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article imageScientists: Strange lizard-like reptile is earliest known turtle

By Megan Hamilton     Sep 4, 2015 in Science
Scientists have now solved an important piece of the turtle evolution puzzle.
Recent fossil evidence has helped paleontologists prove that turtles share a recent common ancestor with birds, lizards, and crocodiles.
This discovery may settle an argument that has long divided scientists about the origins of turtles, YaleNews reports.
The evolution of the turtle's skull provided a bit of mystery for scientists. Through genetic analysis of molecular sequence data, turtles are consistently placed in a group with birds and crocodilians, and some scientists believed that early turtles likely had a diapsid skull, with a pair of openings behind each eye that helped jaw muscles to tighten and flex during chewing. But here was the puzzle: No fossil evidence had been found to confirm this.
Until the discovery of Eunotosaurus africanus, a 260-million-year-old reptile from Africa's Karoo basin.
Researchers from Yale, the New York Institute of Technology (NYIT), and the Denver Museum of Nature and Science discovered that the fossil skull of this little creature (a juvenile) had the tell-tale openings.
"Eunotosaurus is a 'cryptic' diapsid because it closes the skull openings later in life," Yale assistant professor in the Department of Geology and Geophysics and study coauthor Bhart-Anjan Bhullar told YaleNews. "Only the fortuitous discovery of these openings in a very young juvenile allowed us to realize this."
Modern turtles are anapsids, meaning that they have no openings behind the eyes.
The study was published in the journal Nature.
NYIT scientist Gaberiel Bever, leader of the research team, says the extinct reptile is the earliest known branch of the turtle tree of life.
"Eunotosaurus is a critical link connecting modern turtles to their evolutionary past," Bever, an assistant professor of anatomy at the NYIT College of Osteopathic Medicine, told Phys.org. "This is the fossil for which science has been searching for more than 150 years. You can think of it as a turtle, before turtles had a shell."
It was a rather odd-looking creature and its appearance led one scientist to remark that it looked like "a strange, gluttonous lizard that swallowed a small Frisbee," Reuters reports.
The foot-long creature possessed wide, flat ribs that gave it a rounded, turtle-like profile. It combined features of it's lizard-like ancestors with emerging characteristics that were turtle-like and 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," Bever told Reuters. "And teeth, it had a mouthful of them."
Those wide ribs and that distinctively circular torso were the first clues that this fossil was a link to the origin of turtles, Phys.org notes.
However, in their study, Bever and his colleagues focused on the skull of Eunotosaurus. Their findings showed that the complex anatomy of the creature's skull offered convincing evidence of the crucial role Eunotosaurus played in the deep history of turtle evolution.
"Our previous studies showed that Eunotosaurus possessed structures that likely represent the first steps in the evolution of the turtle shell," said Tyler Lyson of the Denver Museum of Science and Nature, and also a coauthor of the study, "but what those studies lacked was a detailed analysis of the skull."
By using high-resolution computed tomography, Bever digitally dissected the bones and internal structures of several Eunotosaurus skulls, all of which reside in South African museums. Then he incorporated his observations into a new analysis regarding the reptile tree of life. Even though the process took nearly four years, the results were definitely worth the effort Bever said.
"Imaging technology gave us the opportunity to take the first look inside the skull of Eunotosaurus, Bever said. "And what we found not only illuminates the close relationship of Eunotosaurus to turtles, but also how turtles are related to other modern reptiles."
Because of the anapsid-diapsid distinction, it was long thought that turtles were the remnants of an ancient reptile lineage and weren't closely related to modern lizards, crocodiles, and birds. This new data provided by Eunotosaurus rejects this hypothesis, Phys.org notes.
"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. "That expectation remained unfulfilled for a long time, but with some help from technology and a lot of hard work on our part, we can now draw the well-supported and satisfying conclusion that Eunotosaurus is the diapsid turtle that earlier studies predicted would be discovered."
Insight into the lineage of turtles received a jumpstart in 2008 with the discovery of Odontochelys, a primitive turtle that lived 220 million years ago in what is now China, Reuters reports.
"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," Bever said.
And with that, a very large piece of the puzzle of the turtle evolution puzzle has been found.
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