Probainognathus meaning “progressive jaw” is an extinct genus of cynodonts that lived around 235 to 221.5 million years ago, during the Late Triassic in what is now Argentina. Together with the genus Bonacynodon from Brazil, Probainognathus forms the family Probainognathidae. Probainognathus was a relatively small, carnivorous or insectivorous cynodont. Like all cynodonts, it was a relative of mammals, and it possessed several mammal-like features. Like some other cynodonts, Probainognathus had a double jaw joint, which not only included the quadrate and articular bones like in more basal synapsids, but also the squamosal and surangular bones. A joint between the dentary and squamosal bones, as seen in modern mammals, was however absent in Probainognathus. Discovery and naming The first specimens of Probainognathus jenseni were discovered in the Chañares Formation in La Rioja Province, Argentina, by a group headed by the Harvard collector James A. Jensen. The specimens were collected north of the terminus of the Rio Chañares at the Campo de Talampaya. Probainognathus jenseni was first described and named by the Harvard paleontologist Alfred Sherwood Romer in 1970. The generic name Probainognathus means “progressive jaw” in Greek, referencing its advanced jaw articulation. The specific epithet jenseni honours James A. Jensen. A juvenile skull (PVSJ 410), from the Ischigualasto Formation of northwestern Argentina, was assigned to cf. Probainognathus in a 1994 paper by José Bonaparte and Alfred W. Crompton. Later studies have shown that the specimen likely belongs to a cynodont more derived than Probainognathus. The specimen was sent through a CT scan at the University of Texas at Austin, and the resulting 3D model was made available to the public. In 1994, the holotype of Probainognathus jenseni, along with several other specimens, were stolen from the National University of La Rioja in Argentina. Description Probainognathus was a small cynodont, the apparent body size of which is inconsistent across sources, ranging from being about the size of a rat, to the size of a small dog. Teeth and diet The diet of Probainognathus is thought to have been carnivorous or insectivorous, which can be extrapolated from its teeth. The teeth included dully-tipped, partially developed canines, postcanines, four upper incisors that were positioned vertically, and three lower incisors that leaned slightly forward. The cheek teeth were slender from the sides, but were lengthened anteroposteriorly, with several cusps along the row. Brain Based on an endocranial cast of Probainognathus, several details have been extrapolated