Half-Shelled Prehistoric Reptile Was Early Ancestor of Turtles

By K. N. Smith | June 24, 2015 12:00 pm

STEM TURTLE

Researchers have found a new species of reptile which they say is an ancestor of modern turtles.

The 240 million-year-old fossils were found in sediments of a Triassic freshwater lake in southern Germany. The species, named Pappochelys (“grandfather turtle”) rosinae, could help settle a long-running debate about how turtles evolved.

Grandfather Turtle

Pappochelys wouldn’t have looked much like modern turtles at first glance. For one thing, it didn’t have a shell; the ancestors of modern turtles didn’t evolve shells until about 26 million years after Pappochelys.

But the early makings of a shell are visible in Pappocheyls’ skeleton. Its ribs are broad, with a T-shaped cross-section, and it has a hard wall of bones along its belly.

It also had a long, whip-like tail that accounted for half of the little reptile’s 8-inch length, quite unlike the stubby tails of its descendants. And the turtles we know today don’t have teeth – they have beaks instead – but Pappochelys had lots of small, sharp teeth: at least 29 in its lower jaw, and up to 21 in its upper jaw.

“The teeth of Pappochelys suggest that it fed on small invertebrates and perhaps small fish,” lead scientist Hans-Dieter Sues told Discover.

turtle

Credit: Gerald A. DeBoer/ Shutterstock

Missing Link

Pappochelys helps zoologists fill in a missing chapter of how turtles got their shells. Before the discovery, there was a gap in the fossil record between about 260 and 220 million years ago.

The flat underside of the shell, called the plastron, evolved first, probably to provide stability while swimming, as well as for protection. Pappochelys had thick abdominal bones, and some of them showed signs of fusing together at the ends. Twenty million years later, Pappochelys’ descendant Odontochelys had a fully-formed plastron, but no dome-like upper carapace yet.

To form the carapace, over time, the ribs and vertebrae of early turtles (or stem-turtles) enlarged and fused. Pappochelys’ broad, T-shaped ribs look like a start down the path toward a carapace. By 214 million years ago, turtles had developed full shells.

Family Ties

Turtles have puzzled zoologists for at least a century. Most of today’s reptiles, including snakes, lizards, crocodiles, birds, and non-avian dinosaurs, are what are called diapsids. They have a pair of openings in either side of their skulls. But turtles are an outlier, the only modern reptile without these holes.

For years, scientists assumed that meant that turtles were the last surviving members of a group of anapsids (“no openings”), which was an older branch on the reptile family tree. That means they wouldn’t be very closely related to other modern reptiles, which was easy to believe, since turtles are so unusual.

DNA studies in recent years have had mixed results. A 1998 study of turtle mitochondrial DNA says they’re definitely diapsids, despite their smooth skulls, and they’re probably most closely related to archosaurs – a group including crocodiles and birds. But in 2012, another group of researchers examined microRNA from turtles and found that they seem to be more closely related to lizards.

Pappochelys provides fossil evidence that supports that latter conclusion. Its skull has the holes, and the overall shape of its skull looks more like the skulls of lizards and snakes than archosaurs. One hole is smaller than expected, though, which Sues and his colleagues say could mean that, when Pappochelys came along, turtle evolution was, for some reason, starting to close up the skull. That would have eventually led to today’s smooth-skulled turtles, which were so easily mistaken for a much older kind of reptile.

 

Top image credit: Rainer Schoch

CATEGORIZED UNDER: Living World, top posts
MORE ABOUT: animals, paleontology
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