Simorhinella (meaning "flattened snout") is a genus of large early therocephalian (an extinct type of therapsid, the group that modern mammals belong to) that lived roughly 262–260 million years ago, during the Middle Permian period, from what is now South Africa. The type and only species is S. baini, first described in 1915 by Robert Broom from a partial juvenile skull discovered during the 19th century at Prince Albert, in Western Cape. The skull being unusual with his extremely short and broad snout, unlike that of other early therocephalians, its systematic position within the group consequently remained uncertain for much of the 20th and early 21st centuries. In 2014, the discovery of a large skull at Victoria West, Northern Cape, identified as belonging to an adult individual of this genus, particularly due to a distinctive bony crest on the vomer of the palate found in both specimens, suggested that Simorhinella may be closely related to the basal therocephalian Lycosuchus. It is therefore assigned to the family Lycosuchidae, though its precise evolutionary relationships remain untested. The adult skull specimen reach approximately 37 cm (15 in) in length, placing Simorhinella among the largest therocephalians described to date, although its size is exceeded by that of another indeterminate lycosuchid specimen. The taxon is known from the upper levels of the Abrahamskraal Formation, more specifically within the Tapinocephalus Assemblage Zone. Unlike its contemporary Lycosuchus, Simorhinella disappeared during the mass extinction at the end of the Middle Permian, which led to the decline of most large therocephalians (including many scylacosaurids), as well as large carnivorous therapsids such as the dinocephalian Anteosaurus.
History of discovery At an indeterminate point during the 19th century, the road engineer and geologist Thomas Charles John Bain discovered a partial fossil skull at the Weltevreden farm in the Prince Albert district of Western Cape Province, South Africa. This specimen was subsequently acquired in 1878 by the British Museum (later the Natural History Museum, London), where it has since been catalogued under the number NHMUK (formerly BM) 49422. The fossil remained undescribed until a 1915 study, in which the palaeontologist Robert Broom examined the museum's collection of carnivorous therapsids. He then regarded it as representing a new genus and species of therocephalian, which he named Simorhinella baini. Although Broom's description of this taxon was brief, he correctly identified the skull as belonging to a young juvenile individual. The generic name Simorhinella is derived from the Ancient Greek σιμός (simós, "flattened nose") and ῥίς (rhís, "snout"), combined with the Latin diminutive suffix -ella, together meaning "little flattened snout", in a clear reference to its cranial morphology. The specific name baini honours Bain for his contribution to the discovery of several fossil specimens described by Broom in his publication. NHMUK 49422 comprises the front half of a skull and lower jaws up to and including the eye sockets. The specimen is weathered and its surfaces are cracked, such that Broom struggled to distinguish the sutures of individual bones. Nonetheless, Broom highlighted the unusual breadth and shortness of the snout, being broader across the eyes than it is long. Broom allied Simorhinella with other small therocephalians such as Ictidognathus and Scaloposaurus, which at the time were classified under the now defunct grouping Scaloposauridae (sometimes known as Scaloposauria)—an artificial collection of small therocephalians often named from juvenile specimens, like Simorhinella.
Little was written about Simorhinella in the decades following its description. This was in part because the specimen was incomplete and missing key parts of the skull (such as the intertemporal region) that could indicate its relationship to other therocephalians, as Lieuwe Dirk Boonstra noted in 1954, for example. Indeed, Boonstra would later regard Simorhinella as a nomen dubium (a doubtfully valid scientific name) in 1969 due to its juvenile nature. Growing doubts over the validity of Scaloposauridae and its genera led to further complications, but some authors, namely Christiane Mendrez in 1975, maintained the genus was distinct and upheld Simorhinella as a valid taxon. This uncertainty over its classification persisted into the early 21st century, even with advancements in understanding therocephalian systematics. In 2014, Fernando Abdala and colleagues described a large skull and a few other associated bones belonging to a lycosuchid therocephalian that was collected from near farm Rheboksfontein 74 in the Victoria West district of Northern Cape Province. Catalogued as BP/1/5592, it was discovered by palaeontologists John Nyaphuli and Bruce S. Rubidge, and is housed at the Evolutionary Studies Institute (formerly the Bernard Price (BP) Institute) of the University of the Witwatersrand in Johannesburg, South Africa. Abdala and colleagues identified BP/1/5592 as an adult specimen of Simorhinella, despite the marked size difference between them (compare its 182 mm (7.2 in) long snout to just 21 mm (0.83 in) in the holotype). Abdala and colleagues found that the two specimens share a unique suite of characteristics suggesting they are adult and juvenile of the same taxon, and that Simorhinella was a valid taxon of lycosuchid. BP/1/5592 comprises a weathered skull missing the lower jaws and most of its teeth, with a few other bones from the postcranial skeleton. The skull of BP/1/5592 has suffered some dorso-ventral compression (from top to bottom), displacing some of the upper surface of the skull backwards relative to the base. Although the two specimens come from widely separated localities, they were both discovered in regions of the southwestern part of the Karoo Basin that expose layers of rock (strata) belonging to the upper Tapinocephalus Assemblage Zone (AZ) of the Abrahamskraal Formation, one of several geological formations that make up the Beaufort Group in the Karoo Basin. In 2020, the upper strata of the Tapinocephalus AZ (encompassing the stratigraphic range of Simorhinella) was formally defined as the Diictodon-Styracocephalus Subzone, which has been constrained to date between 262 and 260 million years ago during the late Capitanian stage of the Guadalupian (or middle Permian).
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