Published online by Cambridge University Press: 05 August 2015
The various mammalian groups, rodents in particular, have developed a wide trophic range shown first and foremost by a significant morphological differentiation of the masticatory apparatus (skull, mandible, teeth and musculature). The mammalian masticatory apparatus is a highly plastic region of the skull, which explains why the associated features are frequently used as diagnostic phylogenetic attributes. In the first place, studying this masticatory apparatus requires precise knowledge of the extent to which its associated morphological features vary. It is only afterwards that one can focus on the factors most likely to have influenced its morphological evolution. Among mammals, the radiation of rodents constitutes a special case. Rodents are considered to be one of the great successful groups in the evolutionary history of mammals, and few mammal clades have been studied as extensively as the order Rodentia. The modern representatives of the order, around 2200 species, are spread across every continent barring Antarctica (Wilson and Reeder, 2005). Their fossil record is very rich, which makes rodents an unavoidable biostratigraphic tool for Paleogene and Neogene deposits.
Justifying the choice of a “rodent” model in a study of evolutionary biology is therefore easy, as it allows the integration of study results in such varied fields as palaeontology, anatomy, ecology or development. Although intensively studied, the phylogenetic relationships between the different groups of rodents have been a matter of debate for over 150 years. While exceptional for an intense diversification of lineages, all rodents share one of the most extreme specializations of the masticatory apparatus characterized by the reduction of the upper and lower incisor series to a single pair. This diprotodonty (i.e. single pairs of upper and lower incisors highly specialized for gnawing) is a hallmark of the rodent masticatory apparatus and is accompanied by a reduction of the number of cheek teeth in association with the development of anteroposterior movements of the mandible for gnawing and chewing (Becht, 1953).