| Pappochelys | |
|---|---|
 | |
| Theholotype specimen ofPappochelys | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Pantestudines |
| Genus: | †Pappochelys Schoch & Sues 2015 |
| Type species | |
| †Pappochelys rosinae | |
Pappochelys (παπποχέλυς[πάππος(grandfather) + χέλυς(turtle)] meaning "grandfather turtle" inGreek) is an extinctgenus ofdiapsid reptile possibly related toturtles. The genus contains only one species,Pappochelys rosinae, from theMiddle Triassic of Germany, which was named by paleontologistsRainer Schoch [de] andHans-Dieter Sues in2015. The discovery ofPappochelys provides strong support for the placement of turtles withinDiapsida, 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-turtleOdontochelys from theLate Triassic of China andEunotosaurus, a reptile from theMiddle Permian of South Africa.[1][2][3]
Pappochelys had 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 (8 in). The skull is pointed with large eye sockets. Several turtle-like features are present, including expanded ribs andgastralia that seem to be precursors of a shell. As is the case inEunotosaurus, 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 theplastron of turtles. Unlike turtles,Pappochelys has teeth in its jaws and two pairs of holes in the back of the skull calledtemporal fenestrae. The presence of two pairs of fenestrae make the skull ofPappochelys diapsid, as opposed to theanapsid skulls of turtles that lack any temporal fenestrae.[1][3]
Fossils ofPappochelys come from a rock group in Germany called theLower Keuper, which dates to theLadinian stage of the Middle Triassic, approximately 240 million years ago (Ma), and are restricted to a 5 to 15 centimetres (2 to 6 in) layer of organic-richclaystone in an outcrop of theErfurt Formation in the town ofVellberg. 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 ofPappochelys were found. Since then, excavations by theStaatliches Museum für Naturkunde Stuttgart have uncovered 20 specimens ofPappochelys representing most of the skeleton.[1]
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 withinParareptilia, which is abasalclade or evolutionary group withinSauropsida (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,[4][5][6][7]) 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 theLepidosauromorpha (which includes lizards and snakes)—supported by one microRNA analysis—or the cladeArchosauromorpha (which includes crocodilians and birds)[1]—supported by almost all molecular analyses.
Of the reptiles that most closely resemblePappochelys,Eunotosaurus was originally classified as a parareptile andOdontochelys 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).[3] SinceEunotosaurus possesses both turtle-like and parareptile-like features, it has often been used to justify a parareptilian ancestry for turtles. The discovery ofPappochelys, which is clearly a diapsid, provides the first strong evidence from the fossil record that turtles belong within Diapsida. In 2015, Schoch and Sues incorporatedPappochelys,Eunotosaurus, andOdontochelys into aphylogenetic analysis along with parareptiles, turtles, and many other reptilian taxa to elucidate their relationships. Their analysis found support for a diapsid clade containingEunotosaurus,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-basedphylogenies (hypotheses of evolutionary relationships), Schoch and Sues's phylogeny was in agreement with molecular data. Below is acladogram or evolutionary tree showing the results of their analysis, with stem-turtles denoted by the green bracket:[1]
| stem-turtles |
In their description ofChinlechelys, Lichtig & Lucas (2021) criticized Schoch & Sue's hypothesis of turtle origins, particularly the idea shells evolved from broadening ribs with no osteoderms involved (asPappochelys apparently indicates). Instead, they proposedPappochelys was actually asauropterygian related toplacodonts while turtles were derivedparareptiles.[8]
The claystone bed in which fossils ofPappochelys were found was likely deposited in a lake setting,[3] suggesting thatPappochelys may have been semi-aquatic like modern turtles. AlthoughPappochelys 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.[1] However, otherwise the anatomy has no signs of a fully aquatic lifestyle[3] and only few adaptations for swimming. In addition, ahistological study found that its limb bones had a thick outer wall and small, open (rather than spongy)medullary cavity, like only a few aquatic reptiles and completely unlike modern aquatic turtles. These features have also been recorded in terrestrial reptiles such as the modernlizardSceloporus andEunotosaurus, another genus ofpantestudine with burrowing adaptations. This may indicate thatPappochelys had a burrowing or modestly aquatic lifestyle, rather than a fully aquatic one.[9]
Media related toPappochelys at Wikimedia Commons.png)
