Hypnomys morpheus is a giant endemic dormouse from the Pleistocene deposits of Mallorca and Menorca (Balearic Islands, Spain). The present paper aims to interpret the morphological divergence between the mandibles of Hypnomys and of its extant relative Eliomys, the outline of the mandible being used as a marker of the morphological divergence. By comparison with the mandible of Eliomys, the more massive mandible of Hypnomys has recorded an ecological shift of the insular lineage towards a more abrasive diet, including hard vegetable matter, and a different niche. A microwear analysis of the teeth of Hypnomys was simultaneously performed as it can shed light on the diet, and is independent from the comparison of the mandibles. Hypnomys possibly ate harder food items than Eliomys, and likely occupied most of the island environments. Hypnomys appears to have differentiated from its ancestral type toward a more generalized morphology because of the lack of competitors.

Key words: Rodentia, Gliridae, Hypnomys morpheus, Fourier analysis, microwear, morphological evolution, paleodiet, Balearic Islands, Spain.

Lionel Hautier [Lionel.Hautier@univ−montp2.fr], Laboratoire de Paléontologie, Institut des Sciences de l’Evolution, Université de Montpellier 2, UMR−CNRS 5554, CC 064; 2, Place Eugène Bataillon, F−34095 Montpellier Cedex 5, France; Pere Bover[pbover@amnh.org], Department of Mammalogy, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, USA; Josep Antoni Alcover [vieapba@uib.es], Institut Mediterrani d’Estudis Avançats (CSIC−UIB), Cta de Valldemossa km 7,5, 07122 Palma de Mallorca, Balearic Islands, Spain; Jacques Michaux [michaux@isem.univ−montp2.fr], Laboratoire EPHE de Paléontologie des Vertébrés, et Institut des Sciences de l'Evolution, Université de Montpellier 2, UMR−CNRS 5554, CC 064; 2, Place Eugène Bataillon, F−34095 Montpellier Cedex 5, France.