The lower jaw and dentition of the traversodontid Exaeretodon riograndensis Abdala, Barberena & Dornelles, from the Brazilian Triassic (Santa Maria 2 Sequence, Hyperodapedon Assemblage Zone)

Abstract

Exaeretodon riograndensis Abdala, Barberena, & Dornelles, 2002 is the most abundant traversodontid preserved in the basal Santa Maria 2 Sequence (Hyperodapedon Assemblage Zone), southern Brazil and is closely related to Exaeretodon argentinus Cabrera, 1943 from the Ischigualasto Formation, Argentina. Previous anatomical studies of E. riograndensis have focused mainly on cranial material and little is known about the morphology of its lower dentition or postdentary bones. We describe the first fairly complete postdentary series of the mandible of E. riograndensis and provide additional information on its lower dentition. The postdentary bones of E. riograndensis include a complex coronoid, an angular with a delicate reflected lamina and a stout retroarticular process of the articular, contrasting with the morphology reconstructed for Argentinean specimens, which possess a small retroarticular process. Apart from that, the postdentary bones do not differ significantly from those known for E. argentinus, a fact expected due to the great similarity between other skeletal features of these species. Furthermore, the lower postcanines of E. riograndensis have virtually the same structure as those of E. argentinus, with an approximate quadrangular shape in occlusal view. Moreover, the transverse cusp row is placed anteriorly and comprises a lingual and a buccal cusp, and the occlusal basin delimited by the four main cusps is relatively deep. The new material does not add any taxonomically diagnostic features to E. riograndensis. However, the fossils greatly improve our understanding of the anatomy of the Brazilian species.

巴西三叠纪(Santa Maria 2 地层序列, Hyperodapedon组合带)的traversodontid Exaeretodon Riograndensis Abdala, Barberena & Dornelles 的下颌 和牙列

Exaeretodon riograndensis Abdala, Barberena & Dornelles, 2002 是保存在巴西南部Santa Maria 2 地层基部(Hyperodapedon 组合带0的最丰富的traversodontid, 与阿根廷Ischigualasto组的Exaeretodon argentinus Cabrera, 1943关系较近。 对E. riograndensis 进行的解剖学研究主要注重于颅骨材料,而对它的下齿列或齿后骨骼的形态所知甚少。我们描述了E. riograndensis下颚的第一个较完整的齿后骨骼系列,提供了更多的关于其下齿列的资料。E. riograndensis 的齿后骨骼包括一个复合冠突、一个具细小反曲椎板的角和一个大的反关节突起,这与阿根廷标本的重建形态形成反差,后者的反关节突起小。除此以外,齿后骨骼与E. argentinus的已知齿后骨骼并没有很大差别;这点在意料之内,因为这些种之间的其它骨骼很相似。进一步说,E. riograndensis 的下后犬齿与E. argentines 的在结构上几乎完全一样,它们的咬合视都呈近四边形。另外,横向齿尖排位于前部,由一个舌齿尖和一个颊齿尖组成;由四个主齿尖为界的咬合面较深。新材料并没有增加E. roigrandensis 的分类鉴定特征。 然而,这些化石大大增进了我们对巴西种解剖学方面的了解。

Key words:

• Exaeretodon riograndensis
• Traversodontidae
• Upper Triassic
• Santa Maria Supersequence
• postdentary bones

Acknowledgements

We thank Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Programa de Pós-Graduação em Geociências/UFRGS (PPGGEO/UFRGS) for the Financial Grant, and Dr Maria Claudia Malabarba, Dr Alejandro Kramarz and Dr Marcelo Reguero for access to specimens from the MCP, MACN and MLP collections, respectively. We also thank Dr Carolina Saldanha Scherer for the photos of MLP specimens and Mr Luís Flávio Lopes for the photos of UFRGS-PV-1177-T. Tomaz Panceri Melo provided some insightful comments that improved the discussion. Dr Stephen McLoughlin and two anonymous reviewers are thanked for valuable suggestions and comments that greatly improved the content of this paper.