Scymnosaurus
Scymnosaurus is a dubious genus of therocephalian therapsids from the Middle Permian of what is now South Africa based upon the fossils of large, but indeterminate, early therocephalians. The genus and its type species S. ferox was named by Robert Broom in 1903, followed by S. watsoni in 1915. A third species, S. major, was named by Lieuwe Dirk Boonstra in 1954, who also referred many more specimens to the genus as Scymnosaurus sp.
The genus Scymnosaurus and its species are all considered nomina dubia today, meaning the fossils have no distinguishing traits to define each species or to unite them together as a distinct genus. Indeed, Scymnosaurus includes specimens that have since been determined to belong to two separate families of carnivorous early therocephalians. Most specimens, including those of S. ferox and S. major, represent indeterminate specimens from the family Lycosuchidae, while the sole skull of S. watsoni belongs to an indeterminate member of Scylacosauridae. Though Scymnosaurus watsoni represents a scylacosaurid, because the type species S. ferox is identifiable as a lycosuchid, the genus Scymnosaurus itself is regarded as a dubious lycosuchid.
Most specimens attributed to Scymnosaurus are fragmentary, often only partial snouts and jaws. The genus was originally defined in part upon a specific shared tooth count, which is now known to be unreliable for diagnosing early therocephalians and the same dental formula is now known to be typical of lycosuchids in general. The only other consistent feature to unite these fossils is their large size, and though not diagnostic, they notably include some of the largest known specimens of any therocephalians, with the largest specimens representing skulls estimated to exceed in length.
History and taxonomy
''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. It is a tightly closed partial snout and jaws consisting of only the very front portion around the incisors and canines up to and including the third postcanine tooth, with the entirety of the snout above the roots of the teeth missing such that the internal anatomy of the palate and roots of the teeth are exposed. Though fragmentary, the specimen had a unique dental formula —five incisors, one canine, and only three small postcanines on each side—that Broom used to defined the genus. SAM-PK-632 was notable for its large size, described by Broom as the second largest theriodont known at the time after Titanosuchus and belonging to an animal he believed to be the size of a hyena. Indeed, it remains one of the largest therocephalian skulls on record, as though it is incomplete the preserved portion of its snout is proportionately comparable to that of the related Simorhinella, one of the largest therocephalians recognised today with a total skull length of.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. Both SAM-PK-3430 and 4341 are similar specimens to the holotype, preserving occluded snouts and jaws as far as the postcanines, while SAM-PK-9084 is a more complete but distorted skull and jaws that includes most of the jaws but only the snout and eyes of the skull, missing the temporal region, along with partial limb bones. Boonstra referred all three to S. ferox on the basis of sharing the same dental formula and estimating that all three were of similar size to the holotype. 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''
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. This skull had already been published upon the year prior by David M. S. Watson, who described the bones of its palate and doubtfully assigned the specimen to Lycosuchus. 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 and reassigned S. watsoni to this genus as the new combination Pristerosaurus watsoni, claiming that it could not be included under Scymnosaurus. Boonstra later backtracked on this assignment and reverted S. watsoni to Scymnosaurus once again in a 1969 paper without comment. When revising the taxonomy of scylacosaurids in 2023, Christian Kammerer commented that S. watsoni indeed closely resembles Mairasaurus 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. Both skulls were collected by Boonstra in 1929 from two separate expeditions into the Karoo, the first recovering the holotype SAM-PK-9005 at the Kleinkoedoeskop locality and later the second skull SAM-PK-10556 from Knoffelfontein, where it was originally discovered by the farm's owner. 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, a supposedly steeper "chin" at the mandibular symphysis, and the frontal bone not reaching the edge of the eye socket.Other specimens
Boonstra referred five more specimens to Scymnosaurus in 1954, though he was unable to determine if they belonged to either S. ferox or S. major and so only referred them to Scymnosaurus sp. Most specimens were collected by Boonstra himself and his colleagues on various trips during the first half of the 20th century; SAM-PK-9126 collected from Voelfontein in 1929, SAM-PK-11459 from Buffelsvlei in 1939, SAM-PK-11833 from Lammerkraal in 1947, and SAM-PK-11961 from Dikbome. It is not known when the fifth specimen in the museum's collections SAM-PK-8999 was collected, or by who. Like most other specimens under Scymnosaurus they consist of only the snout and occluded lower jaws to varying degrees of completeness and preservation quality, and were referred to Scymnosaurus mostly due to their dental formula.In 1964 Boonstra referred many more specimens to Scymnosaurus sp.; SAM-PK-11577, 11695, 11957, 12118, 12193, 12262, K352 and K353, with another specimen SAM-PK-11558A only doubtfully referred to "Scymnosaurus?". Almost all of these specimens are based upon only postcranial bones from the skeleton, mainly limbs and their girdles, with SAM-PK-11695 being the only specimen among them to preserve part of the skull comparable to other Scymnosaurus specimens. However, this skull was identified as that of Glanosuchus by Kammerer in 2023, without any reference to the postcrania. Similarly, Boonstra had previously referred SAM-PK-11957 to Pristerognathus in 1954 and originally included a partial skull in the same specimen, but excluded the skull in 1964. The skull was later reidentified as Ictidosaurus, another early therocephalian now regarded as dubious. Unlike the cranial specimens, none of the postcranial specimens have been re-examined and had their taxonomic affinities reassessed since Boonstra's work.
A fourth species once included under Scymnosaurus was S. warreni, named by Broom in March 1907 from a specimen discovered in KwaZulu-Natal, NM 188. Like most other Scymnosaurus specimens NM 188 is an occluded, and somewhat crushed, partial snout and lower jaws. Broom assigned it to Scymnosaurus due to its similar dental formula, but regarded the assignment as provisional. 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.