Probainognathidae is an extinct family of insectivorous cynodonts which lived in what is now South America during the Middle to Late Triassic. The family was established by Alfred Romer in 1973 and includes two genera, Probainognathus from the Chañares Formation of Argentina and Bonacynodon from the Dinodontosaurus Assemblage Zone of Brazil. Probainognathids were closely related to the clade Prozostrodontia, which includes mammals and their close relatives.
Description
thumb|left|Restoration of Bonacynodon
Members of Probainognathidae were relatively small-bodied animals, with skull lengths of around . The temporal region (area behind the eye sockets) was rather wide, and longer than the snout. The secondary palate was well-developed compared to earlier cynodonts, and the portion made up by the maxilla was larger than the part made from the palatine bone. The dentary, the tooth-bearing bone of the lower jaw, was quite tall when seen from the side. The mandibular symphysis (the joint between the two halves of the dentary) was fused in Probainognathus, but unfused in Bonacynodon. In addition to the ancestral quadrate-articular jaw joint found in most non-mammalian cynodonts, probainognathids also had an incipient contact between the squamosal and surangular bones. The canine teeth were large and labio-lingually compressed, and in Bonacynodon they bore a serrated edge. The postcanines had a typical "triconodont" shape, with four main cusps placed in a straight line. Unlike in other basal probainognathians like Chiniquodon, the cusps were not recurved. The lower postcanines bore a discontinuous cingulum which was restricted to the mesiolingual and distolingual sides of the teeth. The shape of the postcanines indicates that probainognathids most likely were insectivorous.