Bonacynodon

Article

August 13, 2022

Bonacynodon is an extinct genus of cynodont therapsids that lived during the Triassic (Ladinian-Carnian ages) in what is now southern Brazil. The genus is monotypic, containing only the type species Bonacynodon schultzi, described in 2016 from two fossil specimens consisting of two partial skulls and some poorly preserved postcranial bones, found in the Santa Maria formation, within the basin of the Plate. This site yielded many fossils of other species of cynodonts, dicynodonts, as well as many sauropsids. Bonacynodon is a small cynodont that was probably insectivorous and estimated to be around 30 cm in length. It is distinguished from other cynodonts by its large, serrated, saw-like canines. Together with the genus Probainognathus, found in Argentina, it constitutes the family Probainognathidae, one of the basal lineages of the taxon Probainognathia. It is a close relative of the mammaliaformes, which later gave rise to the mammals, the only remaining group of cynodonts.

Discovery and naming

Bonacynodon schultzi is known from two fossil specimens, the holotype, cataloged MCT-1716-R, and the specimen referenced MCT-1717-R. They were both discovered in the 1940s by Brazilian paleontologist Llewellyn Ivor Price in two separate rocky outcrops, in the Pinheiros region, about 12 km south of the town of Candelária, Rio Grande do Sul. The rocks belong to the Pinheiros-Chiniquá Zone of the Santa Maria Formation, which is dated to the mid to late Triassic, more precisely between the Ladinian and the Carnian. The holotype was discovered in an outcrop known as Sanga do Janguta, and is preserved with a specimen of the dicynodont Dinodontosaurus. It mainly consists of a partial skull, including parts of the cranial roof, mandible, upper and lower dentition, base of the skull, and palate. The cranial roof was badly damaged during the preparation. The holotype also includes very fragmentary postcranial elements, which have not been described in detail. As for the second referenced specimen, it was found in a different outcrop, known as Sanga do Forno. As on the holotype, parts of the skull, the mandible and the upper and lower dentition are preserved. The specimen is heavily deformed due to sediment pressure during fossilization, but some parts of the skull missing from the holotype are present. Based on the lack of wear facets on the teeth, the researchers suggest that both specimens represent subadult individuals. Both specimens were described in 2016 and given a binomial name given by Agustín G. Martinelli and colleagues. The first part of the genus name Bonacynodon is derived from the surname of José Bonaparte, an Argentinian paleontologist specializing in Mesozoic vertebrates from South America. The second part is derived from the ancient Greek word κύων / kuōn, which means "dog", and refers to the fact that it is a cynodont, the name of the group literally meaning "dog's tooth". The specific epithet schultzi honors Cesar L. Schultz, Brazilian paleontologist and professor at the Federal University of Rio Grande do Sul.

Description

Simplified description

Since the known postcranial remains of Bonacynodon remain very fragmentary, the skeletal anatomy of the animal is poorly understood. The skull is only known from the holotype and the second referenced specimen. However, based on the size of other contemporary cynodonts, the total length of Bonacynodon would be around 30 cm.

Skull

Bonacynodon's skull measures approximately 6"