The evolution of bunodont South American native ungulates

GELFO J. N.

Origins and Evolution of Cenozoic South American Mammals

SPRINGER

Año: 2017;

During the Cenozoic of South America a wide radiation of eutherian mammals led to the development of several endemic ungulates orders (e.g. Notoungulata, Litopterna, Astrapotheria, Xenungulata), which have converged on artiodactyls and perissodactyls in several morphological traits. Although the monophyly of South American native ungulates (SANU) was never tested, they were joined together in the Meridiungulata (McKenna, 1975), one of the divisions of the Grandorder Ungulata together with the Eparctocyona and Altungulata (McKenna y Bell, 1997). Since molecular phylogenies have presented Ungulata as a polyphyletic group by dividing it between Laurasiatheria and Afrotheria (Murphy et al., 2001; Waddell et al., 2001; Springer et al., 2004), South American endemic ungulates have been ignored in most higher rank classifications of placental mammals and have not been included in recent analyses. It has been suggested that most of South American native ungulates evolved in situ from a hypothetical eutherian mammal, closely related to the North American ?Condylarthra? or ?archaic ungulates? from the late Cretaceous and early Paleocene (Simpson, 1948; Cifelli, 1983a, 1993). At least two South American groups, Kollpaniinae and Didolodontidae, have been considered condylarths and associated, by several authors, with North American forms. Despite that Kollpaniinae and Didolodontidae could be distinguished by the presence of several evolutionary novelties (Gelfo, 2004, 2006), they share a low crowned and bunodont dentition, which was usually considered per se as evidence of a primitive evolutionary condition. In contrast, most of the South American native ungulates developed buno-lophodont, buno-selenodont, lophodont or selenodont dentitions, together with an important trend to increase crown height starting with middle Eocene forms. Most native ungulates show a derived dentition with respect to this alleged primitive bunodont pattern but two groups of litopterns, the Protolipterniidae and Megadolodinae, retained brachydont and bunodont cheek teeth, combined with a derived postcranial anatomy. These groups highlight the presence of different evolutionary trends between the locomotor (postcranial) and digestive system (dentition). Native bunodont forms were never as diverse and numerous as other native ungulates. Despite they moderate diversity compared to other groups, they are key-stone to understand the evolutionary relationships and the origin of SANU. Particularly, because all morphological type developed by SANU evolved from a primitive bunodont type. The persistence of several bunodont lineages through the Cenozoic, suggest that they are potencial tools for paleoecological inferences. The main scope of this chapter is to review all the bunodont SANU history, discussing actual taxonomic and paleobiological consensus, principal systematic problems and future investigation lines.