Scymnosaurus

S. ferox Scymnosaurus and its type species S. ferox were named in 1903 by Robert Broom for the holotype specimen SAM-PK-632, originally collected on an expedition by J. R. Joubert from an unknown locality in 1881 and now housed in the Iziko South African Museum (SAM). Additional specimens were later referred to S. ferox by Lieuwe Dirk Boonstra; SAM-PK-9084 in 1953, collected by Boonstra from locality Riet Kuil 387 in 1929, and SAM-PK-3430 and 4341 in 1954, collected by Sidney H. Haughton in 1916 from Janwillemsfontein and Stinkfontein, respectively. However, Boonstra misinterpreted the dental formula of SAM-PK-4341, and it has been reidentified as a specimen of the scylacosaurid therocephalian Glanosuchus. S. watsoni PV R 4100, an indeterminate scylacosaurid skull and holotype of S. watsoni In 1915, Broom named the species S. watsoni from a skull in the collections of the Natural History Museum, London catalogued NHMUK PV R 410, originally purchased in 1878 from Thomas Bain, a pioneering road engineer from South Africa with an interest in Karoo geology. Having examined the skull after further preparation, Broom instead assigned NHMUK PV R 410 to Scymnosaurus as a new species. He did so the basis of it sharing a similar dental formula as S. ferox, in spite of the fact the skull is missing the front end of the snout, including the incisors and most of the canines, while S. ferox is known mostly from just this region. Broom acknowledged there was room for doubt in assigning S. watsoni to Scymnosaurus because of this, but nonetheless believed it and S. ferox to be "certainly closely allied". In his 1932 book The Mammal-like Reptiles of South Africa and the Origin of Mammals Broom remained doubtful of its placement in Scymnosaurus, but retained it in the genus nonetheless. S. watsoni was further examined by Watson in 1921 and again by Boonstra in 1934, who together revised the anatomy of its occiput, braincase and palate. In 1954, Boonstra named a new genus and species of early therocephalian Pristerosaurus microdon (now a junior synonym of Mairasaurus) When revising the taxonomy of scylacosaurids in 2023, Christian Kammerer commented that S. watsoni indeed closely resembles Mairasaurus (i.e. "Pristerosaurus") proportionately, but as the latter is known only by one similarly sized specimen more information on its anatomy would be needed to investigate any potential affinities as Boonstra had suggested. S. major In 1954 Boonstra named a third species S. major from two incomplete but very large partial snouts he each estimated to belong to skulls over long when complete. SAM-PK-9005 is also associated with some postcranial bones, namely parts of the shoulder and hip girdles and the ends of several limb bones that Boonstra went on to describe in 1964. Boonstra included S. major in Scymnosaurus on the basis of its large size and similar dental formula, but only differentiated it from S. ferox by larger size and subtle perceived differences of the jaw and the bones of the skull. These included slightly fewer postcanines (2-3 versus 3-4), a supposedly steeper "chin" at the mandibular symphysis (where the two jaw bones meet), and the frontal bone (a bone on the roof of the skull between the eyes) not reaching the edge of the eye socket. In 1932 he recognised that the specimen actually belonged to the akidnognathid therocephalian Moschorhinus and reassigned the species to the genus as the new combination M. warreni. This species is regarded by subsequent researchers to be synonymous with the type species of Moschorhinus, M. kitchingi. Taxonomy and validity Named in 1903, Scymnosaurus was among the first genera assigned to Therocephalia after Broom had not long named the order earlier that year. At the time, Therocephalia also included the dinocephalian Titanosuchus (among other genera now known to not be therocephalians), which Broom believed to "undoubtedly" be the closest relative of Scymnosaurus. Scymnosaurus would even be used as the basis of the subfamily Scymnosaurinae that was proposed by Boonstra in 1969 for large "pristerognathids" like Scymnosaurus. While the source localities for the holotype as well as SAM-PK-8999 (S. sp.) are unknown, lycosuchid fossils are restricted to the Tapinocephalus AZ and into the lower layers of the subsequent Endothiodon AZ from rocks of the overlying Teekloof Formation, though they are mostly known from the Tapinocephalus AZ. ==Description==

Specimens of Scymnosaurus represent large to very large therocephalians, among the largest of any therocephalians known. All the skulls referred to Scymnosaurus are incomplete, often only partial snouts and associated jaws with only a few preserving the orbital region around the eyes. However, from comparing their proportions to related, more complete therocephalians, the estimates for the complete skull length have ranged from comparable to Simorhinella (i.e. long) for specimens referred to S. ferox and up to between and long for S. major—though these estimates are based upon reconstructions made prior to modern classifications. More direct comparisons between the equivalent portions of the snout show that the type specimen of S. ferox ( snout) is similar in size to Simorhinella ( snout), while the snout of SAM-PK-9005 (S. major) measures long. Apart from size, Scymnosaurus was also historically diagnosed by a dental formula of five large upper incisors, a very large single canine, and only two to four small postcanines, all serrated. Otherwise, specimens under Scymnosaurus have no uniquely distinguishing traits beyond the typical characteristics of the families they belong to (and hence are nomina dubia). Most specimens of Scymnosaurus, including those of S. ferox and S. major, are identifiable as lycosuchids (Lycosuchidae incertae sedis). Like all lycosuchids their snouts are broad and proportionately short compared to scylacosaurids (albeit some specimens are distorted). All of these specimens have five incisors in each premaxilla and less than five small postcanines behind the large canine, both characteristic of lycosuchids compared to scylacosaurids, which have more of each. Notably, no specimens attributed to Scymnosaurus have the "double canines" seen in many other lycosuchid specimens. However, lycosuchid "double canines" are now recognised to be a product of tooth replacement rather than being the typical appearance, and indeed the broken or resorbed roots of the alternate canines are observed in several specimens of Scymnosaurus. In at least one large specimen of S. major the upper margin of the orbit is thickened and rugose, obscuring sutures and hence why Boonstra initially thought the frontal did not reach the edge. This appears to be a typical feature of large and mature specimens of lycosuchids, as it is also seen in Lycosuchus S. watsoni In contrast, S. watsoni is identifiable as an indeterminate scylacosaurid. Scylacosaurid skulls are proportionately longer and narrower than lycosuchids, mostly in the snout, and they typically have more teeth, though the skull of S. watsoni is distorted and the missing front end of the snout precludes identifying its incisor count. However, it does preserve the pterygoid bones of the palate, which lack palatal pterygoid teeth. This is a trait found in scylacosaurids but not lycosuchids, which do have pterygoid teeth. S. watsoni is somewhat smaller than S. ferox and S. major, and in 1915 Broom estimated its complete skull length to be approximately . Broom highlighted the large temporal fenestra and very tall and thin sagittal crest between them as distinctive, but tall sagittal crests appear typical of all large scylacosaurid specimens and both its height and the broad fenestra have probably been distorted during fossilisation (taphonomy). Though relatively complete, its poor state of preservation hinders identification any further than Scylacosauridae indet., though its proportions are similar to those of the rare scylacosaurid Mairasaurus. However, unlike definitive Mairasaurus specimens its orbits are visible when viewed from above, rather than facing mostly out to the side as is characteristic of Mairasaurus, though the potential taxonomic significance of this difference is unclear. ==Notes==