L'évolution de la pentalophodontie chez les rongeurs caviomorphes (Mammalia, Rodentia)

Abstract

The origin of Caviomorph rodents whose oldestrepresentatives are first recorded from the Late Oligocene of South America is still a matter of debate. This debate rests in a large part on the homology of the molar cusps and on the polarity of some molar structures. A detailed analysis of these structures and the reevaluation of former data, as compared with those relative to other fossil Hystricomorph rodents, indicate that the three genera Branisamys0Incamys-Sallamys from the Bolivian Deseadan (late Oligocene?) illustrate three successive stades of a morphocline which leads from a pentalophodont condition to a tetralophodont one, considered here to be derived for these taxa. The third crest of the tetralophodont forms is neither the metaloph, as suggested by Wood, nor the mesoloph, as suggested by Hoffstetter and Lavocat, but corresponds to a distinct crest resulting from the fusion of these two structures. In our opinion, this structural rearrangement of the occlusal structures is the consequence of the development, in these forms, of an asymmetric hypsodonty of the upper molars, similar to the one which occurs also in some other groups of Hystricomorphs, like the Ctenodactylidae and the Theridomyidae. The mode of origination of the mesoloph among these Caviomorphs seems to have been distinct from the one illustrated by their african vicariant, the Phiomyidae, and by the Theridomyidae. Phylogenetic implications of this new interpretation indicate that in the case of an african origin of the Caviomorphs, the origination must date from a period of time during which the african ancestors still retained a primitive tetralophodont structure, which must have been earlier than late Eocene. In such a case, one would have to admit an unknown land area where these earliest Caviomorphs have evolved, because they do not appear in South America before the Deseadean (Late Oligocene?). These problems of homology and polarity of dental structures can also be investigated from the examination of the dental development in the extant Caviomorphs. Morphogenetic studies would confirm or invalidate these interpretations, based only on fossils.