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
thumb|left|The landscape of [[La Rioja Province, Argentina|La Rioja Province, in which remains of Probainognathus have been found]]
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 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
thumb|right|Dentition of Probainognathus jenseni
The diet of Probainognathus is thought to have been carnivorous or insectivorous, which can be extrapolated from its teeth. The cheek teeth were slender from the sides, but were lengthened anteroposteriorly, with several cusps along the row. These were among other, less pronounced, structures such as anterior colliculi. In comparison to close relatives, such as Chiniquodon, the braincase of Probainognathus was broader in the parietal region, and the occipital region of the skull was deeper. Likewise, the parietals had more depth, the occipital crest was higher, and the orbits were proportionately larger relative to the rest of its head. The sagittal crest was also lengthened posteriorly, and split into the occipital crests. This led to a straighter posterior margin, and a more concave occiput in Probainognathus. Like some other cynodonts, Probainognathus possessed a double jaw joint, as indicated by a socket (glenoid) in the squamosal bone. Romer (1970) interpreted this socket as articulating with the dentary bone of the lower jaw, as seen in modern mammals and early mammaliaforms like Morganucodon. Once the dentary-squamosal articulation became established, the former bones involved in jaw articulation, the articular and quadrate, could become integrated into the inner ear as the malleus and incus, respectively.
Classification
When Romer first described Probainognathus in 1970, he assigned it to the family Chiniquodontidae based on the animal's overall similarities with that group. In 2016, a second probainognathid, Bonacynodon, was described from Brazil.
Paleoecology
Probainognathus was collected from the Chañares Formation in La Rioja Province, Argentina. When Probainognathus inhabited this area, it is thought to have been a lacustrine region within a rift basin that got a large influx of sedimentary debris and volcanic ash.