A new nomenclatural system for the study of sigmodontine rodent molars: First step towards an integrative phylogeny of fossil and living cricetids

Abstract

We propose a new nomenclatural system to study 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 on one side of the molar are a mirror image, partly rotated, of the cuspal areas on the other side. With this nomenclatural system, occlusal morphology is interpreted within a consistent theory including cuspidogenesis and ontogeny. Our system also shows that some structures in previous nomenclatures (e.g. median murid) do not correspond to just one element of the occlusal surface (i.e. hybrid structures). Furthermore, we propose a hypothesis of primary homology for the structures recognized in our system, providing a topological approach rather than the topographical assessment of previous proposals. The evaluation of occlusal patterns for phylogenetic information should be focused on unworn or slightly worn molars. We provide an evolutionary explanation for why certain parts of specific molars (e.g. conulids on the first lower molar) do not follow the logic of the new system, and recognize that more work may be needed to develop a universal system applicable to all cricetid rodents.