Viatkogorgon

Viatkogorgon Temporal range: Ma PreꞒ Ꞓ O S D C P T J K Pg N
Holotype skeleton exhibited in Dinosaurium, Prague
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Synapsida
Clade:	Therapsida
Clade:	†Gorgonopsia
Genus:	†Viatkogorgon Tatarinov, 1999
Type species
Viatkogorgon ivachnenkoi Tatarinov, 1999
Synonyms
Viatkogorgon ivakhnenkoi Tatarinov, 1999

Viatkogorgon is a

The holotype specimen is about 80 cm (31 in) long, including the 14 cm (5.5 in) long skull, making Viatkogorgon a relatively small gorgonopsian. The assigned specimen is larger, with a 17 cm (6.7 in) 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

Gorgonopsians were a group of

The almost complete Viatkogorgon holotype specimen is about 80 cm (31 in) long, including the skull. The skull is 14 cm (5.5 in), the preserved part of the tail is approximately 17 cm (6.7 in), the forelimb is approximately 24 cm (9.4 in), and the hindlimb is more than 26.5 cm (10.4 in).

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 sclerotic ring. The diameter of the eye socket was 2.8 cm (1.1 in), while the outer diameter of the sclerotic ring was 2.3 cm (0.91 in), and its inner diameter 1.5 cm (0.59 in). 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.^[5][2]

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.^[5]

The palatal structure of Viatkogorgon was typical for gorgonopsians. It had teeth on the palatine bone (bony palate) placed on stout tubercles (also called bosses,^[5] with 15–18 teeth on each), which formed two rows that extended along the outer and inner edges of each tubercle. Each tooth row formed two lines, with the outer line being longer than the inner. The surface of the tubercle was weakly concave and smooth between the teeth. There were also tooth-bearing tubercles on the pterygoid bone behind the palatine bone, though less developed, and the teeth here (12–13 on each) were similar to those of Sauroctonus. They were much smaller than the palatine teeth, covered the entire surface of the tubercles, and formed three rows. There were also teeth on the transverse processes of the pterygoid, which formed only one row mainly on the inner part, unlike in Sauroctonus. Unlike the teeth on the tubercles, these teeth were strongly worn and probably not replaced in adults.^[2]

The lower jaw of Viatkogorgon was typical for gorgonopsians, unlike that of Nochnitsa, with a tall

Since gorgonopsians have been described mainly from remains of their skulls, with scant known postcranial remains, it is uncertain how widespread the postcranial features of Viatkogorgon were in other members of the group. Viatkogorgon had seven

The thoracic vertebrae of Viatkogorgon were somewhat shorter than the lumbar vertebrae, and their neural spines were moderately tall, with horizontal zygapophyses. The neural spines became somewhat taller beginning at the second third part of the thoracic region, and were vertical in side view, though in the hind part of this region they were inclined rearwards and their front edge became convex (showing the transition from thoracic to lumbar vertebrae). The front ten or eleven thoracic ribs were very long and directed back and down. They were closely adjoined and attached at the front edge of the centrum by two closely positioned articular heads, as in the other ribs. The hindmost four or five thoracic rib pairs shortened gradually, transitioning into the lumbar ribs, whose articular heads got closer together. The sternum (breast bone) was 13 mm (0.51 in) long, shaped like a rectangular plate, and had three costal processes on each side, similar to Aelurognathus. The gastralia of Viatkogorgon formed an unusual, latticed frame of segmentally arranged narrow bars, located under the frontmost thoracic ribs. The bars were arranged in two layers at an angle to each other, with at least thirteen bars extended frontwards and down at the deeper layer. The four at the upper layer continued the thoracic vertebrae and extended hindward and down. The left bars overlay the lower ends of the right bars in some cases, and these elements were much thinner and denser spaced than ribs. No element connecting the right and left ribs has been identified.^[3]

The lumbar region was slightly differentiated from the thoracic region in having shorter ribs. The first and second lumbar ribs curved slightly hindward, were directed almost to the sides in the third, and curved slightly forwards in the hindmost two vertebrae. The lumbar ribs appear to have been single headed, unlike the thoracic ribs. The lumbar vertebrae were massive, longer than the thoracic ones, and had horizontally positioned zygagophyses and somewhat thickened neural spines, whose front edges narrowed towards the top. The three frontmost lumbar vertebrae had horizontal diapophyses (processes which projected from the sides of the

