| Eunotosaurus | |
|---|---|
 | |
| Fossil specimen, on display at Karoo National Park | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Reptiliomorpha |
| Clade: | Amniota |
| Clade: | Sauropsida |
| Genus: | †Eunotosaurus Seeley, 1892 |
| Species: | †E. africanus |
| Binomial name | |
| †Eunotosaurus africanus Seeley, 1892 | |
Eunotosaurus (Latin: Stout-backed lizard) is anextinctgenus ofamniote, possibly a close relative ofturtles.Eunotosaurus lived in the lateMiddle Permian (Capitanian stage) and fossils can be found in theKaroo Supergroup of South Africa andMalawi.Eunotosaurus resided in the swamps of what is now southern Africa.[1] Its ribs were wide and flat, forming broad plates similar to a primitiveturtle shell, and the vertebrae were nearly identical to those of some turtles. Accordingly, it is often considered as a possibletransitional fossil between turtles and their prehistoric ancestors.[2][3] However, it is possible that these turtle-like features evolved independently of the same features in turtles, since other anatomical studies and phylogenetic analyses suggest thatEunotosaurus may instead have been aparareptile,[4] an early-divergingneodiapsid unrelated to turtles,[5] or asynapsid.[6]
Eunotosaurus reached up to 30 cm (12 in) in total body length.[7] It had a broad body formed by nine pairs of widened ribs that overlap each other. The forward-most ribs are angled slightly backward and the backward-most ribs angle slightly forward. The ribs are T-shaped in cross section, each having a broad, flat surface on the top and a narrow ridge running along its length on the bottom. The upper surface is convex, giving the body ofEunotosaurus a rounded shape. Each pair of ribs connects to an elongateddorsal or back vertebra. Most ribs are fused to the vertebrae, but some smaller specimens ofEunotosaurus have rib pairs that connect with the vertebrae but are not fused to them. There are nine dorsal vertebrae, far fewer than what is seen in other parareptiles. The neck ofEunotosaurus is short, consisting of six shortcervical vertebrae.[8][7]
Histological analysis of cross-sections of the ribs indicate that they grew in three different phases as an individual developed. As is the case in most land vertebrates, the first phase involves the growth of a ribprimordium thatossifies into a rib bone. The second phase, which deviates from most other land vertebrates, is the development of a shelf of bone above the main shaft of the rib to form the T-shape. The third and final phase is the widening of the lower ridge into a teardrop-like shape, reinforcing the rib. While the third phase is unique toEunotosaurus, the second phase is also seen in modern turtles. In turtles, the shelf of bone that forms from the rib shaft becomes a plate of the shell orcarapace. In each rib ofEunotosaurus, the posterior surface of the lower ridge hasSharpey's fibers embedded in it. Sharpey's fibers help anchor muscles to bone. Mostamniotes have Sharpey's fibers on the posterior and anterior edges of the ribs because the ribs are connected to each other byintercostal muscles, which are muscles that assist in breathing. The lack of Sharpey's fibers on the anterior side of the ribs ofEunotosaurus suggests that it lacked functional intercostal muscles. Turtles also lack intercostal muscles and instead have muscles that connect to the undersides of the ribs for the purpose of locomotion. IfEunotosaurus is close to the ancestry of turtles, it may have had similar sets of muscles.[8]
Even thoughEunotosaurus has been traditionally considered an anapsid, it is considered to possess a lowertemporal fenestra, though without the temporal bar. Moreover, a juvenile specimen also shows upper temporal fenestrae, meaning the skull demonstrates a fully diapsid condition. In the adult, the upper fenestra is covered by thesupratemporal bone.[9]
Eunotosaurus was named in 1892 for a specimen (now NHMUK PV R 1968 in the Natural History Museum, London) that he had obtained from Mr L. Pienaar at the farm Weltevreden nearBeaufort West, during Seeley's visit to South Africa in 1889. Seeley was uncertain of the systematic position of Eunotosaurus, but postulated that it was likely referable to the Mesosauria, based on the pubis.[10] It was not until 1914 that it was proposed to be an ancestor ofChelonia, the turtleorder. English zoologistD. M. S. Watson claimed thatEunotosaurus was transitional betweencotylosaurs (now referred to ascaptorhinids) and Chelonia.[11] 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 ofEunotosaurus show that the pelvis is in the normal tetrapod position and is placed over the ribs rather than within them, as in modern turtles.[12] Many fossils have been found showing a semi-rigid, turtle-like rib cage, one which presumably necessitated a tortoise-like fashion of walking.[13]
