The Cenozoic Radiation of Mammals

A review of Cenozoic vertebrate ichnology may serve as a starting point for in-depth ichnologic analyses, literature compilations, and evaluation of the biologic, ethologic, and ecologic information provided by footprints within the framework of the evolutionary history of their producers. Identifying the problems and promises of the mammalian paleoichnologic record in each continent will help to develop a research program that will permit global comparisons. In this contribution we summarize Mesozoic and Cenozoic footprints attributed to mammals, and specifically address the evolutionary implications of the Cenozoic ichnologic record of South American mammals. One of the major features of the continental Cenozoic faunas of South America is the presence of native lineages of mammals, for the most part having no counterpart in living faunas. The Eocene and Oligocene ichnologic assemblages mostly represent forms of uncertain affinity, including small caviomorph rodents or typothere notoungulates, medium-size undetermined tetradactyl mammals, small toxodontid notoungulates or macraucheniid litopterns, large toxodontid notoungulates, and large dinomyids caviomorph rodents. Footprints assigned to ground sloth, macrauchueniiid, and proterotheriid litopterns, caviid caviomorph rodents and hegetotheriid notoungulates are recorded for the first time in the Miocene. The late Miocene-Pliocene and late Pleistocene assemblages include footprints of both native South American mammals and North American lineages that arrived as part of the Great American Biotic Interchange. Footprints of native South American lineages recorded for the first time include a variety of ground sloths, glyptodonts, hydrochoerid caviomorph rodents, and carnivorous marsupials. The North American lineages are equids, gomphotheriids, camelids, cervids, and several carnivoran taxa such as felids, mustelids, and bears. The ichnologic records of these unique lineages provide valuable paleobiologic information on locomotion capacity, serve as proxy for body size, and are useful tools for paleocommunity analysis and paleoenvironmental interpretations. Some remarkable cases include the locomotion capacity of megatheres discussed based on the record of their trackways, and the record of peculiar rodent-like footprints that could expand the record of caviomorph rodents to the early Eocene.