Gorgonopsia

'' (reconstructed with long lips and some hair) Earlier gorgonopsids in the Middle Permian were quite small, with skull lengths of , Nonetheless, small gorgonopsians remained abundant until extinction (though small species may actually represent juvenile specimens of other taxa). Like other Permian therapsids, gorgonopsians had developed several mammalian characteristics. These might have included a parasagittal gait (the limbs were vertically oriented and moved parallel to the spine) as opposed to the sprawling gait of amphibians and earlier synapsids. This gait change in therapsids was possibly related to the reduction in tail size and phalangeal formula). Other developments included fibrous lamellar cortical bone and deeply-set teeth. Many species are distinguished by vague proportional differences, and consequently smaller species may actually represent juveniles of larger taxa. Notably, the vomer at the tip of the snout varies among species in terms of the degree of its expansion, as well as the positions, degree of splay, and shape of the 3 ridges. Teeth '' showing tooth arrangement, dual canines, and canine root depth Like many mammals, gorgonopsians were heterodonts, with clearly defined incisors, canines, and postcanine teeth homologous with premolars and molars. The serration pattern of gorgonopsians was most similar to those of theropod dinosaurs than to other synapsids. The palate also features tuberosities and ridges which oftentimes have functional teeth, which may have been used to hold onto struggling prey, diverting these powerful forces away from the fragile canines. Similar ridges have been identified on the machairodont Homotherium. Like some therapsids, while there was one functional canine, another canine was growing to replace it when it inevitably broke off. The left and right sides of the jaws did not have to be synchronous, so, for example, the first canine on the left side could be functional while the first canine on the right side was still growing. The joint between the ankle and the heel bones may have been somewhat mobile. The fifth digit for both the hands and feet was not attached to the carpus/tarsus, and instead connected directly to the ulna/heel bone. ==Taxonomy==

Fossil bearing sites '' In 1876, the first gorgonopsian remains were identified in the Beaufort Group of the Karoo Supergroup of South Africa, by the biologist and paleontologist Richard Owen. He classified the fossils as Gorgonops torvus, combining the Greek Gorgon, a mythological beast, with the word (), meaning 'aspect'. In 1979, Chinese palaeontologist Yang Zhongjian described a Chinese gorgonopsian "Wangwusaurus tayuensis" based on teeth from the Late Permian Jiyuan Formation, but in 1981, palaeontologists Denise Sigogneau-Russell and Ai-Lin Sun found the assigned material to be a random assemblage of which only two have even a remote similarity to Gorgonopsia. In 2022, paleontologists Jun Liu and Wan Yiang confirmed the presence of a gorgonopsian in China based on an incomplete set of fossil teeth discovered in the Turpan Depression, Xinjiang. In 2003, Indian palaeontologists Sanghamitra Ray and Saswati Bandyopadhyay assigned some skull fragments from the Late Permian Kundaram Formation of the Pranhita–Godavari Basin to a medium-sized gorgonopsian, though the gorgonopsian characteristics have also been documented in some therocephalians. A second low-latitude gorgonopsian was described in 2024 from the Port des Canonge Formation of Mallorca in the western Mediterranean. Notably, it also likely represents the oldest gorgonopsian yet known in the fossil record, constrained to a minimum age before the middle Wordian, if not pre-Wordian. This suggests that the absence of gorgonopsians from low-latitudes reflects uneven sampling rather than a true restriction to high latitudes. Classification Upon discovery, Owen presumed that Gorgonops and several other taxa he described from the Karoo Supergroup were cold-blooded reptiles, despite bearing teeth resembling those of carnivorous mammals. He proposed classifying all of them under the newly coined order Theriodontia (which he placed in the class Reptilia). He decided to subdivide Theriodontia into families based on the anatomy of the nostrils (the bony narials)—"Mononarialia" for those with one opening in the skull for the nose as in mammals, "Binarialia" for those with two openings as in reptiles, and "Tectinarialia" for Gorgonops because its opening was overshadowed by a thick bone roof (tectus is Latin for "covered, roofed, decked"). In 1890, English naturalist Richard Lydekker made Gorgonops the type species of the family Gorgonopsidae. British palaeontologist Harry Seeley in 1894 believed Gorgonops lacked an opening in the temporal