Eunotosaurus was considered the ancestor of turtles up until the late 1940s. In his 1956 bookOsteology of the Reptiles, American paleontologistAlfred Sherwood Romer claimed thatEunotosaurus could not be included within Chelonia based on the available evidence. He placed it withinAnapsida in its own orderincertae sedis.[12][14]
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 theBernard Price Institute for Palaeontological Research inJohannesburg and theNational Museum, Bloemfontein. While relatively rare,Eunotosaurus is common enough in the Karoo to be used as a biostratigraphic marker. It is present in the upperTapinocephalus Assemblage Zone and in all parts of the succeedingPristerognathus Assemblage Zone.[15] In 2024, an articulatedEunotosaurus fossil was described from the collections of theCultural & Museum Centre Karonga inMalawi, having been discovered in 2016 by a herdsman in theMwesia Beds ofKaronga. This marked the first occurrence ofEunotosaurus outside South Africa, and confirm that the Mwesia Beds correspond with theTapinocephalus andPristerognathus zones.[16]
The ribs ofEunotosaurus were very wide and flat, touching each other to form broad plates similar to thecarapace 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 its many similarities to turtles,Eunotosaurus has a skull that shares many characteristics with the skulls of more primitive reptiles, resulting in many studies placing it in the extinct groupParareptilia.Phylogenetic analyses that use only the physical features of fossils and living species to determine evolutionary relationships have often shown strong support for bothEunotosaurus and turtles being descendants of parareptiles, in which caseEunotosaurus.[8] However, analyses which also include genetic data from living reptiles strongly support the idea that turtles fall within a group calledDiapsida, as close relatives of either lizards (in which case they would belepidosauromorphs) or birds and crocodiles (making themarchosauromorphs). According to this view, the expanded ribs and similar vertebral columns ofEunotosaurus and turtles may be a case ofevolutionary convergence.[17] However, the discovery ofPappochelys, a prehistoric species whose fossil remains show a mixture of features found inEunotosaurus and the toothed stem-turtleOdontochelys, helped to resolve the issue. Though an analysis which included data fromPappochelys found weak support for the idea thatEunotosaurus was a parareptile, it found stronger support for the hypothesis thatEunotosaurus was itself a diapsid closely related to turtles, and that its apparently primitive,anapsid skull was probably developed as part of the turtle lineage, independently of parareptiles.[18][7]
Eunotosaurus was assigned to its own family,Eunotosauridae, in 1954.[19] However, this name has fallen into disuse. In 1969, it was placed in the parareptile suborderCaptorhinomorpha,[20] which is now[when?] considered to be within the cladeEureptilia.[21] In 2000,Eunotosaurus was placed in the clade Parareptilia, separate from turtles and cotylosaurs.[22] A 2008 phylogenetic analysis of parareptiles foundEunotosaurus to be thesister taxon ofMilleretta and thus within the familyMillerettidae.[23]
Eunotosaurus was incorporated in a recent 2010 phylogenetic analysis that sought to determine the origin of turtles.[24][7] Turtles have recently been considereddiapsids on the basis of genetic and phylogenetic evidence, and thus more closely related to modernlizards,snakes,crocodiles, andbirds than parareptiles. However, with the inclusion ofEunotosaurus and theLate Triassicstem turtleProganochelys, the resulting phylogenetic tree placed turtles outside Diapsida in a position similar to turtles' original placement as parareptiles. This study claimed thatEunotosaurus shared derived features of its ribs and vertebrae with the earliest turtles, thus making it a transitional form. The study identified several features that unitedEunotosaurus with turtles in a true clade.[7] 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 ofEunotosaurus and turtles was calledPan-Testudines (defined as all animals more closely related to turtles than to any other living group). More derived pan-testudines, such as the earliest turtleOdontochelys, have aplastron.[24]
The followingcladogram shows thephylogenetic position of theEunotosaurus, from Rutaet al., 2011.[25]
The cladogram below follows the most likely result found by another analysis of turtle relationships, published by Rainer Schoch and Hans-Dieter Sues in 2015. This study foundEunotosaurus to be an actual early stem-turtle, though other versions of the analysis found weak support for it as a parareptile.[18]
The following cladogram is adapted from a 2022 study by Simõeset al. Here,Eunotosaurus was recovered as neither a parareptile or a stem-turtle, but as a basal neodiapsid located outside the reptilian crown group.[5]
| Sauropsida | Traditional Parareptilia | |
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