Abstract
Recent geological studies demonstrate that the Isthmus of Panama emerged some 10 m.y. earlier than previously assumed. Although absent today in Panama, Central American savanna environments likely developed in connection with the onset of Northern Hemisphere glaciations. As is widely recognized, most of the mammals crossing the isthmus since 2.5 Ma lived in savannas. Could climate-induced vegetational changes across Panama explain the delayed migration of mammals, rather than terrestrial connectivity? We investigate the congruence between cross-continental mammal migration and climate change through analysis of fossil data and molecular phylogenies. Evidence from fossil findings shows that the vast majority of mammals crossed between South and North America after ca. 3 Ma. By contrast, dated mammal phylogenies suggest that migration events started somewhat earlier, ca. 4–3 Ma, but allowing for biases toward greater ages of molecular than geologic dating and uncertainties in the former, we consider this age range not to be significantly earlier than 3 Ma. We conclude that savanna-like environments developed in response to the vast Laurentide ice sheet at the first Quaternary glaciation triggered the initiation of the Great American Biotic Interchange in mammals.
Highlights
Two virtually simultaneous events occurred at the end of the Pliocene Epoch, ca. 2.5 Ma: the onset of recurring ice sheets covering North America and Fennoscandia (e.g., Haug et al, 1999; Jansen and Sjøholm, 1991; Shackleton et al, 1984) and the intensification of an interchange of mammals across the Isthmus of Panama between North and South America (Fig. 1A)—the Great American Biotic Interchange (GABI) (e.g., Simpson, 1980; Stehli and Webb, 1985)
The “vast majority” (Webb, 1985, p. 378) or “overwhelming majority” (Jackson and O’Dea, 2013, p. 786) of the GABI participants have been assumed to be savanna-adapted mammals (e.g., Leigh et al, 2014; Marshall, 1985; Stehli and Webb, 1985; Webb, 1978, 1991, 2006; Woodburne, 2010). Because both geological (e.g., Montes et al, 2012a, 2012b, 2015) and biological (e.g., Bacon et al, 2015a, 2015b) evidence suggest that the isthmus had emerged long before 3 Ma, another mechanism needs to be sought for explaining the GABI in mammals
The most commonly cited cases based on fossil data, referred to as “heralds” by Webb (1976), include three genera of ground sloths, Megalonyx, Pliometanastes, and Thinobastides, that moved from South to North America at 9.5–9 Ma (Marshall et al, 1979), and the procyonid carnivore Cyonasua that crossed to South America at ca. 7 Ma (e.g., Marshall, 1985, 1988; Marshall et al, 1979; Webb, 1978, 1985; Woodburne, 2010)
Summary
Two virtually simultaneous events occurred at the end of the Pliocene Epoch, ca. 2.5 Ma: the onset of recurring ice sheets covering North America and Fennoscandia (e.g., Haug et al, 1999; Jansen and Sjøholm, 1991; Shackleton et al, 1984) and the intensification of an interchange of mammals across the Isthmus of Panama between North and South America (Fig. 1A)—the Great American Biotic Interchange (GABI) (e.g., Simpson, 1980; Stehli and Webb, 1985). 2.5 Ma: the onset of recurring ice sheets covering North America and Fennoscandia (e.g., Haug et al, 1999; Jansen and Sjøholm, 1991; Shackleton et al, 1984) and the intensification of an interchange of mammals across the Isthmus of Panama between North and South America (Fig. 1A)—the Great American Biotic Interchange (GABI) (e.g., Simpson, 1980; Stehli and Webb, 1985).
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