Gorgonopsia
Gorgonopsia is an extinct clade of sabre-toothed therapsids from the Middle to the Upper Permian, roughly between 270 and 252 million years ago. They are characterised by a long and narrow skull, as well as elongated upper and sometimes lower canine teeth and incisors which were likely used as slashing and stabbing weapons. Postcanine teeth are generally reduced or absent. For hunting large prey, they possibly used a bite-and-retreat tactic, ambushing and taking a debilitating bite out of the target, and following it at a safe distance before its injuries exhausted it, whereupon the gorgonopsian would grapple the animal and deliver a killing bite. They would have had an exorbitant gape, possibly in excess of 90°, without having to unhinge the jaw.
They markedly increased in size as time went on, growing from small skull lengths of in the Middle Permian to bear-like proportions of up to in the Upper Permian. The latest gorgonopsians, Rubidgeinae, were the most robust of the group and could produce especially powerful bites. Gorgonopsians are thought to have been completely terrestrial and could walk with a semi-erect gait, with a similar terrestrial locomotory range as modern crocodilians. They may have been more agile than their prey items, but were probably inertial homeotherms rather than endotherms unlike contemporary therocephalians and cynodonts, and thus were probably comparatively less active. Though gorgonopsians were able to maintain a rather high body temperature, it is unclear if they would have also had sweat glands or fur. Their brains were reminiscent of modern reptilian brains, rather than those of living mammals. Most species may have been predominantly diurnal though some could have been crepuscular or nocturnal. They are thought to have had binocular vision, a parietal eye, a keen sense of smell, a functional vomeronasal organ, and possibly a rudimentary eardrum.
The major therapsid groups had all evolved by 275 million years ago from a "pelycosaur" ancestor. The therapsid takeover from pelycosaurs took place by the Middle Permian as the world progressively became drier. Gorgonopsians rose to become apex predators of their environments following the Capitanian mass extinction event which killed off the dinocephalians and some large therocephalians after the Middle Permian. Despite the existence of a single continent during the Permian, Pangaea, gorgonopsians have only been found in the Karoo Supergroup, the Moradi Formation of Niger, western Russia, the Port des Canonge Formation on the island of Mallorca in Spain, and in the Turpan Basin of Xinjiang, China, with probable remains known from the Kundaram Formation in the Pranhita–Godavari Basin of India. These places were semi-arid areas with highly seasonal rainfall. The oldest gorgonopsian specimen, a middle Permian gorgonopsian, was described from the Port des Canonge Formation on the island of Mallorca, western Mediterranean. Gorgonopsian genera are all very similar in appearance, and consequently many species have been named based on flimsy and likely age-related differences since their discovery in the late 19th century, and the group has been subject to several taxonomic revisions.
Most gorgonopsians became extinct during a phase of the Permian–Triassic extinction event taking place at the very end of the Permian, in which major volcanic activity and resultant massive spike in greenhouse gases caused rapid aridification due to temperature spike, acid rain, frequent wildfires, and potential breakdown of the ozone layer. However, some smaller taxa like Cyonosaurus may have survived up to the Early Triassic. The large predatory niches would be taken over by the archosaurs in the Mesozoic.
Description
Earlier gorgonopsids in the Middle Permian were quite small, with skull lengths of, whereas some later genera attained massive, bear-like sizes with the largest being Inostrancevia up to in length and in body mass. Nonetheless, small gorgonopsians remained abundant until extinction.Like other Permian therapsids, gorgonopsians had developed several mammalian characteristics. These might have included a parasagittal gait 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. Like reptiles, gorgonopsians lack a secondary palate separating the mouth from the nasal cavity, prohibiting chewing.
Skull
Anatomy varies incredibly little between gorgonopsians. 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. They typically feature a long and narrow skull. Juvenile Rubidgea appear to have had snouts wider than long. Unlike eutheriodonts, the occipital bone is rectangular and concave, as opposed to triangular.The gorgonopsian brain, like other non-mammaliaform therapsids, lacks an expansion of the neocortex, has a relatively large hindbrain compared to the forebrain, a large epyphysial nerve, an enlarged pituitary gland, and an overall elongated shape; all-in-all resembling a reptilian brain. The braincase was also rather reptilian, and is also comparatively smaller and not as thick as those of mammals. The flocculus, a lobe of the cerebellum, is proportionally large, and is related to the vestibulo-ocular reflex. Judging by the orientation of the semi-circular canals in the ear, the head of the gorgonopsian specimen GPIT/RE/7124 would have tilted forward by about 41°, increasing the overlap between the visual fields of the two eyes and improving binocular vision—useful to a predator. Unlike either reptiles or mammals, the semi-circular canals are flat, probably because they were wedged between the opisthotic and supraoccipital bones.
Teeth
Like many mammals, gorgonopsians were heterodonts, with clearly defined incisors, canines, and postcanine teeth homologous with premolars and molars. They had five incisors in the upper jaw and four on the bottom.In the majority of gorgonopsians, the incisors were large, and the upper canines were elongated into sabres, much like those of later sabre-toothed cats. Some gorgonopsians had exceptionally long upper canines, such as Inostrancevia, and some of them had a flange on the lower jaw to sheath the tip of the canine while the mouth was closed. Sabres are generally interpreted as having been used as stabbing or slashing weapons, which would have required an extremely wide gape. Both the upper and lower canines of Rubidgea were elongated, and the animal would have needed an even greater gape. 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. The postcanine teeth were reduced in both size and number; many rubidgeines did not have postcanines in the lower jaw, and Clelandina lacked them entirely.
Gorgonopsians were polyphyodonts, and teeth grew continuously throughout an individual's life. 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. Such a method might have been in play so as always to have a set of functional canines, as having a single or no canines would have severely impeded hunting, and growing such large teeth took a long time. On the other hand, because the functional canine is typically found in the foremost tooth socket, it is possible that canine replacement occurred a finite number of times, and the animal would eventually be left with a single, permanent set of functional canines in these sockets. In 1984, British palaeontologists Doris and Kenneth Kermack suggested that the canines grew to match the size of the skull, and continually broke off until the animal stopped growing, and that gorgonopsians featured an early version of finite tooth replacement exhibited in many mammals. The tooth replacement patterns of the other teeth are unclear. The postcanine teeth were replaced more slowly than the other teeth, likely due to their lack of functional significance.
Postcranium
The seven cervical vertebrae are all the same size as each other except for the last one, which is shorter and lower; there is one atlas and one axis. Like sabre-toothed cats, the neck is long with well-developed muscles, which would have been especially useful when the canines were sunk into an animal. Like other early synapsids, gorgonopsians have a single occipital condyle, and the articulation of the cervical vertebrae is overall reptilian, permitting side-to-side movement of the head but restricting up-and-down motion. The last cervical is shaped more like the dorsal vertebrae.The dorsals are spool-shaped and all appear about the same as each other. The spinous processes jut out steeply from the centra, and feature sharp keels on the front and back sides. Unlike eutheriodonts, gorgonopsians do not have distinguished lumbar vertebrae. Nonetheless, the dorsals equating to that series are similar to the lumbars of sabre-toothed cats with steeply oriented zygopophases, useful in stabilising the lower back especially when pinning down struggling prey.
There are three sacral vertebrae, and the series attached to the pelvis by the first vertebra. The pelvis is reptilian, with separated ilium, ischium, and pubis. The femur is slightly s-shaped, and is short but longer and slenderer than the humerus. For most, the tibia and fibula strongly curve into each other, and the tibia is more robust than the fibula. 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.