bone (temporal fenestra), which is a diagnostic feature of Theriodontia, and so elevated Gorgonopsidae to Gorgonopsia, distinct from Theriodontia. He classified all South African materials bearing both reptilian and mammalian traits into the order "Theriosuchia", and considered Gorgonopsia and Theriodontia suborders of it. American palaeontologist Henry Fairfield Osborn completely reworked the classification of Reptilia in 1903, and erected two major groups: Diapsida and Synapsida, and in 1905, South African palaeontologist Robert Broom created a third group, Therapsida, to house the "mammal-like reptiles", including Theriodontia. He also challenged Seeley's claim and relegated Gorgonops back to Theriodontia, but he placed it into his newly erected subgroup Therocephalia, dissolving Gorgonopsia. In 1913, especially in light of an almost complete G. torvus skull discovered by the Reverend John H. Whaits, Broom reinstated Gorgonopsia. The number of South African genera rapidly grew in the 20th-century, headed principally by Broom, whose extensive work on the Karoo therapsids—from the beginning of his career in the country in 1897 to his death in 1951—led to his description of 57 gorgonopsian holotype specimens and 29 genera. Many of Broom's taxa would later be invalidated. Many other contemporary workers created wholly new species or genera based on single specimens. Among the first attempts to organise the clade was carried out by British zoologist David Meredith Seares Watson and American palaeontologist Alfred Romer in 1956, who split it into twenty families, of which the members of three (Burnetiidae, Hipposauridae, and Phthinosuchidae) are not considered gorgonopsians anymore. Sigogneau-Russell split Gorgonopidae into three subfamilies—Gorgonopsinae, Rubidgeinae, and Inostranceviinae—and reduced the number of genera to twenty-three. In 2002, Russian palaeontologist Mikhail Feodosievich Ivakhnenko, considering the Russian taxa, instead considered Gorgonopsia a suborder, and grouped it together with Dinocephalia into the order "Gorgodontia". He divided Gorgonopsia into the superfamilies "Gorgonopioidea" (families Gorgonopidae, Cyonosauridae, and Galesuchidae) and "Rubidgeoidea" (Rubidgeidae, Phtinosuchidae, and Inostranceviidae). In 2007, biologist Eva V. I. Gebauer, in her comprehensive review of Gorgonopsia (her PhD dissertation), rejected Ivakhnenko's model in favour of Sigogneau-Russell's, and further reduced the number of genera to fourteen in addition to the Russian genera: Aloposaurus, Cyonosaurus, Aelurosaurus, Sauroctonus, Scylacognathus, Eoarctops, Gorgonops, Njalila (formally described in 2026), Lycaenops, Arctognathus, Aelurognathus, Sycosaurus, Clelandina, and Rubidgea. and sunk Scylacognathus and the next year Eoarctops into it. }} Evolution Synapsida has traditionally been split into the basal "Pelycosauria" and the derived Therapsida. The former comprises cold-blooded creatures with a sprawling gait and presumably lower metabolism which evolved in the Upper Carboniferous. Through the middle to late 20th-century, American palaeontologist Everett C. Olson investigated synapsid diversity in the Middle Permian San Angelo, Flowerpot, and Chickasha Formations in North America, and noted that pelycosaur diversity reduced from six to three in these formations, and that they coexisted with several fragmentary specimens which he interpreted as therapsids. He then suggested the adaptive shift from pelycosaur-grade to therapsid-grade took place during the Middle Permian (Olson's Extinction); however, the classification of those "therapsids" and the age of the formations have since been challenged. Thus, the exact timing of the therapsid takeover is unclear, but the six major therapsid groups (Biarmosuchia, Dinocephalia, Anomodontia, Gorgonopsia, Therocephalia, and Cynodontia) had evolved by 265 million years ago during the Wordian. of Mallorca, currently the oldest record of the group The oldest definitive gorgonopsian fossil worldwide comes from the Port des Canonge Formation of Mallorca in the western Mediterranean. The rocks of this formation are imprecisely dated, however magnetostratigraphy, palynology and ichnology constrain its age to the earliest Wordian at minimum. The stratigraphic position of the gorgonopsian itself and surrounding fossil track record may suggest it is significantly older, potentially Roadian of the early Guadalupian (Middle Permian) or even of the Kungurian or Artinskian stages of the Cisuralian (Early Permian). Such an age would not only make it the oldest gorgonopsian yet known, but the oldest therapsid altogether. Although of indeterminable species, the anatomy of the Mallorcan gorgonopsian suggests it is more derived than the earliest-diverging recognised