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.