Viatkogorgon is a genus of gorgonopsian (a type of therapsid, the group that includes modern mammals) that lived during the Permian period in what is now Russia. The first fossil was found at the Kotelnich locality near the Vyatka River and was made the holotype of the new genus and species V. ivakhnenkoi in 1999. The generic name refers to the river and the related genus Gorgonops—the gorgons of Greek mythology are often referenced in the names of the group. The specific name honors the paleontologist Mikhail F. Ivakhnenko. The holotype skeleton is one of the most complete gorgonopsian specimens known and includes rarely preserved elements such as gastralia (abdominal ribs) and a scleral ring (a bony ring inside the eye). A larger, but poorly preserved specimen has also been assigned to the species.
The holotype specimen is about long, including the long skull, making Viatkogorgon a relatively small gorgonopsian. The assigned specimen is larger, with a long skull, and the holotype may have been young. As a gorgonopsian, it would have been skeletally robust with a somewhat dog-like stance, though with outwards-turned elbows. The snout was high, and the teeth were generally recurved (curved backward), pointed, and serrated. The canines (the saber teeth) were much larger than the incisors at the front and postcanines behind but relatively small for a gorgonopsian. Viatkogorgon was characterized by its unusually large eye socket and scleral ring. It was distinct from other gorgonopsians in that the lower end of the postorbital bar was narrow, and it had a very large sulcus (or furrow) at the back of the skull on each side. Since gorgonopsians have been described mainly from skulls, it is uncertain how widespread the postcranial features of Viatkogorgon were in other members of the group. The skeleton of Viatkogorgon was unusual in having gastralia, in that the tail was differentiated with a front and hind part, with the former less flexible, and in that some of the foot bones and its digits were reduced in size and interconnected.
Gorgonopsians were a group of carnivorous stem mammals with saber teeth that disappeared at the end of the Permian. Phylogenetic analysis has found Viatkogorgon to be one of the earliest-diverging gorgonopsians, after Nochnitsa, also from Kotelnich. All other gorgonopsians appear to belong to two, later-diverging Russian and African groups. Viatkogorgons proportionally large scleral ring suggests nocturnal habits. Gorgonopsians would have been relatively fast predators, killing their prey by delivering slashing bites with their saber teeth. The skeleton of Viatkogorgon had features such as a vertebral column with increased vertical curvature, including the hind part of the tail, and restricted mobility of some digits of the feet, likened to a flipper-like structure indicating it may have been a relatively good swimmer. The age of the Vanyushonki Member of the Kotelnich succession, from which Viatkogorgon is known, is not determined but is thought to date to either the late or middle Permian epoch.
Discovery
In 1999, the paleontologist Leonid P. Tatarinov described and named a new genus and species of gorgonopsian, Viatkogorgon ivakhnenkoi. The holotype specimen (cataloged as PIN 2212/61 in the Paleontological Institute of the Russian Academy of Sciences in Moscow), on which the scientific name is based, was found in 1992 at the Kotelnich locality of Kotelnichsky District, in Russia's Kirov Oblast. The Kotelnich locality contains a series of Permian exposures on the banks of the Vyatka River.
The generic name Viatkogorgon refers to the Vyatka River, and to Gorgonops, the name of a related genus. The name "gorgon", referring to the monstrous hags of Greek mythology, is often used in the generic names of gorgonopsians. The specific name ivakhnenkoi honors the paleontologist Mikhail F. Ivakhnenko. The specific name was spelled V. ivachnenkovi in a 2001 article by the paleontologists Elena G. Kordikova and Albert J. Khlyupin, a spelling which Tatarinov modified slightly in 2004 as V. ivachnenkoi, while some other researchers have continued using the original spelling. Russian gorgonopsid discoveries began in the 1890s with the finding of Inostrancevia, one of the largest members of the group. There were fewer finds during the 20th century, with Viatkogorgon being the first recognized gorgonopsian from Russia, the only place outside Africa where the group is definitely known, since 1974.
The holotype skeleton is one of the most complete gorgonopsian specimens known (a 2023 study called it the most complete), and preserves almost the entire postcranial skeleton (the rest of the skeleton apart from the skull). This includes elements rarely preserved intact in therapsids (the group in which gorgonopsians belong), such as the gastralia (or abdominal ribs, a feature found among reptiles, for example). The skull is relatively poorly preserved in comparison, with the left side and the palate badly broken. The skull roof is reconstructed in plaster, except for the snout tip and left postorbital bone (the bone that formed the hind border of the eye socket). Overall, the skull is also compressed from side to side, which makes it narrower than it would have been in life when viewed from above. The scleral rings (bony rings inside the eye), particularly the right one, are preserved, which is otherwise uncommonly preserved in theriodont (the sub-group of therapsids to which gorgonopsians belong) fossils. The proatlas, atlas and axis (the first bones of the vertebral column) are attached to the skull.