gorgonopsian, Nochnitsa, and is consistent with a position in or just outside of the base of the "African" and "Russian" clades. This suggests that not only were gorgonopsians present in the earliest Wordian, if not the late Cisuralian, but that their diversification was well underway by this time and that the clade as a whole likely originated before this interval. Furthermore, it suggests that early gorgonopsian (and indeed therapsid) diversity was not restricted to high-latitudes of temperate Pangaea, as previously suggested from literal readings of the fossil record, but also included the low equatorial latitudes of the supercontinent (where Mallorca was located). distribution and temperature belts through the Upper Permian and Early Triassic The Permian progressively became dryer and dryer. In the Upper Carboniferous and Lower Permian, pelycosaurs seem to have clung to the everwet coal swamp habitats near the equator (fossils known within 10° of either side of the palaeoequator); beyond this to about 30° was an expansive desert which extended all the way to the coast, separating the swamps from the temperate regions. By the Middle Permian, the equatorial forests had switched to a seasonal wet/dry system, but the swamps were connected to the temperate zones via coastal passages along East Pangaea, allowing cross-continental migration from what is now South Africa to what is now Russia. Therapsids appear to have evolved in this seasonally humid/dry landscape, expanding even into the temperate zones. At this point, synapsids were the only large terrestrial animals of their environment; and pelycosaurs may not have been able to adapt to the aridification. At about the time of pelycosaur extinction, therapsids experienced a major adaptive radiation (all carnivores) continuing into the Upper Permian. The rubidgeans were the most derived gorgonopsians, and consequently the most massive and heavily built. ==Palaeobiology==

Bite '' has the proportionally longest canines of any gorgonopsian. The elongated canines have generally been thought to have been instrumental in their hunting tactics. The gorgonopsian jaw hinge was double jointed and made up of somewhat mobile and rotatable bones, which would have allowed them to open their mouths incredibly wide—perhaps in excess of 90°—without having to unhinge the jaw. It has alternatively been suggested (first in 2002 by biologists Blaire Van Valkenburgh and Tyson Secco, though in reference to cats) that sabres evolved primarily due to sexual selection as a form of mating display. This is exhibited in some modern deer species, but is difficult to test given the lack of living sabre-toothed synapsid predators. In sabre-toothed cats, long-sabred ("dirk-toothed") taxa are thought to have been pursuit hunters, whereas short-toothed ("scimitar-toothed") taxa are thought to have been ambush predators. Among the dirk-toothed cats, these predators are suggested to have killed with a well-placed slash to the throat after grappling prey, but gorgonopsians may have been less precise with bite placement, armed with reptilian jaws and tooth arrangements. Instead, gorgonopsians possibly used a bite-and-retreat tactic: the predator would ambush its quarry and take a sizable and debilitating bite out of it, and then follow as the prey tried to escape before succumbing to its injury, whereupon the gorgonopsian would deliver a killing bite. Because the postcanines are reduced or entirely absent, meat would have been forcibly torn away from the carcass and swallowed whole. Unlike mammalian carnivores, gorgonopsians (and therocephalians) had reduced or completely lacked postcanines, and the jaw likely could not exert shearing pressure necessary for crushing bones open to access the bone marrow. It has largely been unclear if bone marrow had even evolved yet in Permian synapsids (fish and many amphibians lack this in present day), but in 2021 it was shown that the Early Permian amphibians Seymouria and Discosauriscus likely had haematopoietic (red blood cell-producing) bone marrow in their limbs. Locomotion '' walking with a sprawling gait Gorgonopsians are considered to have been strictly terrestrial. They are thought to have been able to move with an erect gait similar to that used by crocodilians, the limbs positioned almost vertically as opposed to horizontally as in the sprawling gait of lizards. The glenoid cavity on the shoulder blade is strongly angled tailwards, so the limbs had limited forward movement, and they may have had a short stride length. Lizards often move their spines side to side to increase stride length, but the more vertically orientated facet joints connecting the vertebrae in gorgonopsians would have made the spine more rigid and stable, encumbering