Functional morphology of the forelimb of Early Miocene caviomorph rodents from Patagonia

Abstract

Caviomorph rodents (New World Hystricognathi) are represented in the Santa Cruz Formation of Patagonia (Early Miocene, Santacrucian) by four superfamilies. From this unit, excellent cranial and associated postcranial remains of Neoreomys, Eocardia, Schistomys (Cavioidea), Perimys (Chinchilloidea) and Steiromys (Erethizontoidea) are known. To interpret their use of substrate, a comparative description of limb bones (scapula, humerus, radius and ulna) of the five genera was performed within a taxonomic and ecologically diverse sample of extant rodents. Using palaeobiological inferences based on the habits and functional morphology of members of the extant sample, hypotheses were generated for the use of substrate by the Santacrucian taxa. Neoreomys would have had a marked flexo-extension capacity of the humerus and a moderate flexion of the manus. This genus and Eocardia would have had a complete and stabilized forearm extension, with wide stride and moderate extension of the forearm for the latter. Schistomys presents features similar to Eocardia. Perimys would have had strong external rotation of the humerus, moderate flexion of the manus and moderate, complete and stabilized extension of the forearm. Steiromys would have had good pronation/supination capacity, strong internal rotation of the humerus and flexion of the forearm and manus. Neoreomys would have been ambulatory and an occasional runner. Eocardia and Schistomys would have been good runners. Perimys would have been a digger and Steiromys a climber. This morphological disparity reflects a high ecological diversity, compatible with differential use of space during the Santacrucian.