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. Later studies have reinterpreted the socket as articulating with the surangular, one of the postdentary bones, instead of the dentary. 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. This had not yet happened in the case of Probainognathus, but the reduced size of the quadrate, as well as its loose association with the squamosal and proximity to the stapes indicates the quadrate to incus process was underway. The jaw of Probainognathus is suggested to have had mammal-like soft tissue features. The jaw muscles in Probainognathus are thought to have been positioned farther forward, with the masseter splitting into two separate muscles: the superficial masseter and the deep masseter. 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 a 1973 paper, he decided that Probainognathus was distinct enough from other chiniquodontids to deserve its own family, and thus Probainognathidae was erected. In 2016, a second probainognathid, Bonacynodon, was described from Brazil. Phylogenetic analyses have generally found probainognathids to be early-diverging members of the clade Probainognathia, being closer to mammals than to the traversodontids and other cynognathians. The cladogram below is from a 2023 paper by Stefanello et al., and shows the position of Probainognathus within Probainognathia. Paleoecology Probainognathus was collected from the Chañares Formation in La Rioja Province, Argentina. This locale is known for its preservation of tetrapods, which are largely fossilized in volcanic concretions. 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. Various pieces of evidence suggest tetrapod mass mortality was the cause of death in the Chañares Formation. Fossils of both young and adult specimens were found at this locale, and these are not limited to Probainognathus, but rather represent many taxa that fell victim to the event. There is also an unusual concentration of herbivores, carnivores, and omnivores in this formation. Numerous fossils of each type were