Pappochelys
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''Pappochelys'' (''παπποχέλυς'' (grandfather) + χέλυς (turtle)">€Î¬Ï€Ï€Î¿Ï‚ (grandfather) + χέλυς (turtle)/span> meaning "grandfather turtle" in
Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...
) is an extinct
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of
diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The earliest traditionally identified diapsids, the araeosc ...
reptile possibly related to
turtle Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...
s. The genus contains only one species, ''Pappochelys rosinae'', from the
Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geology), period or the middle of three series (stratigraphy), series in which the Triassic system (stratigraphy), system is di ...
of Germany, which was named by paleontologists and
Hans-Dieter Sues Hans-Dieter Sues (born 1956) is a German-born American palaeontologist who is a Senior Research Geologist and Curator of Vertebrate Paleontology at the National Museum of Natural History of the Smithsonian Institution in Washington, DC. Career ...
in
2015 2015 was designated by the United Nations as: * International Year of Light * International Year of Soil __TOC__ Events January * January 1 – Lithuania officially adopts the euro as its currency, replacing the litas, and becomes ...
. The discovery of ''Pappochelys'' provides strong support for the placement of turtles 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 The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...
of China and ''
Eunotosaurus ''Eunotosaurus'' (''Latin (language), Latin'': Stout-backed lizard) is an extinct genus of amniote, possibly a close relative of turtles. ''Eunotosaurus'' lived in the late Middle Permian (Capitanian stage) and fossils can be found in the Karoo ...
'', a reptile from the
Middle Permian The Guadalupian is the second and middle series/epoch of the Permian. The Guadalupian was preceded by the Cisuralian and followed by the Lopingian. It is named after the Guadalupe Mountains of New Mexico and Texas, and dates between 272.95 ± 0. ...
of South Africa.


Description

''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 . 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 The turtle shell is a shield for the ventral and dorsal parts of turtles (the Order (biology), order Testudines), completely enclosing all the turtle's vital organs and in some cases even the head. It is constructed of modified bony elements such ...
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.


Discovery

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 layer of organic-rich
claystone Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone and shale. Most of the particles of which the stone is composed are less than and are too small to ...
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.


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 Parareptilia ("near-reptiles") is an extinct group of basal sauropsids (" reptiles"), traditionally considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near the en ...
, which is a basal
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
or evolutionary group within
Sauropsida Sauropsida (Greek language, Greek for "lizard faces") is a clade of amniotes, broadly equivalent to the Class (biology), class Reptile, Reptilia, though typically used in a broader sense to also include extinct stem-group relatives of modern repti ...
(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)—supported by one microRNA analysis—or the clade
Archosauromorpha Archosauromorpha ( Greek for "ruling lizard forms") is a clade of diapsid reptiles containing all reptiles more closely related to archosaurs (such as crocodilians and dinosaurs, including birds) than to lepidosaurs (such as tuataras, lizards, ...
(which includes crocodilians and birds)—supported by almost all molecular analyses. 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 In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...
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. Below is a
cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
or evolutionary tree showing the results of their analysis, with stem-turtles denoted by the green bracket: In their description of '' Chinlechelys'', Lichtig & Lucas (2021) criticized Schoch & Sue's hypothesis of turtle origins, particularly the idea shells evolved from broadening ribs with no osteoderms involved (as ''Pappochelys'' apparently indicates). Instead, they proposed ''Pappochelys'' was actually a sauropterygian related to
placodonts Placodonts ("Tablet (pharmacy), tablet tooth, teeth") are an Extinction, extinct order (biology), order of marine reptiles that lived during the Triassic period, becoming extinct at the end of the period. They were part of Sauropterygia, the gro ...
while turtles were derived parareptiles.


Paleobiology

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 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. However, otherwise the anatomy has no signs of a fully aquatic lifestyle and only few adaptations for swimming. In addition, a
histological Histology, also known as microscopic anatomy or microanatomy, is the branch of biology that studies the microscopic anatomy of biological tissue (biology), tissues. Histology is the microscopic counterpart to gross anatomy, which looks at large ...
study found that its limb bones had a thick outer wall and small, open (rather than spongy)
medullary cavity The medullary cavity (''medulla'', innermost part) is the central cavity of bone shafts where red bone marrow and/or yellow bone marrow (adipose tissue) is stored; hence, the medullary cavity is also known as the marrow cavity. Located in the ma ...
, like only a few aquatic reptiles and completely unlike modern aquatic turtles. These features have also been recorded in terrestrial reptiles such as the modern
lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...
'' Sceloporus'' and ''
Eunotosaurus ''Eunotosaurus'' (''Latin (language), Latin'': Stout-backed lizard) is an extinct genus of amniote, possibly a close relative of turtles. ''Eunotosaurus'' lived in the late Middle Permian (Capitanian stage) and fossils can be found in the Karoo ...
'', another genus of pantestudine with burrowing adaptations. This may indicate that ''Pappochelys'' had a burrowing or modestly aquatic lifestyle, rather than a fully aquatic one.


References


External links

* {{Taxonbar, from=Q20220069 Pantestudines Triassic reptiles of Europe Triassic Germany Transitional fossils Fossils of Germany Fossil taxa described in 2015 Prehistoric reptile genera