such movement. These adaptations may have made gorgonopsians swifter and more agile than their prey. Senses '' with proportionally large scleral rings Unlike eutheriodonts, but like some ectothermic creatures today, all gorgonopsians possessed a pineal eye on the top of the head, which is used to detect daylight (and thus, the optimal temperature to be active). It is possible that other theriodonts lost this due to the evolution of either endothermy, intrinsically photosensitive retinal ganglion cells in the eyes—in tandem with the loss of colour vision and a shift to nocturnal life—or both. Nocturnal behaviour has long been assumed to have originated in mammals (nocturnal bottleneck), but the large orbit size and presence of scleral rings in many early synapsids, stretching as far back as the Carboniferous, would suggest that the ability to venture out in low-light conditions evolved much earlier. Based on these aspects, the specimen SAM-PK-K10034 may have had mesopic vision, and Cyonosaurus scotopic or photopic vision. The diameters of the scleral rings for the small Viatkogorgon are proportionally large, with an inner diameter of and outer diameter of , compared to a diameter of for the orbit itself, which suggests it made predominantly nocturnal excursions. Early theriodonts (including gorgonopsians) may have possessed an eardrum, unlike earlier pelycosaurs, indicated by the reduction of the connection between the quadrate bone (at the jaw hinge) and the pterygoid bone (at the palate), allowing the quadrate to independently vibrate to a degree. This may have allowed the detection of air-borne sounds with a low amplitude of less than , but the eardrum would have been supported by cartilage or ligaments instead of bone. If correct, then the postdentary bones (which in early mammals form the middle ear bones) would have needed to become detached from the dentary (jawbone); the gorgonopsian fossil record seems to indicate the postdentary-dentary connection was reduced. Though, given the specialisations required for biting, the condition of an isolating quadrate in gorgonopsians could alternatively be explained as streptostyly (rotatable quadrate) in order to widen the gape rather than facilitate hearing. Thermoregulation A major anatomical shift occurred between earlier pelycosaurs and therapsids, which is postulated to have been related to an increasing metabolism and the origins of homeothermy (maintenance of a high body temperature). The evolution of a secondary palate, and the separation of the mouth from the nasal cavity, may have increased ventilation efficiency associated with high levels of aerobic activity; gorgonopsians did not have a bony secondary palate, but possibly had one of soft tissue. Nonetheless, the secondary palate could have instead aided in eating large quantities of food at once rather than in ventilation. The reorganisation of the skeleton (from a sprawling to a parasagittal gait) has been postulated to be indicative of the presence of a diaphragm, and thus also enhanced ventilation for aerobic activity; but it could have instead been to increase acceleration or agility, which does not necessarily equate to intense aerobic activity, much like in crocodiles. Fibrous lamellar cortical bone, which all early therapsids had, would indicate an increased growth rate, but this may not be linked to metabolic rate. Modern large reptiles naturally give off body heat at a slower rate than smaller ones, and are considered "inertial homeotherms", but they maintain a low body temperature of . If therapsids required a higher body temperature of , they would either have needed to have been endotherms (generating their own body heat) or have had greater control over heat loss (that is, better homeothermy). The parasagittal gait may have aided the latter, as it would have kept most of the body off the ground as well as allowed blood to stay in the abdomen instead of having to circulate through the appendages, both of which would reduce heat transfer to the ground and stabilise core temperature. The reduced tail would have also reduced the total surface area of the animal, further minimising heat loss. Among therapsids, only eutheriodonts (not gorgonopsians) have respiratory nasal turbinates, which help retain moisture while breathing in large quantities of air, and its evolution is typically associated with the beginning of "mammalian" oxygen consumption rates and the origins of endothermy. If gorgonopsians were inertial homeotherms, it is not impossible that they had hair. The snout is typically riddled with foramina (small holes which confer with blood vessels), which could potentially point to the existence of loose skin (as opposed to scales), hair, various skin glands (such