Exaeretodon

Exaeretodon is an extinct genus of fairly large, low-slung traversodontid cynodonts from the southern parts of Pangea. Four species are known, from various formations. "E. argentinus" is from the Carnian-age (Late Triassic) Cancha de Bochas Member of the Ischigualasto Formation in the Ischigualasto-Villa Unión Basin in northwestern Argentina. "E. major" and "E. riograndensis" are from the Carnian-age portion of the Santa Maria Formation of the Paraná Basin in southeastern Brazil. "E. statisticae" is from the Carnian-age Lower Maleri Formation of India.
Description.
This genus was an herbivore up to long, with a specialized grinding action when feeding.
An analysis of the size of the bones of calves collected in Paleorrota concluded that the mother "Exaeretodon" had one or two calves, for one pregnancy.
Taxonomy.
When he first named the species, Argentine paleontologist José Bonaparte mentioned several features that distinguish it from all other traversodontids. The tooth rows of the upper jaw are more parallel to each other in "I. sudamericanus" than they are in "Exaeretodon", and they are also inset closer to the inside of the mouth. There are also more postcanine teeth oriented toward middle of the subtemporal fenestrae (two holes in the bottom of the skull) and ascending rama of the dentary (projections of the lower jaw that extend up to the skull). The ascending rama are also wider and taller in the "I. sudamericanus" specimen. The orbit or eye socket is longer than that of "Exaeretodon", as are the palatine bones.
Despite the differences, a 2007 study concluded that the "I. sudamericanus" skull represents the same species as "E. argentinus". Using allometry, paleontologist Jun Liu found "I. sudamericanus" to be the largest known example of a growth series in "E. argentinus". Thus, "Ischignathus" is now regarded as a junior synonym of "Exaeretodon". As the animal grew, the proportions of bones changed. These differing proportions were initially seen as species-distinguishing characters, but are now regarded as natural ontogenic variation.