Morphofunctional Transformations of the Jaw Muscles in Rodent Evolution

Abstract

The paper summarizes results of long-term research on the jaw apparatus of modern rodents. The studied material includes original data on 22 families of the order Rodentia. The structure of the jaw muscles and the pathways of transformations during the evolution of the order, as well as the functional and phylogenetic aspects of its diversity in this group, are considered based on the concept of the jaw apparatus as an integral system. The structure of the jaw muscles has been studied in detail. It is shown that, despite their external diversity, aponeurotic carcasses have the same structure type; this can serve as a reliable basis for the identification and homologization of the elements of the internal muscle differentiation. The morphofunctional specialization of the jaw muscles is discussed. The morphotypes of the jaw apparatus, which reflect its adaptability to crushing and to vertical cutting are highlighted. Particular attention is paid to the description of two directions of specialization in the longitudinal grinding that is typical only for rodents. The first variant is manifested in hypertrophy of the anterior parts of the masseter, simplification of its differentiation, and a more horizontal orientation of its lateral and medial portions against the background of weakening of the temporal muscle. The second variant is associated with enhancement of internal differentiation of the masseter due to a special transformation of the lower jaw: an increase in its height, a shift of the angular process up to the dentition level, and a highly raised articular process. The correspondence of the types of jaw apparatus to the basal branches of Rodentia phylogenetic tree has been revealed. It is assumed that the typological diversity of this system evolved at the early stages of adaptive radiation of rodents. The similarity of these types within the phyletic lines results from the relatedness of the species. The functional specialization of the jaw apparatus within the basal groups was based on their structural type. It occurred mainly due to changes in the mandible and zygomatic arch and was expressed in similar spectra of its morphofunctional transformations in different rodent taxa.