CONICET | Buscador de Institutos y Recursos Humanos

We propose a new nomenclatural system describing and interpreting the occlusal pattern in Sigmodontine (Rodentia, Cricetidae) molars. The proposal is based on the hypothesis that the occlusal pattern is best understood as the repetition of a single basic cuspal area. Cuspal areas located on one side of the molar are a mirror image, partly rotated, of the cuspal areas located on the other side. The new nomenclatural system has many advantages over previous ones, in part because occlusal morphology is interpreted within a consistent theory including cuspidogenesis and ontogeny. Our system also shows that a number of structures interpreted as singular in previous nomenclatures (e.g., median murid, mesolophid) are in fact hybrid in nature (i.e., do not correspond to only one element of the oclusal surface). Furthermore, we propose a hypothesis of primary homology for the different structures recognized in our system, providing it a topological purpose support rather than the topographical assessment that transpired previous proposals. Because the occlusal pattern is significantly modified by modest wear, especially in clades that have developed mesodont or hypsodont molars (e.g., Phyllotini, Reithrodontini), the evaluation of occlusal patterns for phylogenetic information should be focused on unworn or slightly worn molars. We provide an evolutionary explanation why certain parts of specific molars (e.g., conulids on the first lower molar) do not seem to follow the logic of the new system, and recognize that more work may need to be accomplished in order to develop a universal system applicable to all cricetid rodents. Examples are provided demonstrating the utility of the new system.