Prozostrodon brasiliensis, a probainognathian cynodont from the Late Triassic of Brazil: second record and improvements on its dental anatomy

Abstract

Probainognathian cynodonts are conspicuous elements of the Assemblage Zones of the Triassic Santa Maria Supersequence in southern Brazil. Within this group, the derived clade Prozostrodontia, in which the crown group Mammalia is included, is taxonomic diverse in the Hyperodapedon and Riograndia AZs. We describe here the second known specimen (CAPPA/UFSM 0123) of Prozostrodon brasiliensis, until now only represented by its holotype. CAPPA/UFSM 0123 includes a right dentary with dentition. As in the holotype of P. brasiliensis, it has four lower incisors, pc4 with conspicuous cusp a, and small cusps b, c, and d, pc5-pc6 of ‘triconodont’ type with cusps a > c > b > d, with continuous lingual cingulum bearing up to six small discrete cusps, length of the lower tooth row more than half the length of the dentary, and relatively deep horizontal ramus of the dentary. The new specimen is about 25% smaller than the holotype and there is not a diastema between the canine and postcanine teeth, indicating its juvenile condition. Based on both known specimens of P. brasiliensis, a discussion on tooth replacement is presented, showing that the adult individual has more postcanine tooth morphotypes than the juvenile one.

Keywords:

• Cynodontia
• Prozostrodontia
• lower jaw
• Hyperodapedon AZ

Acknowledgements

We are thankful to Marchezan family for his kindness permitting our fieldwork on his property. We are grateful to the group of the palaeontologists and students of the Universidade Federal de Santa Maria (UFSM) and Universidade Luterana do Brasil (ULBRA) for their assistance during the excavation of the block containing the specimen here described. We also thank Flavio Pretto (CAPPA) for the preparation of the new specimen, Leonardo Kerber (CAPPA) and Rodrigo Müller (CAPPA) for comments and photograph assistance of the new specimen, and Luiz Flavio Lopes (UFRGS) for photograph assistance of the Prozostrodon holotype. We thank the reviewers Téo V. Oliveira, Cesar L. Schultz, and the Editor in Chief Gareth Dyke for comments and suggestions that greatly improve the MS.