The geometry of marmot (Rodentia: Sciuridae) mandible: phylogeny and patterns of morphological evolution [Géométrie de la mandibule de marmotte : phylogénie et canevas de l'évolution morphologique].

Résumé

Marmots have a prominent role in the study of mammalian social evolution, but only recently has their systematics received the attention it deserves if sociobiological studies are to be placed in a phylogenetic context. Moreover, sciurid morphology offers a fascinating model to test the congruence between morphological change and phylogeny as sciurid skeletal characters are considered to be inclined to convergence. In spite of this, no morphological study involving all marmot species has ever been undertaken. Therefore, geometric morphometric techniques were applied in a comparative study of the marmot mandible. The adults of all 14 living marmot species were compared and mean shapes of their mandible used to investigate the morphological evolution of the genus Marmota. Three major trends can be outlined. 1) The phylogenetic signal in the variation of landmark geometry, which describes mandible morphology, seems to account for the shape differences at intermediate taxonomic levels. The subgenera Marmota and Petromarmota, recently proposed on the basis of mitochondrial cyt b sequence, receive support from mandible morphology. Moreover, when other sciurid genera are included in the analysis, the monophyly of the genus Marmota and that of the tribe Marmotini (i.e., marmots, prairie dogs and ground squirrels) are strengthened by the morphological data. 2) The marmotine mandible may have evolved as a mosaic of characters and does not show convergence determined by size similarities. 3) Allopatric speciation in peripheral isolates may have acted as a powerful force for modelling shape. This is strongly suggested by the peculiar mandible of M. vancouverensis and, to a lesser degree, by that of M. olympus, both thought to have originated as isolated populations in Pleistocene ice-free refugia.