Paleogeographic Setting During The Beginning Of The Age Of Mammals

The evolution and dispersal of mammals during the early Cenozoic were strongly influenced by the positions of the continental plates, the connections among them, the amount and distribution of subaerial exposure, and the marine barriers separating or dividing continents. The salient aspects of paleogeography at that time summarized here are based primarily on McKenna (1972, 1975b, 1980a, 1983) and Smith et al. (1994).

At the end of the Cretaceous, a wide epicontinental sea extended between the Arctic Ocean and the western Atlantic, dividing North America into eastern and western landmasses (Fig. 1.8A). The western portion was joined to Asia across Beringia (site of the present-day Bering Strait), whereas the eastern part was more closely approximated to Greenland, which was close or joined to northwestern Europe. North America and South America were separated by a wide seaway that connected the Pacific and Atlantic oceans. During the Late Cretaceous and early Paleocene, an epeiric sea apparently divided South America into northern and southern faunal provinces, limiting faunal exchange between the two regions (Pascual et al., 1992; Wilson and Arens, 2001). The southern parts of South America and Australia were close to Antarctica but lacked subaerial connections to that continent. South America and Africa were much closer to each other than they are today, though still separated by a sizable marine barrier. A narrow seaway split northwestern Africa from the rest of that continent, and the Tethys Sea (predecessor of the Mediterranean) came between northern Africa and Europe, which consisted of several islands. The Tethys extended eastward, south of Asia, where it was continuous with the Indian Ocean. India had recently separated from Madagascar and begun its drift northward. The rest of Asia was a large landmass separated from Europe by an epicontinental seaway (the Obik Sea to the north and the Turgai Straits at the southern end), which joined the Arctic Ocean to the Tethys Sea. This was the pale-ogeographic setting at the beginning of the Age of Mammals. Interchange of land mammals between any of the landmasses separated by marine barriers could only have occurred by Simpson's sweepstakes dispersal (Simpson, 1953; McKenna, 1973).

By the end of the early Paleocene a major lowering of sea level was under way, exposing more extensive land areas. North America was now a single landmass, as the epicontinental sea had diminished to a narrow extension from the Caribbean northward to the middle of the continent. Land bridges joined North America to northern Europe and to Asia, allowing faunal exchange. The Eurasian epicontinental sea also receded, exposing land bridges or islands between Europe and western Asia (Iakovleva et al., 2001). India was almost halfway to its junction with Asia.

The brief interval of global warming at the beginning of the Eocene (the Initial Eocene Thermal Maximum) resulted in increased continental temperatures as well as surface warming of high-latitude oceans (Sloan and Thomas, 1998). These changes turned the high-latitude North Atlantic land bridge (and, to a lesser extent, the North Pacific Bering bridge) into a hospitable corridor for mammalian dispersal. Geophysical evidence in fact suggests the presence of two North Atlantic land bridges during the late Paleocene-early Eocene: the northern De Geer Route and the southern Thulean Route (Fig. 1.8B, numbers 2 and 3). The De Geer Route—which was probably farther south in the early Tertiary, near the present-day Arctic Circle—joined northern Scandinavia, Svalbard (including Spitsbergen), northern Greenland, and northern Canada in the region of Ellesmere Island, and could have served as a direct passage between northwestern Europe and the Western Interior of North America. The Thulean bridge would have connected the British Isles to Greenland via the Faeroe Islands and Iceland, a geothermal "hot spot" in the early Cenozoic (Knox, 1998).

Although little fossil evidence is known from along these proposed land bridges, Simpson (1947: 633) long ago established that the extent of exchange between Europe and North America indicated that these land masses were "zoo-geographically essentially a single region at this time." About 50-60% of early Eocene mammal genera from northwestern Europe are shared with western North America (Savage, 1971; McKenna, 1975b; Smith, 2000). In contrast, only one-third of earliest Eocene genera were shared by northern and southern Europe, suggesting that the continent was sporadically divided by some kind of barrier during the Paleo-gene, but whether it was geographic or climatic is unknown (Marandat, 1997). Ellesmere Island, which was within the Arctic Circle and at about the same latitude in the Eocene as it is today, has produced early-to-middle Eocene mammals and reptiles (crocodilians) that indicate a warm climate (Dawson et al., 1976; West et al., 1977; McKenna, 1980a). Several of the mammalian taxa are similar at the generic or family level to those found on both continents and suggest dispersal across Ellesmere in both directions (Eberle and McKenna, 2002). The effect of highly variable periods of daylight (and seasonal darkness) on the biota at such high latitudes remains problematic. By the middle Eocene (Lutetian), faunal disparities indicate that the opening of the North Atlantic by sea floor spreading had already interrupted the Euramerican land bridges.

