Lycosuchidae

'' from above and the side, illustrating lycosuchid proportions Lycosuchids are characterised by several physical features of their skulls, including a low and wide snout that is proportionately short, typically half or less of the skull's whole length. The number of teeth are also distinctive, as lycosuchids only ever possess five or less incisors on each side of the upper jaw, a pair of canines, and very few postcanines behind them, typically only two or three behind each. In the lower jaws, there are only three incisors, a canine, and around five postcanines in each mandible. The large, almost "sabre-tooth" like canines are especially striking similarities. This anatomy suggests lycosuchids were capable of relatively strong bites with their large canines and incisors, and were resistant to the twisting of prey in their powerful jaws. All lycosuchids, including dubious genera and indeterminate specimens, are very similar superficially and share all of these features with little variation. The only accepted valid genera, Lycosuchus and Simorhinella, are mostly told apart by details of palate and the jaw joint. Lycosuchids are large therocephalians, with the largest known Lycosuchus skull measuring almost long and the even larger skull of Simorhinella measuring long. The largest lycosuchid is an indeterminate specimen previously named as the holotype of "Scymnosaurus major" labelled SAM-PK-9005. It is a poorly preserved and incomplete snout measuring long, but proportionately it exceeds the snout dimensions of Simorhinella (which measures long). SAM-PK-9005 is both the largest known lycosuchid and also one of the largest therocephalians known altogether. A complete skeleton of a large therocephalian (UCMP 42667) that was previously described as a specimen of "Cynariognathus platyrhinus" (a taxon synonymous with Glanosuchus) in 1967 was alluded by palaeontologist Christian Kammerer to be a lycosuchid in a 2023 paper on scylacosaurid therocephalians, though it has yet to be compared in detail with other lycosuchid remains. "Double Canines" Historically, "double canines" in the upper jaw were regarded as the most defining trait of lycosuchids, and sometimes even the sole trait to distinguish them. Unlike other predatory therapsids (such as gorgonopsians and other therocephalians), which only ever have one functioning pair of upper canines at a time, lycosuchids were thought to have two distinct pairs, each with its own tooth socket (alveolus) one immediately behind the other, that were both simultaneously functional and independently replaced. Notably, these other therapsids also have two positions for each canine and the active tooth alternated between the alveoli every time it was replaced. The distinction made for lycosuchids was that two distinct sets teeth supposedly occupied both positions at the same time, rather than a single pair that alternated between them. This was thought to be a primitive characteristic retained by lycosuchids from earlier sphenacodont (or "pelycosaur") ancestors such as Dimetrodon, which were also thought to have two distinct sets of canines (now also known to be incorrect). Though not truly representing two functional pairs of canines as originally thought, the frequency of overlap between the alternating old and replacement canines suggests they were both still functional together, at least to some degree, during the animals' lifetimes. This is unlike other predatory therapsids, where the old and worn canine is shed to be functionally replaced by its alternate. How two concurrent large canines in each upper jaw functioned is not clear, and it has been argued that such "double canines" acting as a single unit would be less efficient at both puncturing and tearing flesh due to their bulk and by obscuring the serrations of the other tooth. ==Taxonomy and classification==