Tatarinov only described the skull of Viatkogorgon in the 1999 article, wherein he also named the new scylacosaur genus Kotelcephalon, because the article was restricted in volume, but he preliminarily described the postcranium in 2004. In 2018, the paleontologists Christian F. Kammerer and Vladimir Masyutin redescribed the skull of Viatkogorgon and stated that a detailed description of the postcranium would greatly improve understanding of gorgonopsian skeletal anatomy, but noted it was unavailable for study during their research, as it was part of a traveling exhibition. They also named the new gorgonopsian Nochnitsa from Kotelnich in the article. In 2002, Ivakhnenko reported an additional, larger Viatkogorgon specimen (cataloged as PIN 4678/5), which is very poorly preserved.
Description
The almost complete Viatkogorgon holotype specimen is about long, including the skull. The skull is , the preserved part of the tail is approximately , the forelimb is approximately , and the hindlimb is more than . It was relatively small for a gorgonopsian; for comparison, Inostrancevia and Rubidgea measured at least in length. Ivakhnenko suggested in 2002 that the holotype specimen was possibly a young individual, based on it having a narrow postorbital bar; the width of this bar widened with age in Estemmenosuchus, for example. For comparison, the skull of the larger assigned specimen is long, and its postorbital bar is wide, while that of the holotype is . The skull length was therefore increased 1.3-fold, and the width of the postorbital bar was increased by a factor of 1.6. Gorgonopsians were skeletally robust, yet long-limbed for therapsids, with a somewhat dog-like stance, though with outwards-turned elbows. It is unknown whether non-mammaliaform therapsids such as gorgonopsians were covered in hair or not.
Skull
The snout of Viatkogorgon was high and narrow, though much of the skull's narrowness was due to sideways compression of the fossil. The premaxilla (the frontmost bone of the upper jaw) was little exposed on the side surface of the snout, as in Nochnitsa. The internarial bar (which divided the nostrils) curved slightly hindward in side view, so that the lower front margin of the snout was blunt rather than pointed. The upper tip of the premaxilla extended to the upper front edge of the bony nostril. Few of the upper incisors (front teeth) are preserved intact in the holotype, but the partial tooth roots and tooth sockets show it would have had five incisors on each side, as is typical for gorgonopsians. The upper incisors were weakly curved and spatulate (spoon-shaped), with clear serrations towards their tips. It is uncertain if they decreased in size further back in the tooth-row, because the holotype's only two intact incisors are the same tooth of each side (I5). In general, the teeth were pointed, flattened on the outer side, and somewhat convex on the inner side, and the serrated area of each tooth was displaced towards the outer edge of the crowns. The septomaxilla (a small bone between the nasal bone and the maxilla, the main bone of the upper jaw) had a shorter side facing process than Nochnitsa.
The maxilla was proportionally taller and shorter from front to back than in Nochnitsa, and whereas the upper margin of the latter's maxilla was rounded, there was a broad process that extended hindward between the nasal and lacrimal bone (a bone that formed the front border of the eye socket) in Viatkogorgon. The hindward directed process that formed the lower margin of the maxilla was shorter than in Nochnitsa, not reaching the midpoint of the eye socket. The "step" by the diastema (gap) between the incisors and the canine (the saber-tooth) was deeper than in Nochnitsa, and the part of the maxilla that bore the canine was very convex, which gave the impression of a "flange" when seen in side view. The canine was relatively small for a gorgonopsian, as was the case in Nochnitsa, and bore serrations on its hind edge. The tooth row behind the canine was short and consisted of four closely packed postcanines, which were recurved (curved backwards), unlike those of Nochnitsa. The postcanines were substantially shorter than the incisors, a regular feature of gorgonopsians, and almost three times shorter than the canines. There was a weak margin above the postcanine tooth-row on the maxilla, to a larger extent than Nochnitsa, but less than in the gorgonopsian Eriphostoma and rubidgeines.
The nasal bone of Viatkogorgon was somewhat broader at the front (by the hind edge of the bony nostril) than in Nochnitsa. The prefrontal bone (at the forehead, above and in front of the eye) was proportionally shorter than in Nochnitsa and contributed less to the upper front margin of the eye socket, where the lacrimal bone formed a larger part instead. Viatkogorgon was characterized by its unusually large eye socket with its proportionally large scleral ring. The diameter of the eye socket was , while the outer diameter of the scleral ring was , and its inner diameter . The ring consisted of 15 ossicles (small bones also termed plates) which overlapped each other with no gaps, and the rims of the ring were regularly rounded. The jugal bone, which formed the lower border of the eye socket of Viatkogorgon, formed a more extensive part of the side of the face than in Nochnitsa.
Viatkogorgon was distinct from all gorgonopsians except Nochnitsa in that the lower end of the postorbital bar (between the eye socket and the temporal fenestra opening behind the eye socket) was narrow, forming a straight rod when seen in side view. It was broader and lacked the curvature seen in the bar of Nochnitsa. In other gorgonopsians, the lower end of the postorbital bar was expanded where it contacted the jugal bone, even if the rest of the upper length of this bar was narrow. The skull of Viatkogorgon was also distinct in having a very large sulcus (or furrow) on the squamosal bone on each side at the back of the skull, which extended onto a squamosal flange which impinged on the lower edge of the temporal fenestra. Apart from being much larger and expansive than the squamosal sulcus of Nochnitsa, this feature had a distinct boundary and a flange that pointed back and down in both.