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Pappochelys rosinae
Topic Started: Jun 26 2015, 12:39 AM (1,671 Views)
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Pappochelys rosinae

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Temporal range: Middle Triassic, 240 Ma

Scientific Classification
Species: †P. rosinae

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Pappochelys (meaning "grandfather turtle" in Greek) is an extinct genus of diapsid reptile closely related to turtles. The genus contains only one species, Pappochelys rosinae, from the Middle Triassic of Germany, which was named by paleontologists Rainer Schoch and Hans-Dieter Sues in 2015. The discovery of Pappochelys provides strong support for turtles' placement within Diapsida, a hypothesis that has long been suggested by molecular data but never previously by the fossil record. It is morphologically intermediate between the definite stem-turtle Odontochelys from the Late Triassic of China and Eunotosaurus, a reptile from the Middle Permian of South Africa whose relationships were unclear prior to the discovery of Pappochelys but now likely lie with stem-turtles as well.

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Pappochelys has a wide body, small skull, and a long tail that makes up about half of the total body length, which is up to 20 centimetres (7.9 in). The skull is pointed with large eye sockets. Several turtle-like features are present, including expanded ribs and gastralia that seem to be precursors of a shell. As is the case in Eunotosaurus, each rib is flattened into a broad blade-like structure with bumps and ridges covering its outer surface and a ridge running down its inner surface, forming a T-shape in cross section. The gastralia (rib-like bones covering the abdomen) are tightly packed and occasionally fused together, forming a structure similar to the plastron of turtles. Unlike turtles, Pappochelys has teeth in its jaws and two pairs of holes in the back of the skull called temporal fenestrae. The presence of two pairs of fenestrae make the skull of Pappochelys diapsid, as opposed to the anapsid skulls of turtles that lack any temporal fenestrae.

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Fossils of Pappochelys come from a rock group in Germany called the Lower Keuper, which dates to the Ladinian stage of the Middle Triassic, approximately 240 million years ago (Ma), and are restricted to a 5 to 15 centimetres (2.0 to 5.9 in) layer of organic-rich claystone in an outcrop of the Erfurt Formation in the town of Vellberg. Paleontologists have studied the Lower Keuper extensively since the early nineteenth century and the claystone layer has been subject to intensive fossil collecting since 1985, yet it was not until 2006 that the first fossils of Pappochelys were found. Since then, excavations by the Staatliches Museum für Naturkunde Stuttgart have uncovered 20 specimens of Pappochelys representing most of the skeleton.

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Relationship to Turtles
The placement of turtles on the reptile evolutionary tree has been a point of contention in the past few decades because of a disagreement between morphological and molecular data. Based on anatomical data alone, turtles appear to fall within Parareptilia, which is a basal clade or evolutionary group within Sauropsida (Sauropsida is the reptile clade). Parareptiles are generally characterized by the lack of temporal openings in their skull (but now most of them are known to have at least a lower temporal fenestra and lie outside the main group of reptiles, Diapsida, which includes all other living sauropsids (lizards, snakes, crocodilians, and birds) and is characterized by two pairs of temporal openings. In contrast, molecular data suggests that turtles lie within Diapsida, either as a subset of the Lepidosauromorpha (which includes lizards and snakes) or the clade Archosauromorpha (which includes crocodilians and birds).
Of the reptiles that most closely resemble Pappochelys, Eunotosaurus was originally classified as a parareptile and Odontochelys has always been classified as a stem-turtle (stem-turtles are taxa more closely related to turtles than they are to any other living reptile group, but are not themselves turtles). Since Eunotosaurus possesses both turtle-like and parareptile-like features, it has often been used to justify a parareptilian ancestry for turtles. The discovery of Pappochelys, which is clearly a diapsid, provides the first strong evidence from the fossil record that turtles belong within Diapsida. In 2015, Schoch and Sues incorporated Pappochelys, Eunotosaurus, and Odontochelys into a phylogenetic analysis along with parareptiles, turtles, and many other reptilian taxa to elucidate their relationships. Their analysis found support for a diapsid clade containing Eunotosaurus, Pappochelys, Odontochelys, and turtles, and placed this clade within Lepidosauromorpha. This clade was only distantly related to parareptiles, which was recovered as the most basal group within Sauropsida. Unlike previous morphology-based phylogenies (hypotheses of evolutionary relationships), Schoch and Sues's phylogeny was in agreement with molecular data.

