A new nomenclatorial system for the study of sigmodontine molars.

F. BARBIERE; P.E. ORTIZ; R. MARTIN; C. RONEZ; U.F.J. PARDIÑAS

Jornada; XXXI Jornadas Argentinas de Mastozoología; 2018

Sociedad Argentina para el Estudio de los Mamíferos

We propose a new nomenclatorial system describing and interpreting the occlusal pattern in sigmodontine molars. The proposal is based on the hypothesis that the occlusal pattern is best understood as the repetition of a single basic cuspal area. Labial cuspal areas in upper molars and lingual cuspal areas in lower molars have the same underlying morphology as cuspal areas on the opposite side, but rotated and forming mirror images. The direction of rotation is clockwise on upper left and lower right molars, and counterclockwise on upper right and lower left molars. The new nomenclatorial system, developed on the base of the study of 59 genera belonging to all the tribes of sigmodontines, has many advantages over previous ones, in part because cuspal morphology is interpreted within a consistent theory including cuspidogenesis, ontogeny and evolutionary history. Our system also shows that a number of structures interpreted as singular in previous nomenclatures (e.g., median murid, mesolophid) can instead be hybrid in nature, representing developmental fusion of two or more distinct elements. Because the occlusal pattern is significantly modified by modest wear, especially in clades that have developed mesodont or hypsodont molars, the evaluation of occlusal patterns for phylogenetic information should be limited to unworn or slightly worn molars. We provide an evolutionary explanation of why certain parts of specific molars (e.g., conulids on m1) 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. A few examples are provided for demonstrating the utility of the new system.