Evolutionary contexts

Abstract

In order to understand the timing of metatherian radiations, adaptations, and extinctions, it is important to review the variety of strongly interrelated contexts that defined them. (1) During the Cenozoic Era, global climates shifted from Greenhouse to Icehouse conditions; this major change was quite obvious by the Eocene–Oligocene boundary (ca. 33 Ma). Other large-scale changes also occurred prior and subsequent to this shift: hyperthermal events, as the Paleocene−Eocene Thermal Maximum, or cooling phases, such as that triggered by the late Miocene closing of the Panama isthmus. (2) Many elements precursor to the Cenozoic South American ecosystems were already in place by the late Mesozoic Era. By the Paleocene−early Eocene, several of the most important types (e.g., Neotropical forest, broad-leaved forest types) had developed their modern versions, as is the case of the. Grasslands seem to have been established, at least in southern South America, by the late Oligocene. (3) A recent biogeographical review supports the proposal that the southernmost tip of South America (the Andean Region) belongs to a distinct biogeographical unit, the Austral Kingdom. In turn, most of the remaining areas of South America, as well as southernmost North America and the Caribbean, comprise the Neotropical Region of the Holotropical Kingdom. South America’s Arid Diagonal is the great biogeographic divide between the Neotropical Region and the Andean Region. The distribution of this arid-semiarid belt, originally placed in much of southwestern Gondwana, closely matches the distribution of the Subtropical Seasonal Dry climatic belt since early Mesozoic times. (4) Reinforcing this climatic divide of the continent, paleogeographic reconstructions of South America suggest that the continent was split into northern and southern portions by means of epeiric seas due to marine transgressions. In southernmost South America, the paleogeography resulting from marine transgressions led to a very complex, almost archipelagic continental configuration. (5) At least six successive phases can be recognized in the evolution of Mesozoic–Cenozoic South American mammals: Early Gondwanian, Late Gondwanian, Early South American, Late South American, Interamerican, and Hypoamerican.