Prozostrodon is an extinct genus of probainognathian cynodonts that was closely related to mammals. The remains were found in Brazil and are dated to the Carnian age of the Late Triassic. The holotype has an estimated skull length of 6.7 centimetres (2.6 in), indicating that the whole animal may have been the size of a cat. The teeth were typical of advanced cynodonts, and the animal was probably a carnivore hunting reptiles and other small prey.
Discovery and naming Prozostrodon brasiliensis was originally described as a species of Thrinaxodon in a 1987 paper by Mário C. Barberena, José F. Bonaparte and A. M. Sá Teixeira. The holotype (UFRGS-PV-0248-T) includes a well-preserved skull preserving the front half of the cranium, a mostly complete lower jaw and all of the teeth, but missing most of the braincase, sagittal crest and zygomatic arches. It also preserves multiple postcranial elements, including parts of the vertebral column, ribs, interclavicle, humeri, right ilium, femora and right foot. The specimen was found at the Faixa Nova locality in the municipality of Candelária, Rio Grande do Sul, in layers belonging to the Candelária Sequence of the Santa Maria Supersequence. It is deposited in the palaeontology museum of the UFRGS. The assignment of the species to Thrinaxodon was disputed by Battail (1991), who instead suggested that it might be a member of Chiniquodontidae, a group of cynodonts closer to mammals than Thrinaxodon. A 2001 redescription by Bonaparte & Barberena indicated that T. brasiliensis was indeed more closely related to mammals than to the other Thrinaxodon species; as a result, it was reclassified into the new genus Prozostrodon. The generic name references the well-developed cingulum of the lower postcanines, being derived from the Greek word zoster, meaning a girdle or band, and the Latin prefix pro-, meaning "before". A 2018 histological analysis found that the holotype represents a subadult, as its bones were still growing when it died. A second specimen (CAPPA/UFSM 0123) was described in 2018 from the Marchezan locality in the municipality of São João do Polêsine. It consists of a right dentary bone preserving three postcanine teeth. Because of its small size and lack of a diastema behind the canine, this specimen is thought to be a juvenile. A third specimen (UFRGS-PV-0543-T) was described in 2020 from the Faixa Nova locality, consisting of the front part of the cranium and fragments of the right dentary. This specimen notably preserves a natural endocast of the nasal cavity. In 2023, a nearly complete skull (CAPPA/UFSM 0210) was described from the Marchezan locality. Unlike any of the other known specimens, it preserves the back portion of the skull, including the basicranium.
Description With the holotype having an estimated skull length of 6.7 centimetres (2.6 in), Prozostrodon was considerably larger than the other prozostrodontians (Alemoatherium and Therioherpeton) from its locality.
Skull On the snout tip, the premaxilla bore a narial process that connected with the nasal bone. Further back, the premaxilla had a thin posterodorsal process that was wedged between the septomaxilla and maxilla. Above the eye socket (orbit), Prozostrodon retained small prefrontal and postorbital bones, unlike in most other prozostrodontians where these bones are completely absent. The postorbital bar, which had divided the eye socket from the temporal fenestra in earlier cynodonts, was missing in Prozostrodon, as in most other prozostrodontians. Within the eye socket, there was a broad contact between the frontal, palatine and orbitosphenoid bones. A small hole called the sphenopalatine foramen was present within the eye socket, as in other prozostrodontians. In Prozostrodon the foramen was completely enclosed by the palatine bone, unlike in some later prozostrodontians like Brasilodon and Morganucodon, where the foramen was bordered by both the orbitosphenoid and palatine. The secondary (hard) palate extended slightly behind the last postcanine tooth. On the lingual (inner) side of the upper postcanines, there was a deep groove where the lower teeth would have fit when the mouth was closed. The lower jaw consisted primarily of the dentary bone. The dentary was fairly robust, with an upturned tip. The symphysis (joint) between the two halves of the dentary was unfused. More basal probainognathians generally had a fused symphysis, and an unfused symphysis is one of the main synapomorphies (shared derived features) of Prozostrodontia. At the rear end of the dentary, behind the tooth row, a somewhat tall and broad coronoid process extended upwards. Behind the coronoid process, there was a backwards-pointing projection called the articular process. In modern mammals, the articular process bears a dentary condyle that articulates with the squamosal bone of the cranium, but this condyle was absent in Prozostrodon. On the lingual side of the dentary, a meckelian groove stretched from near the symphysis to the rear part of the bone.
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