Morphology of the lower deciduous premolars of South American hystricomorph rodents and age of the Octodontoidea

Abstract

We analyse the crest homologies of lower deciduous premolars (Dp4) of South American caviomorphs in a comparative context including other hystricomorphs and under a dynamic topological criterion. An hexalophodont pattern in which the three anteriormost crests are assumed to be the anterolophid, metalophulid I and metalophulid II, respectively, is proposed as ancestral morphology. Simplified pentalophodont morphologies would have resulted from loss of the metalophulid I, and distinct transformation pathways are recognised especially for Octodontoidea and Cavioidea. A basal octodontoid morphological pattern can be recognised in the fossil record from as early as the middle Eocene, supporting that an initial divergence among major clades of caviomorphs had already occurred by this time. Programs for identifying potentially useful dental characters in phylogeny and taxonomy of caviomorphs need to be revised. Interpreting lophids as dynamic components in the morpho-functional variation of molars, rather than as static landmarks, is central for understanding dental evolution in these rodents.

Keywords:

• Caviomorphs
• dental homologies
• hexalophodonty
• octodontoids

Acknowledgements

The authors thank F Anaya, J Oliveira, M de Vivo, A Kramarz, R Martínez, C Montalvo, E Tonni, M Reguero, G Vucetich, E Ruigómez, M Pérez, J Patton, E Lacey, N Sánchez for granting access to materials under their care. A Candela and M Pérez assisted with the identification of some materials. Drawings are by F Ballejo and AIO. We especially acknowledge the thoughtful suggestions of two anonymous reviewers which greatly improved the ms.