Sauroctonus

Diagram of PIN 156/5, the holotype skull of S. progressus

In 1938, the Soviet paleontologist Aleksandra Paulinovna Hartmann-Weinberg published an article which described fossil remains of gorgonopsians having been discovered in Tatarstan, in the present-day Russia. One of the fossils described in his paper is an incomplete but well-preserved skeleton, cataloged PIN 156/51, which includes the skull, dorsal and cervical vertebrae, ribs, cervical rib, shoulder girdle, humerus, and radius. Hartmann-Weinberg considers this specimen to be the holotype of a new species of the genus Arctognathus, designating it as Arctognathus progressus. The specific epithet progressus is named so because of the species' morphological closeness to the cynodonts, which have a large number of progressive and more mammal-like features than other gorgonopsians.[1] Two years after the publication of the article by Hartmann-Weinberg, in 1940, Ivan Yefremov reclassified the taxon as a new species of the genus Inostrancevia, renaming it as Inostrancevia progressus.[2] In 1955, after the analysis of the material, Alexey Bystrow proposed a separate genus for the species, which he named Sauroctonus,[3] a proposal that will be quickly recognized later.[4][5][6][2][7][8][9] The genus name Sauroctonus comes from the Ancient Greek σαῦρος (saûros, "lizard") and κτόνος / (któnos, "murderer" or "killer"), to literally give "lizard killer", in reference to its carnivorous diet.[3]

The specimen GPIT/RE/7113, previously assigned to a second species of the genus Sauroctonus

All confirmed remains of Sauroctonus are only recorded in Russian territory and belong to the species S. progressus, mainly represented by isolated and fragmentary skulls.[8] In 1950, a relatively complete skeleton, discovered in the Usili Formation in Tanzania, cataloged GPIT/RE/7113, was described by the German paleontologist Friedrich von Huene as a representative of the species Scymnognathus parringtoni, being named in honor of Francis Rex Parrington.[10] Today, the genus Scymnognathus is considered as a junior synonym of Gorgonops. In 1970, the skeleton was reidentified by the French paleontologist Denise Sigogneau-Russell as a specimen of Aelurognathus, being renamed Aelurognathus parringtoni.[4] It is from 2007 that the paleontologist Eva V. I. Gebauer reclassifies it as an African representative of the genus Sauroctonus,[6] a claim that she would confirm again in 2014.[7] However, this affiliation is formally rejected in 2018, as new classifications based on cranial anatomical traits show that derived gorgonopsians are separated into two clade of Russian and African origin, leaving GPIT/RE/7113 as an incertae sedis within the latter group.[8]

The skull size of Sauroctonus is 22.5 cm long, indicating that it should be a medium-sized gorgonopsian. The skull is narrow posteriorly with small orbits. The temporal fossa is elongated, the cranial arches are narrow, and the dentary bone is moderately high.[11] Sauroctonus also has high palatal tuberosities that have many teeth, the bones poorly sculpted, and incisors only slightly larger than the postcanine teeth. There are also 4 to 6 postcanine teeth on the maxillary bones.[12][2]

Life restoration of S. progressus

The taxonomic position of Sauroctonus has changed many times over the decades. In 1974, the Russian paleontologist Leonid Petrovich Tatarinov classified Sauroctonus within the family Gorgonopsidae, and in the subfamily Cynariopinae, alongside various African genera such as Cynariops, Scylacognathus or Scylacops.[5] In a book published in 1989, Sigogneau-Russell leaves Sauroctonus as a gorgonopsian whose placement is undetermined.[11] Mikhail Ivakhnenko still classifies Sauroctonus within the Gorgonopsidae in 2003, without assigning it to any subtaxon.[12] In his 2007 thesis, Gebauer considers Sauroctonus to be a basal Gorgonopsidae.[6] These repetitive classification changes have left Sauroctonus in an uncertain position among the gorgonopsians, with some seeing it as close to the African genera, while others consider it to belong to a lineage in its own right.[8]

It is from 2018 that paleontologists Christian Kammerer and Vladimir Masyutin definitively reclassify Sauroctonus in a group of Russian gorgonopsians, alongside the genera Inostrancevia, Pravoslavlevia and Suchogorgon, in particular for certain shared cranial characteristics, more precisely for close contact between the pterygoid and the vomer.[8] This classification will be immediately followed by the next studies.[9][13]

The following cladogram showing the position of Sauroctonus within Gorgonopsia follows Kammerer and Rubidge, 2022:[13]

S. progressus is known from the locality of Sjomin Ravine, located in the republic of Tatarstan, in the northeast of European Russia. This fossil site, discovered in 1938 and dating from the end of the Middle Permian (Capitanian), is mainly represented by clays and silts with separated layers of fine polymictic sandstone, which contains fossils of tetrapods, represented by isolated bones and fragmentary skeletons, as well as rare flora remains.[14]

Apart from S. progressus, the main tetrapods identified in the area include the dicynodont therapsid Idelesaurus, the reptiliomorph Chroniosuchus, the temnospondyl Dvinosaurus, and even sauropsids, including the pareiasaur Scutosaurus and the archosauriform Eorasaurus.[14]

• List of therapsids