The tail region of Viatkogorgon was clearly differentiated into a front and hind part, the front part consisting of five caudal vertebrae which together measured 5.7–5.8 cm (2.2–2.3 in) in length. The ribs were short and massive in this part of the tail region. The zygapophyses were unusually inclined upwards, with their hind edges raised in relation to their front edges. The position of the zygapophyses would have restricted sideways movement at the base of the tail. The zygapophyses of the fourth and fifth caudal vertebrae were less inclined. The ribs of two of the front caudal vertebrae were 12 mm (0.47 in) long, whereas they shortened abruptly so that the one of the fifth vertebra was only 4–5 mm (0.16–0.20 in) long. All the ribs here were double-headed, and all the neural spines were relatively massive, though they abruptly decreased in length hindward. The hind part of the tail consisted of 15 vertebrae, which lacked caudal ribs. In at least four of these vertebrae, the zygapophyses were almost raised to the extent of those of the vertebrae that adjoined their front, while they were positioned almost horizontally towards the back. The neural spines here were low and narrow, and the space between the vertebrae had well-developed hypapophyses (processes that project down from the vertebrae) that did not taper. The hypapophyses were inclined hindward and adjoined two vertebrae at the front, and covered the hypapophyses hindward adjoining vertebrae from below.[3]

Limbs and limb girdles

The scapula (shoulder blade) was 7 cm (2.8 in) long, nearly 1 cm (0.39 in) wide at the joint with the

The lower

The femur was proportionally long, 10.3 cm (4.1 in), 70% of the skull's length. It was more massive than that of Lycaenops, and curved along its long axis, to the same extent as in Sauroctonus. The head of the femur was round, about 12 mm (0.47 in) in diameter, directed inward at about a 30-degree angle, and its surface was well-built. The femoral head narrowed towards its back, and it likely entered the hind part of the acetabulum. The greater trochanter descended to about the femur's mid-length. Two long crests extended along the bone, above and below the front of the greater trochanter. The first was probably the inner trochanter, which was typically present in gorgonopsians, but the other has not been identified previously in the group. The lower leg was shorter than the femur, 9 cm (3.5 in) at most, and its surface sculpturing was more pronounced than in other gorgonopsians, resembling cynodonts. The tibia (lower leg bone) of Viatkogorgon had structures not identified in other gorgonopsians, such as a flattened surface on the preaxial epicondyle, and a crest at the upper half back of the bone that bordered a broad area. The fibula (lower leg bone) was more massive than those of other gorgonopsians and had a massive sculptured crest at its middle; this surface sculpturing diminished towards the lower end of the fibula, and the crest abruptly decreased in height, features not seen in other gorgonopsians.^[3]

Viatkogorgon's foot was 7.5 cm (3.0 in) long, and its general structure was similar to those of other gorgonopsians, though some features were notable. The proximal row of bones towards the heel consisted of only two bones as in mammals—a massive astragalus and a fibulare. In the distal row at the front of the foot, the fourth and fifth tarsals were fused, as in other gorgonopsians, but the row of intermediate centralia in Viatkogorgon was unusually well-developed; gorgonopsians usually had only one centrale, whereas Viatkogorgon had an additional one. Another unusual feature was that the articular facets of the hindmost foot bones extended onto the front side of the fibula and tibia of the lower leg. The tarsus (the hindmost cluster of bones in the foot) was 2.5 cm (0.98 in) long, and the metatarsal bones (those between the tarsus and the digits) ranged from 0.8 cm (0.31 in), the first, to 2.1 cm (0.83 in), the fourth.^[3]

The foot's digits were short, the fourth being 2.5 cm (0.98 in), a little longer than the third. The digits varied slightly in length, with the second and fifth digits being only slightly shorter than the third and fourth. The shortening of the metatarsals added to the impression that the digits were short. The first and fifth digits were somewhat hooked; the first was relatively short, about 1.8 cm (0.71 in) long. The phalangeal formula (the number and distribution of phalanx bones in the digits) of gorgonopsians was similar to that of mammals, while the study of Viatkogorgon indicates there was a reduction in phalanges (leading to phalanges becoming disk-like) combined with fusion of them. In the first digit of the foot, two phalanges were almost completely fused. In the third digit the second and third phalanges were shortened and fused. The fourth digit had three phalanges that were shortened, almost disk-like, and fused. Therefore, the phalangeal formula was 1-2, 3, 3-4, 3-5, 3. There was contact between the metatarsals towards the back of the foot, as well as contacts between some metatarsals and phalanges and contacts between the bases of the second and third digits.

