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. Throughout the Middle Permian, the often gigantic dinocephalians were the dominant animals of their ecosystems. They disappear from the fossil record during the Capitanian mass extinction event caused by volcanic activity which has formed the Chinese Emeishan Traps. The exact cause of their extinction is unclear, but they were replaced by gorgonopsians and dicynodonts (which began to greatly increase in size) and the smaller therocephalians. The rubidgeans were the most derived gorgonopsians, and consequently the most massive and heavily built. Palaeobiology Bite Gorgonopsians were likely active predators. The rubidgeines have an especially robust skull among gorgonopsians, comparable to those of enormous macropredators which use their skulls as their primary weapon, such as mosasaurs or some theropod dinosaurs. Less robust gorgonopsians with longer canines and much weaker bite, such as Smilesaurus or Inostrancevia, instead probably used their canines for slashing, much more similar to sabre-toothed cats. The postcanines of Clelandina were replaced by a smooth ridge unlike dicynodonts which have a blade-like keratinous ridge, and it may have predominantly gone after prey it could swallow whole. Gorgonopsian taxa did coexist with each other—as many as seven at one time–and the fact that some rubidgeines possess postcanines while some other contemporary ones do not suggests that they practiced niche partitioning and pursued different prey items. 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