First described as a strange, gluttonous lizard that swallowed a small Frisbee, Eunotosaurus, a shell-less lizard-like reptile, is now recognized to be the earliest known ancestor of turtles.
Eunotosaurus, about a foot long, possessed mixed features of its lizard-like ancestors with emerging turtle-like characteristics that evolved over tens of millions of years into familiar turtle traits, Reuters reported.
Eunotosaurus Africanus existed in the very late Paleozoic Era. During that time, the supercontinent Pangea broke up into smaller continents. Plants became widespread. The first vertebrate animals colonized the land.
Sophisticated skull analysis showed that the reptile that lived in southern Africa 260 million years ago fits perfectly into the turtle family even though it had no shell, researchers said on Wednesday.
The team found a diapsid skull, characterized by having a pair of openings behind the animal's eyes. The diapsid skull allows the muscles of the jaw to flex while the reptile is chewing. Modern lizards, snakes and crocodilians have diapsid skulls. Modern turtles are anapsid. The openings are fully enclosed by bone.
Eunotosaurus is vital in connecting modern turtles to their evolutionary past, according to study leader Gabe Bever, an assistant professor of anatomy at the NYIT College of Osteopathic Medicine. "This is the fossil that science has been searching for more than 150 years. You can think of it as a turtle before turtles had a shell," he said.
"If turtles are closely related to the other living reptiles then we would expect the fossil record to produce early turtle relatives with diapsid skulls," Bever said.
The diapsid skull was left undiscovered in the earlier studies because only the skull of the juvenile Eunotosaurus displays clear evidence of the diapsid holes. As the reptile matures, the diapsid skull becomes obscured.
The obscuring of the diapsid skull as the Enotosaurus grows explains how modern turtles became anapsid. Over the generations, the growth may have been more pronounced. Eventually, the diapsid skull evolved into an anapsid one. Bone closing the holes also appeared.
Previously, Bevel had led a study which revealed that the turtle's shell may have evolved from the animal's wide and flat ribs, which gave it a distinctly rounded and turtle-like profile. The Eunotosaurus' vertebrae were nearly identical to those found in some later turtles. However, there was a lack of research into its skull then.
Over four years, Bever and his team used high-res CT scans to analyze several skull fossils housed in museums in South Africa, CNET reported.
"Imaging technology gave us the opportunity to take the first look inside the skull of Eunotosaurus, and what we found not only illuminates the close relationship of Eunotosaurusto turtles, but also how turtles are related to other modern reptiles," Bever said.
The next step in the research is to study other diapsid reptiles, seeking to expand the turtle family tree.
"The beauty of scientific discoveries is that they tend to reveal more questions than they answer, and there is still much we don't know about the origin of turtles," Bever said.
The study was published in Nature.