Gorgonopsids were a group of creatures that were at one time called "mammal-like reptiles"; though in most current classification systems, they were not true reptiles, instead much more closely related to true mammals. Their mammalian specialisations included differentiated tooth shape, the fully developed temporal fenestra, pillar-like rear legs and even ear bones, as well as other traits associated with its mammalian descendants. Arguments have even been made for synapsids of its time being endothermic. Gorgonopsids had two sabre-teeth that were larger than the others.
Gorgonopsids or gorgonopsia (name meaning "Gorgon Face") was a therapsid synapsid suborder, which existed from 265 to 252 million years ago approx. Gorgonopsids evolved in the Mid-Permian period (at small sizes), and became the Late Permian's apex predators following the dinocephalians' extinction. Gorgonopsia, as synapsids in the theriodont group, possessed several mammalian characteristics, in their palate, temporal fenestra, and ear-bone development. The largest Gorgonopsid species was Inostrancevia (at the size of a large bear), and the Gorgonopsids were among the Late Permian's largest carnivores. Gorgonopsia died out at the end of the Permian in the P-Tr Mass Extinction.