as sweat glands), and whiskers; however, some reptiles present a similar patterning of foramina, which are instead related to dental development rather than skin. Analysis on inner ear biomechanics found that gorgonopsians probably had a body temperature , suggesting the clade was ectothermic. Palaeopathology The anterolateral aspect of the left radius (a forearm bone) of the gorgonopsian specimen NHCC LB396 presents a circular bony lesion, featuring irregular-to-radial spikes made of cortical bone surrounded by a thin layer of subperiosteal bone, which grew rapidly over a single growing season. This is consistent with periostitis most likely stemming from subperiosteal haematoma. This specific condition as well as the fast growth rate are more reminiscent of mammals and dinosaurs than crocodilians or monitor lizards. Among early synapsids, the only other pathology noted is osteomyelitis in several pelycosaur groups. The labial (lip/cheek) side of the tooth root of a functional canine of RB382 presents as many as 8 lesions, clustering along the midline of the tooth, which resemble miniature teeth with a pulp, dentine, and a thin enamel coating. They are roughly circular—with diameters varying from —though they become less circular at around the middle point of the root until passing the cervix of the tooth. This is roughly consistent with the human ailment odontoma, the most frequent type of odontogenic tumour, which previously only extended a few million years back in the fossil record. At 255 million years old, RB382 presents the oldest-known case of odontoma. The adult snout SAM-PK-11490 from an indeterminate Middle Permian gorgonopsian species has an imbedded tooth from an unidentifiable animal. The bone developed a callus around the tooth, indicating it healed and the individual survived the attack. It either came from a predator—namely a biarmosuchian, a therocephalian, or another gorgonopsian—or intraspecific face biting as is commonly exhibited in social predators—such as big cats or monitor lizards, and it has been suggested for several extinct lineages such as theropods, aquatic reptiles, and saber-toothed cats. Social biting is intended to assert dominance or facilitate breeding, and, if correct, suggests at least some Middle Permian gorgonopsians were social carnivores. The tooth was initially overlooked so it is unclear how common this pathology actually is. ==Palaeoecology==

Paleoenvironment hunting Scutosaurus'' Following the extinction of the dinocephalians and (in South Africa) the basal therocephalians Scylacosauridae and Lycosuchidae, gorgonopsians evolved from small and uncommon forms into large apex predators. Through the Middle to Upper Permian, in South Africa the dicynodonts were the most common animals, whereas the pareiasaurs Deltavjatia and Scutosaurus were the most abundant in the gorgonopsian-bearing Russian formations. During the Upper Permian, the South African Beaufort Group was a semi-arid cold steppe featuring large, seasonal (ephemeral) rivers and floodplains draining water sources much farther north into the Karoo Sea, with some occurrences of flash floods after sudden, heavy rainfall; The gorgonopsian-bearing Salarevskian Formation in western Russia was also probably deposited in a semi-arid environment with highly seasonal rainfall, and featured hygrophyte and halophyte plants in coastal areas, as well as more drought-resistant conifers at higher elevations. The Moradi Formation was an arid desert, primarily dominated by the captorhinid reptile Moradisaurus and the pareiasaur Bunostegos. Extinction Prior to the Permian–Triassic extinction event, many gorgonopsians began to go extinct in Africa and Eastern Europe. In Eastern Europe, gorgonopsians went extinct due their inability to adapt to changes in temperature and humidity. Following their extinction the macropredator niches were taken over by large therocephalians and archosauriform reptiles taking over macropredatory niches. Initially, it was thought that some smaller taxa of such as Cyonosaurus up into the Early Triassic, but later analysis found it to have been erroneous. Despite this, it is the youngest occurrence of gorgonopsians within the Karoo Basin, suggesting it was more resilient to the early phases of the mass extinction. Among therapsids, small therocephalians and large herbivorous anomodonts managed to cross the Permian–Triassic boundary, and survived respectively until the Middle and Upper Triassic, but only small-bodied species of cynodonts survived into the Jurassic, whose descendants would include mammals. The large predatory niches gorgonopsians left open were eventually filled by the archosaurs (including crocodiles and dinosaurs) during the early stages of the Triassic. ==See also==