Evolutionary transformation of the subcutaneous muscle in rodents of Ctenohystrica (Rodentia: Diatomyidae, Ctenodactylidae, and Hystricognathi)

The present study examined the subcutaneous muscle (m. cutaneus trunci) in a relict species Laonastes aenigmamus (Diatomyidae) and in two ctenodactylids (Ctenodactylidae: Ctenodactylus gundi and Pectinator spekei ), which are closely related to Laonastes according to current phylogenetic hypotheses. The comparative morphological analysis also included four representatives of Hystricognathi: Echimys sp., Proechimys sp., Cercomys cunicularis , and Hystrix indica . Laonastes and ctenodactylids have a number of common ancestral traits in the morphology of m. cutaneus trunci. This suggests that they are probably related to the ancestral forms of Ctenohystrica. However, the structural similarity of the subcutaneous muscle in Diatomyidae and Ctenodactylidae is based not only on plesiomorphic (presence of the medial head of m. cutaneus trunci), but also on an apomorphic (the development of the thoracoabdominal portion) feature which is common to both branches of Ctenohystrica (Ctenodactiloidea and Hystricognathi). The results of the comparative analysis suggest a new scenario for the evolutionary transformations of m. cutaneus trunci within the clade Ctenohystrica. This scenario differs from earlier suggestions and proposes initial differentiation of the subcutaneous muscle and possibly its secondary simplification, but on a new basis. On this new basis a further secondary increase in complexity of subcutaneous muscle appears possible in response to various adaptations in hystricognaths.