Functional adaptations of the postcranial skeleton of two miocene borhyaenoids (mammalia, metatheria), Borhyaena and Prothylacinus, from South America

Two Santacrucian borhyaenoids, Borhyaena tuberata and Prothylacinus patagonicus, are analyzed from a functional-adaptive perspective. Seven extant placental and marsupial models are examined in order to interpret the locomotor adaptations of the two fossils. These carnivorous models are characterized by various hunting types and locomotor habits, and the association of their skeletal adaptive features with diet, substrate preference, and locomotor performance permits a functional interpretation of the postcranium of Borhyaena and Prothylacinus. The analysis shows that the forelimb of Prothylacinus is modified to provide strength and flexibility for controlled climbing. This taxon exhibits serniplantigrade fore- and hind feet. Its vertebral column was flexible, and the hindlimb suggests an active predatory mode of hunting. The tail was muscular, heavy, and was probably used as a balancing organ. By comparison, the forelimb of Borhyaena indicates a more terrestrial mode of life, with a digitigrade forefoot, and more parasagittal movements. The tail was lighter and less muscular than in Prothylacinus. Both fossils are characterized by a powerful neck musculature related to predatory habits.