| Eunotosaurus Temporal range: Middle Permian |
|
|---|---|
| Restoration of Eunotosaurus africanus | |
| Scientific classification (disputed) | |
| Kingdom: | Animalia |
| Phylum: | Chordate |
| Class: | Sauropsida |
| Subclass: | Parareptilia |
| Family: | Millerettidae |
| Genus: | Eunotosaurus Seeley, 1892 |
| Species | |
|
|
Eunotosaurus is a genus of parareptile from the Middle Permian Karoo Supergroup of South Africa. It was once widely accepted as the missing link between turtles and their prehistoric ancestors.[1] Many fossils have been found showing a semi-rigid, turtle-like rib cage, one which presumably necessitated a tortoise-like fashion of walking.[2] The ribs are very wide and flat, touching each other to form broad plates analogous to the carapace of a turtle. Moreover, the number of vertebrae, the size of the vertebrae, and their structure are nearly identical to those of some turtles. Despite the many similarities to turtles, Eunotosaurus has a skull that shares many characteristics with the skulls of more primitive anapsids. Because of these features, Eunotosaurus is now placed within the Parareptilia and is not widely considered an immediate ancestor of turtles. According to this view, the expanded ribs and similar vertebral column are a case of evolutionary convergence.[3]
Contents |
Eunotosaurus was named in 1892, but it was not until 1914 that it was proposed to be an ancestor of Chelonia, the turtle order. English zoologist D. M. S. Watson claimed that Eunotosaurus was transitional between cotylosaurs (now referred to as captorhinids) and Chelonia.[4] He compared it to "Archichelone", a name he devised for a hypothetical chelonian ancestor, noting that its ribs appeared to be intermediate between those of turtles and other tetrapods. Watson's "Archichelone" had a pelvic girdle that was pushed back on the vertebral column and placed under the shell. However, fossils of Eunotosaurus show that the pelvis is in the normal tetrapod position and is placed over the ribs rater than within them, as in modern turtles.[1]
Eunotosaurus was considered the ancestor of turtles up until the late 1940s. In his 1956 book Osteology of the Reptiles, American paleontologist Alfred Sherwood Romer claimed that Eunotosaurus could not be included within Chelonia based on the available evidence. He placed it within Anapsida in its own order incertae sedis.[1][5]
Over a century after its naming, Eunotosaurus was known from less than a dozen specimens, with very little material known from the skull. Despite the paucity of material, it was well described. Two additional skeletons were unearthed from the Karoo Supergroup and described in 1999. They are now housed in the Bernard Price Institute for Palaeontological Research in Johannesburg and the National Museum, Bloemfontein. While relatively rare, Eunotosaurus is common enough in the Karoo to be used as a biostratigraphic marker. It is present in the upper Tapinocephalus Assemblage Zone and in all parts of the succeeding Pristerognathus Assemblage Zone.[6]
Eunotosaurus was assigned to its own family, Eunotosauridae, in 1954.[7] However, this name has fallen into disuse. In 1969, it was placed in the anapsid suborder Captorhinomorpha,[8] which is now considered to be within the clade Eureptilia.[9] In 2000, Eunotosaurus was placed in the clade Parareptilia, separate from turtles and cotylosaurs.[10] A 2008 phylogenetic analysis of parareptiles found Eunotosaurus to be the sister taxon of Milleretta and thus within the family Millerettidae.[11]
Eunotosaurus was incorporated in a recent 2010 phylogenetic analysis that sought to determine the origin of turtles.[12] Turtles have recently been considered diapsids on the basis of genetic and phylogenetic evidence, and thus more closely related to modern lizards, snakes, crocodiles, and birds than parareptiles or any other anapsid. However, with the inclusion of Eunotosaurus and the Late Triassic stem turtle Proganochelys, the resulting phylogenetic tree placed turtles outside Diapsida in a position similar to turtle's original placement as anapsids. This study claimed that Eunotosaurus shared derived features of its ribs and vertebrae with the earliest turtles, thus making it a transitional form. The study identified several features that united Eunotosaurus with turtles in a true clade. These include broad T-shaped ribs, ten elongated trunk vertebrae, cranial tubercles (small projections on the surface of the skull), and a wide trunk. The clade consisting of Eunotosaurus and turtles was called "Pan-Testudines." More derived testudines, such as the earliest turtle Odontochelys, have a plastron.[12]