Bonacynodon

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Bonacynodon is an extinct small cynodont that lived in what is now southern Brazil during the Triassic period, about the Ladinian to Carnian ages. The genus contains one species, Bonacynodon schultzi.

Two skulls and some fragmentary postcranial bones are known from Bonacynodon schultzi. They were found in the Pinheiros-Chiniquá Sequence, part of the Santa Maria Supersequence in the Paraná Basin, within a faunal zone rich in cynodonts, dicynodonts, and reptiles. The rocks date to the Ladinian–early Carnian interval.

Bonacynodon was a small animal, about 30 cm long, with a skull roughly 6–7 cm long. It stood out for its large, serrated upper canines. Its teeth included four upper incisors, large upper canines, and six postcanine teeth behind each canine. The lower teeth are less well known. The jaws and teeth suggest an insectivorous diet, and a close relationship to Probainognathus, placing Bonacynodon in the family Probainognathidae, one of the early branches near the lineage that would lead to mammals.

The holotype skull (MCT-1716-R) and a referred skull (MCT-1717-R) were collected in the 1940s by palaeontologist Llewellyn Price, from two different outcrops near Candelária, Rio Grande do Sul. The holotype includes parts of the skull and teeth, with the skull roof damaged; the paratype adds more skull elements but is also distorted. Both specimens are thought to represent subadult individuals.

The name Bonacynodon honors Argentine paleontologist José Bonaparte, with cynodon meaning “dog” in part reference to its cynodont nature; the species name schultzi honors Brazilian palaeontologist Cesar L. Schultz.

In phylogenetic analyses, Bonacynodon is usually recovered as the sister taxon to Probainognathus, together forming the Probainognathidae within the broader group Prozostrodontia, which includes mammals. Its relatively unserrated postcanine teeth, compared with some related groups, are interpreted as an adaptation toward insectivory rather than carnivory.

Ecologically, Bonacynodon comes from the Dinodontosaurus Assemblage Zone—the lowermost part of the Santa Maria formation—during a dry period with loess plains. The assemblage also includes other cynodonts, dicynodonts, rhynchosauroids, and various archosaurs and parareptiles, painting a picture of a diverse Triassic ecosystem.