Functional morphology and palaeobiology of the pliocene rodent Actenomys (Caviomorpha : Octodontidae): the evolution to a subterranean mode of life

The Pliocene caviomorph rodent Actenomys has long been recognized as an ear ly fossorial representative of the subfamily Ctenomyinae (Octodontidae), wh ich includes one living genus, Ctenomys (tuco-tucos), and several species w idely distributed in South America. To assess the degree of specialization for digging in Actenomys, we performed morphological comparisons with Other octodontid genera (Octodon, Slalacapus, and Ctenomys) of known mode of lif e and behaviour. As a whole, our results indicate that, in terms of morphol ogical specializations for digging, Actenomys occupies an intermediate posi tion between Octodon, a generalized semi-fossorial rodent which forages abo ve ground, and Ctenomys-Spalacopus, two highly specialized subterranean for ms. The position of the deltoid process (humerus) and the length of the ole cranon process (ulna)-two traits which affect the out-forces exerted by sev eral forelimb muscles-were in Actenomys intermediate between non-subterrane an and subterranean taxa. The skull, particularly the rostrum, appears to b e strong, and sagittal and nuchal crests are well marked. The zygomatic arc hes are as flared as those of the generalized Octodon. Notably, the upper a nd lower incisors of Actenomys were extremely procumbent, as in many highly specialized chisel-tooth digging species. Based upon the stratigraphic and sedimentological analysis of the palaeosoils containing its fossil remains , we suggest that Actenomys lived in an environment of moderate to hig prim ary productivity. The texture of the palaeosoils indicate that theywere har d and highly cohesive. This situation, which contrasts with that observed i n living Ctenomyinae, has relevant implications for burrowing cost. The int egration of morphological and palaeoenvironmental data allowed testing of h ypotheses about the palaeobiological attributes of this ancestral ctenomyin e. (C) 2000 The Linnean Society of London.