Newswise — A team of paleontologists has published a new study that provides clues on the early evolution of one of nature’s unique developments: the turtle shell.
Led by Tyler Lyson of Yale University and the Smithsonian Institution, the researchers maintain that 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. Those ribs, according to the scientists, represent an early step in the evolutionary development of the carapace, the hard, upper part of the shell of today’s turtles.
The team’s findings are published in a paper, “Evolutionary Origin of the Turtle Shell” in Current Biology.
“This is the first real, detailed study of Eunotosaurus and it fills a gap in the turtle fossil record,” said Lyson, who traveled to the Karoo Basin in South Africa to examine specimens in the field and in museums with Gaberiel Bever, an anatomy professor at New York Institute of Technology College of Osteopathic Medicine. “We are the first to thoroughly describe the whole skeleton and we found that Eunotosaurus uniquely shares numerous features with turtles, including development and muscle anatomy.”
Lyson and Bever noted that the origin of the turtle and its shell have been controversial. Scientists disagree on where turtles fit on the tree of life, said Bever, in part because of a disagreement stemming from the lack of clear transitional fossils – those animals whose remains indicate an intermediate state between ancestors and descendants. Bever added that the differences between molecular studies of living animals and anatomical studies of fossils and living animals also contribute to the disputes about turtle origins.
“Our data supports Eunotosaurus as an important link in that evolutionary chain that eventually produced modern turtles,” said Bever, who is also a research associate with the Division of Paleontology in the American Museum of Natural History. “This is an earlier version of the turtle.”
Previously, the oldest known turtle was a 220-million-year-old reptile from China described in 2008. Establishing Eunotosaurus as part of the turtle lineage thus pushes the age of this evolutionary story back another 40 million years.
Interestingly, Eunotosaurus has been known to science since the 19th century, but Bever said its turtle features were either overlooked or dismissed. One reason, he noted, was that the Eunotosaurus did not have a skin containing bones, which scientists long held was a necessary evolutionary step in the development of the turtle shell.
“We were the first to actually include Eunotosaurus in an evolutionary analysis with turtles,” Bever said. “And as soon as we did that, Eunotosaurus was revealed as closely related to everything currently accepted as a turtle.”
However, because Eunotosaurus lacks many of the iconic turtle features, such as the portion of the hard shell covering its belly, or plastron, it will remain a controversial species, at least for now. But Bever maintains that the first steps in an evolutionary transformation are likely to be subtle and that the distinctive ribs are strong evidence that Eunotosaurus is an early chapter in the turtle story.
“This helps fill the morphological gap between a lizard body plan and the highly modified body form found in turtles today,” said Lyson.
This study is part of a larger research project on reptile anatomy and evolution and the team plans to continue to address several related questions. Lyson plans to study turtle respiratory systems and Bever expects to release a study on the evolution of the Eunotosaurus’ skull.
The study’s other authors are: Torsten Scheyer of the Paleontology Institute and Museum in Zurich, and Allison Hsiang and Jacques Gauthier of Yale.
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