Gorgonopsians were a group of carnivorous therapsids that included the

In his 1999 description, Tatarinov found Viatkogorgon to belong in the gorgonopsian subfamily Sycosaurinae within the family Gorgonopsidae, based on features such as small size and the narrowness of the snout. This subfamily was not previously known from Russia. He also noted some similarities to members of the family Rubidgeidae.^[2] Ivakhnenko considered Sycosaurinae, including Viatkogorgon, to be part of Rubidgeidae in 2002.^[9] In his 2004 description of the postcranial skeleton, Tatarinov cautioned that this provided little information about the taxonomic position of Viatkogorgon among gorgonopsians—some features being unique and others occurring in other members of the group. The lack of information resulted from only a few gorgonopsians having had their skeletons examined. Nevertheless, he found the structure of its feet similar to gorgonopsians such as Arctognathus and Aelurognathus.^[3]

In 2018, Kammerer and Masyutin stated that while the early evolution of Gorgonopsia is poorly understood, Viatkogorgon and Nochnitsa expand the knowledge of gorgonopsians from the middle Permian or earliest late Permian of

The following cladogram showing the position of Viatkogorgon within Gorgonopsia follows Kammerer and Masyutin, 2018:^[5]

In contrast, previous analyses had not found gorgonopsians to be grouped geographically, with some studies placing Russian genera such as Inostrancevia in African families. Previously, it had not been suspected that different gorgonopsian groups were

In 2004, Tatarinov interpreted the behavior of Viatkogorgon based on its skeletal features, which, while being generally similar to those of other gorgonopsians, had certain features that are unique or poorly understood. These concerned features of the locomotor apparatus in particular, which indicated a stage of swimming adaptations, while other features were consistent with those commonly seen in its group. Like other gorgonopsians, it had a long lumbar region of the vertebrae, of which the hindmost vertebrae showed an increased capability for vertical curvature, which was also the case for the neck vertebrae and, unusually, the hind part of the tail region.^[3]

The presence of well-developed gastralia was one of its most unusual features among theriodonts, as were the hypapophyses of the tail region. In addition, the broad and somewhat shortened foot with unusual intermetatarsal contacts, which restricted the mobility of its individual digits, indicated transformation of the foot into a flipper-like structure. Tatarinov hypothesized these features to be adaptations for swimming; while Viatkogorgon was not a specialized aquatic predator, Tatarinov suggested its tail and feet enabled it to swim better than most other gorgonopsians. In particular, he thought the hind part of the tail was probably used for swimming, as is the case with reptiles with long tails that have hypapohyses, like Mosasaurus. Tatarinov noted that since the claw on the hindlimb's fifth digit was hooked and somewhat lengthened, it could have been used for protection and grooming, as in modern monotremes. The first digit could have had this function to a lesser degree, as it was free from contacting foot bones. The forelimb of Viatkogorgon was less specialized and could have performed more universal functions, as it lacked contacts between the digits.^[3]

Antón stated in 2013 that while the post-cranial skeletons of gorgonopsians were basically reptilian, their stance was far more upright than in more primitive synapsids, like pelycosaurs, which were more sprawling. Regular locomotion of gorgonopsians would have been similar to the "high walks" seen in crocodilians, wherein the belly is carried above the ground, with the feet pointing forwards, and the limbs carried under the trunk instead of to the sides. The forelimbs had a more horizontal posture than the hindlimbs, with the elbows pointing outwards during movement, but the gait of the hindlimbs would have resembled that of mammals. As in reptiles, the tail muscles (such as the caudofemoralis) were important in flexion of the hindlimb, whereas the tails of mammals are merely for balance. Their feet were probably plantigrade (where the soles were placed flat on the ground), though they were likely more swift and agile than their prey. Their feet were more symmetrical compared to the reptilian condition, making contact with the ground more efficient, similar to running mammals.^[16]

Paleoenvironment

Bibliography

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Viatkogorgon

Wikispecies has information related to Viatkogorgon.

• Antón, Mauricio (2013). Sabertooth. Bloomington:
  OCLC 857070029. Archived
  from the original on 31 July 2021. Retrieved 14 September 2021.