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A History of Savanna Vertebrates in the New World. Part I: North America

1977; Annual Reviews; Volume: 8; Issue: 1 Linguagem: Inglês

10.1146/annurev.es.08.110177.002035

2330-1902

S. David Webb,

Animal Ecology and Behavior Studies

Early in the Cenozoic Era North America was covered almost entirely by forest, predominantly of a mixed mesophytic nature. During the mid-Cenozoic, however, an increasing proportion of the land opened up, forests giving way to woodland savanna, thorn forest, and thorn scrub. By the Late Cenozoic forested areas had decreased still further and much of the savanna was being replaced by grassland steppe and even desert. Today over 25% ofthe natural vegetation of North America consists of such nonforest biomes. Evolution of an open-country fauna naturally followed these growing opportunities, with the locus of its expansion generally near the center of the continent in the zone of prevailing westerlies. A remarkably similar series of changes affected the fauna of temperate South America during about the same 4O-million-year interval. Yet these two American faunas remained totally separated during most of the Cenozoic. Not only were the two continents physically separate, but also the centers of open-country evolution were located deep within each continent. The dramatic climax of this history came about three million years ago, when North and South America were connected by way of the isthmian land bridge. Surprisingly, the biota that then began to interchange through the American tropics included a major component of savanna taxa. The purpose of this review is to elucidate the evolutionary history of open-country biota, and particularly the vertebrates in that biota, in North and South America. A fundamental assumption here is that the New World record of fossil vertebrates is sufficient to reveal much of this history during successive ages of the Cenozoic. It is believed that it can indicate major adaptive tendencies, relative abundance of various