The concept for a family of "double canined" early therocephalians was first put forward by Broom in 1908, when he proposed that the "double canined" early therocephalians recognised at the time (Lycosuchus, Hyaenasuchus and Trochosuchus) formed a separate evolutionary unit from other early therocephalians, then recognised as the "Pristerognathidae" (now known as Scylacosauridae). Broom would continue to be a proponent for this division, but for many decades he did not attribute a name to such a group, even after Lycosuchidae was already made available. The family Lycosuchidae was first established by Baron Franz Nopcsa in 1923, although the name was often misattributed to other authors by later researchers until the end of the 20th century when his precedent was recognised. Other early uses of the name Lycosuchidae were by Samuel W. Williston in his 1925 publication The Osteology of the Reptiles, and a similar concept was used by Sidney H. Haughton and Adrian S. Brink in 1954 catalogue of fossil "reptiles" from the Karoo, though neither of them attributed any authorship to Lycosuchidae. Therein, he also established the taxonomically higher group Lycosuchia (originally "Lycosauria" in his thesis) containing the family, intended to be the equivalent sister group to Scylacosauria, though this higher taxon has not seen use since then. Two genera are considered valid today, Lycosuchus and Simorhinella, each with a single species (monospecific). Both genera were named by Robert Broom, a palaeontologist who worked on and named many Karoo fossils, in the early 20th century (1903 and 1915, respectively), but Simorhinella was not actually identified as a lycosuchid until almost a century after its discovery. Lycosuchus is known from five skulls and mandibles of varying completeness, including the nearly complete holotype. An alternative but functionally identical name for the family, Trochosuchidae, was established by Alfred Romer in 1956, apparently unaware of the pre-existing use of Lycosuchidae by other authors. Romer named the family after Trochosuchus, a now dubious genus of lycosuchid. Curiously, Romer would erect a family for the lycosuchid genera for a second time in 1966, this time as Trochosauridae after the lycosuchid Trochosaurus (also now dubious). Romer likely did this because in 1966 he felt that Trochosuchus was distinct from other lycosuchids and instead assigned it to another family, the Alopecodontidae (a family otherwise made up of what are now scylacosaurids). This is despite the fact that Romer had previously considered the genera Trochosuchus and Trochosaurus synonymous while under Trochosuchidae. Although Lycosuchidae has priority over either name, some authors perpetuated the use of Trochosuchidae and Trochosauridae, including some who used the former well after Romer proposed replacing it with Trochosauridae (e.g. Charles Lewis Camp and colleagues in 1968 and 1972). The dubiousness of all other historically named lycosuchids is in part due to the often poor quality of their type material, which are often incomplete and badly weathered. Another confounding factor is that several were primarily or entirely distinguished based only on features such as the relative proportions of the canines, snout, and number of postcanine teeth—features now known to be individually variable, subject to taphonomic distortion, and associated tooth replacement—as is the case for Trochosuchus, Trochosaurus and Trochorhinus. Not all specimens of Scymnosaurus represent lycosuchids, however, and a third dubious species, S. watsoni, is based on the fossils of an indeterminate scylacosaurid. Nonetheless, a revision of Scylacosauridae in 2023 by Christian F. Kammerer has explicitly defined Scylacosauridae to exclude Lycosuchus as a distinct phylogenetic unit. However, this result was only recovered under the majority rule consensus (i.e. it was not recovered in every iteration of the analysis), and it has only been recovered once since then. Indeed, most subsequent analyses of this dataset (such as Liu and Abdala, 2023, below right cladogram) have found Gorynychus to be placed intermediately between Lycosuchus and Scylacosauria (as would be the suggested position of Simorhinella). Gorynychus was also assigned to Lycosuchidae by Julia Suchkova and Valeriy Golubev in 2018, independently of these phylogenetic results, when they named G. sundyrensis. However, this was based entirely on anatomical similarities and was not supported by any phylogenetic analysis. of the possible lycosuchid Gorynychus with hypothetical fur, note the "double canines" in G. sundyrensis, inferred from its fossils }} ==Evolution and extinction==

Although lycosuchids are believed to have evolutionarily diverged earlier than scylacosaurid therocephalians, the latter appear in the fossil record before lycosuchids do, with representatives from the Eodicynodon Assemblage Zone (AZ), the lowest and oldest biozone of the Beaufort Group. This suggests lycosuchids have a ghost lineage missing from the fossil record that extends at least as far back as them into the Wordian stage of the Middle Permian, roughly between 266.9 and 264.7 million years ago. The fossil record of lycosuchids does not pick up until well into the Tapinocephalus AZ, with the stratigraphically lowest (and therefore oldest) specimens being the holotypes of the dubious Trochorhinus and Trochosaurus intermedius from the locality Abrahamskraal 29 in the Moordenaars Member of the Abrahaamskraal Formation, corresponding to the upper Tapinocephalus AZ (or the Diictodon-Styracocephalus subzone) and so approximately by 262 million years ago. Both Simorhinella and Lycosuchus appear in the upper Tapinocephalus AZ, though Simorhinella appears first and only briefly overlaps with the first appearance of Lycosuchus before it disappears. ==Notes==