Pathways and Level of Morphological Adaptations in Modern Diatomyidae and Ctenodactylidae (Rodentia)

Abstract

Based on published and original data, several morphofunctional systems were compared in the extant species of Diatomyidae and Ctenodactylidae, both families being considered as sister taxa. The subcutaneous and auricular muscles, otic capsule, jaw apparatus, and distal limbs were examined. These groups were shown to differ significantly in both the level and the direction of the morphofunctional transformations of the above systems. Ctenodactylids are a much more specialized group than Laonastes. They have an otic capsule, a jaw apparatus, and distal limb sections that reach the maximum level of morphological and functional specialization in the rodents, whereas in Laonastes, they correspond approximately to the average level of their development. Both groups are characterized by different pathways of morphological transformations of all systems considered, even those of them (jaw apparatus and limbs) that are associated with adaptations to similar ecological conditions, i.e., life on stones and herbivory. In Laonastes, the structure of the above systems retains archaic characteristics that are combined with features specific only to this group. In gundis, the direction of morphological transformations is completely specific with regard to some of the parameters, while in other respects it corresponds to the trends typical of hystricognathous rodents, this having led to a large number of structural parallelisms. In the structure of each morphological system examined, synapomorphies that support the monophyly of Ctenohystrica were revealed. There is no single-valued morphological evidence for the close relationship between Ctenodactylidae and Laonastes relative to Hystricognathi, although in the structure of almost all of the systems examined there are common features distinguishing these taxa from other rodents, in particular from Hystricognathi. Characters reflecting the pattern of differentiation of the subcutaneous muscle and the features of mastoid pneumatization can be regarded as the most significant for assessing the phylogenetic relationships of Diatomyidae, Ctenodactylidae, and Hystricognathi. However, for a more reliable assessment of the relationships based on morphological data, more extensive material is needed to cover the diversity of the structures considered in hystricognathous rodents.