The Evolution of the Cenozoic Terrestrial Mammalian Predator Guild in South America: Competition or Replacement?

Abstract

South America was isolated from other continents during most of the Cenozoic, developing a very particular mammalian fauna. In contrast to North America, Europe, Asia, Africa, and even Australia, the carnivore adaptative zone in South America was filled by crocodiles (Sebecidae), large snakes (Madtsoiidae), large birds (Phorusrhacidae), and metatherian mammals (Sparassodonta). Sparassodonta was diverse and comprised a wide range of sizes (≈ 2-50 kg) and food habits. This diversity decreased towards the late Miocene (Huayquerian) and the group became extinct in the middle Pliocene (≈ 3 Ma, Chapadmalalan). Several authors have suggested that the cause of this decline and extinction was the ingression of placental Carnivores to South America (about 6-7 Ma ago), because they putatively competed with the Sparassodonta. This hypothesis was criticized in recent years. With the intention of testing the hypothesis of ?competitive displacement?, we review the fossil record of South American Sparassodonta and Carnivora, collect data about diversity and first and last appearances, and estimate size and diet of the taxa considered here. The diversity of Sparassodonta is low relative to that of Carnivora throughout the Cenozoic. The highest number is found in the early Miocene (Santacrucian), with eleven species. The fossil record shows overlap of groups during the late Miocene-middle Pliocene, and the Sparassodonta?s richness curve declines beginning with the first record of Carnivora during the Huayquerian. Despite this overlap, carnivores were represented by four species or fewer during the late Miocene-Pliocene, and their diversity reached approximately 20 species only in the early Pleistocene (Ensenadan). Moreover, Carnivora was first represented by small-sized, omnivorous species, with large omnivores first appearing in the Chapadmalalan. During this period, Sparassodonta was represented by large and small hypercarnivores and a single large omnivore species. These data suggest that factors other than competitive displacement may have caused the extinction of the Sparassodonta.