long debate about whether it was part of the turtle lineage.
Bever's team used advanced scanning technology to perform a detailed analysis of its skull anatomy, digitally dissecting each cranial bone in four fossil specimens, to help demonstrate Eunotosaurus was the oldest-known member of the turtle group.
"Where turtles came from, evolutionarily speaking, and how they are related to the other major groups of living reptiles - lizards, snakes, crocodiles and birds - has been a topic of vigorous debate for as long as we've had a theory of evolution," Bever said.
Fresh insight into the turtle lineage came in 2008 when scientists described a primitive turtle from 220 million years ago called Odontochelys from China.
"Turtles have been missing their Archaeopteryx, their missing link to the rest of the vertebrate tree, since Darwin told us that we should be looking for one," said Bever, mentioning the oldest-known bird.
"With Odontochelys, Pappochelys and now Eunotosaurus, we now have a remarkable series of transitional forms that take us from an almost lizard-like creature to the modern turtle body plan that is so interesting and bizarre."
The research appears in the journal Nature.
(Reporting by Will Dunham; Editing by Sandra Maler)