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The claystone bed in which fossils of Pappochelys were found was likely deposited in a lake setting, suggesting that Pappochelys may have been semi-aquatic like many modern turtles. Although Pappochelys lacked a fully formed shell like modern turtles, its thickened bones may have helped reduce the body's buoyancy, making it a more adept swimmer.

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Pappochelys, an early relative of modern-day turtles, had broad dense ribs and a bone-studded underbelly that offered some protection from predators.

"Armored Lizard" was ancestor of today's turtles

By Sid Perkins 24 June 2015 1:00 pm

It’s a primitive turtle, but it looks nothing like today’s dome-shelled reptiles. Resembling a broad-bodied, short-snouted lizard, the 240-million-year-old creature—dubbed Pappochelys rosinae—appears to be a missing link between prototurtles and their modern relatives, according to a new study. If so, the find could fill in a number of pieces about turtle evolution.

The findings are “a very important contribution in addressing who turtles are related to, as well as the evolutionary origin of the turtle shell,” says Tyler Lyson, a vertebrate paleontologist at the Denver Museum of Nature & Science who was not involved with the study. “These have been two vexing questions for evolutionary biologists for the last 200 years.”

About two dozen or so fossils of the creature have been recovered, all of them from 240-million-year-old rocks deposited as sediment on the floor of a shallow, 5-kilometer-long lake in what is now southern Germany. Most of the remains include only bits of bone and are from individuals of various sizes, says Hans-Dieter Sues, a vertebrate paleontologist at the Smithsonian Institution's National Museum of Natural History in Washington, D.C. But between the two most complete specimens yet found, he and Rainer Schoch, a paleontologist at the State Museum of Natural History Stuttgart in Germany, have put together a full skeleton and most of a skull.

P. rosinae adults likely measured about 20 centimeters long, with half of that being a long, whiplike tail. (The species name is a combination of the Greek words for “grandfather turtle” and the person who helped clean rock from the fossils to prepare them for analysis.) Its peglike teeth suggest the animal fed on worms and other soft-bodied prey, Sues says. Yet skeletal anatomy reveals Pappochelys was no run-of-the-mill lizard, Sues and Schoch report online today in Nature.

Unlike lizards, but much like the earliest known relative of turtles (Eunotosaurus, which lived in what is now South Africa about 20 million years earlier), Pappochelys’s ribs are broad, dense, and have a T-shaped cross section. In later, full-shelled species of turtles, those ribs are even wider and have fused with each other and certain bones in the shoulder girdle to form a carapace, or upper shell. But unlike the earlier Eunotosaurus, Pappochelys has gastralia, or belly ribs. These free-floating bones developed within the tissue of the underbelly, Sues says; in more evolved species of turtles, these gastralia broaden and fuse to form a plastron, or lower shell.

Because the fossils were originally entombed in lake floor sediments, the researchers suggest that Pappochelys spent a lot of its time in the water and around the lakeshore—a lifestyle similar to that of today’s marine iguanas, Sues says. So having broad, dense bones and gastralia would have acted like a diver’s weight belt, helping Pappochelys fight buoyancy and forage on the lake’s bottom. But these bones would also have had a beneficial side effect: They would have offered some degree of protection from predators, such as large amphibians or fish living in the lake, by deflecting or blunting their bites.

“In the water, predators can get you from all angles,” Sues notes. Over millions of years, evolution sculpted the bones to create the full set of body armor seen in modern-day turtles. The first full-shelled turtles show up in the fossil record about 205 million years ago.

The two distinctive holes on the side of the head behind each eye of Pappochelys provide vital clues to the evolutionary heritage of turtles, says Torsten Scheyer, a vertebrate paleontologist at the University of Zurich in Switzerland who was not involved in the work. Those holes mark the species as a member of the diapsid (“two arches”) group of reptiles. That diapsid group includes crocodiles, lizards, snakes, dinosaurs, and their surviving kin, birds. But because modern turtle skulls lack these holes, some scientists have proposed that turtles were the last surviving members of an anapsid (“no arches”) lineage of reptiles. But now, he adds, these fossils of turtle progenitors firmly back up the results of genetic analyses of living reptiles: Turtles belong on the diapsid branch of the reptilian family tree.

Scheyer says fossils that are even more complete, or ones that have the bones preserved in more lifelike arrangements, would provide better information about the species. “I’m really looking forward to see more research done on these outstanding fossils.”

Posted in Evolution, Paleontology, Plants & Animals
Edited by Taipan, Jun 26 2015, 09:42 PM.
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