The Bering land bridge (Beringia, Fig. 1.8B, number 1) seems to have been emergent throughout most of the Cenozoic (Marincovich and Gladenkov 1999). However, it was evidently at even higher latitude (about 75° N) during the late Paleocene and early Eocene than it is today and consequently may have acted as a filter rather than a corridor (McKenna, 2003). Nonetheless, similar taxa found on both continents at that time (e.g., arctostylopids, uintatheres, carpolestids, omomyids), many of which are unknown from Europe, indicate faunal exchange. A more southern bridge across the Aleutian area may have existed as well, but probably not before the middle Eocene (McKenna, 1983).

Europe continued to be separated from Asia for part of the early Cenozoic by a marine barrier consisting of the Obik Sea and, at the southern end, the Turgai Strait. Current evidence suggests, however, that occasional subaerial connections may have been present at the northern and southern ends (Fig. 1.8B, numbers 4 and 5), particularly around the Paleocene/Eocene boundary (Iakovleva et al.,

Paleogeographic Map Erectus

Fig. 1.8. (A) Paleogeography during the Late Cretaceous (Maastrichtian), about 70 million years ago. Shaded regions represent subaerial landmasses; white areas are oceans; lines show present-day coastlines. (B) Paleogeography during the early Eocene, about 53 million years ago. Numbered arrows indicate hypothesized dispersal routes during the Early Paleogene: 1, between Asia and North America via Bering land bridge; 2, De Geer route; 3, Thulean route; 4, between Asia and Europe at the northern end of the Obik Sea; 5, across the Turgai Strait; 6, probable sweepstakes dispersal between North and South America via Central America or perhaps a Caribbean archipelago; 7, between southern Europe and north Africa; 8, between South America and Antarctica; 9, between Antarctica and Australia. Some routes shown as marine barriers in this reconstruction might have been intermittently subaerial during the Early Cenozoic. (Modified from Smith et al., 1994.)

Fig. 1.8. (A) Paleogeography during the Late Cretaceous (Maastrichtian), about 70 million years ago. Shaded regions represent subaerial landmasses; white areas are oceans; lines show present-day coastlines. (B) Paleogeography during the early Eocene, about 53 million years ago. Numbered arrows indicate hypothesized dispersal routes during the Early Paleogene: 1, between Asia and North America via Bering land bridge; 2, De Geer route; 3, Thulean route; 4, between Asia and Europe at the northern end of the Obik Sea; 5, across the Turgai Strait; 6, probable sweepstakes dispersal between North and South America via Central America or perhaps a Caribbean archipelago; 7, between southern Europe and north Africa; 8, between South America and Antarctica; 9, between Antarctica and Australia. Some routes shown as marine barriers in this reconstruction might have been intermittently subaerial during the Early Cenozoic. (Modified from Smith et al., 1994.)

2001). A marine recession at the Eocene/Oligocene boundary finally exposed significant land bridges across the former seaway, allowing the immigrations from Asia that characterized the Grande Coupure.

It is now generally thought that the Indian Plate began to collide with Asia in the late Paleocene. Beck et al. (1998) even hypothesized that this collision could have precipitated the CIE (by triggering the release of organic carbon from the northern continental shelf of India) and the concomitant climatic and biotic changes that took place at the Paleocene/ Eocene boundary. Fossil evidence regarding the time of collision is equivocal. Frogs and crocodilians of Laurasian affinity and the mammal Deccanolestes (see Chapter 10) have been cited as evidence of limited contact with Asia as early as the Late Cretaceous (e.g., Jaeger et al., 1989; Sahni and Bajpai, 1991; Prasad et al., 1994), but other records (fishes, turtles, and dinosaurs) imply that some animals dispersed from Madagascar or Africa to India in the Late Cretaceous (about 80 Ma; Sahni, 1984).

South America was isolated from other continents through much of the Cenozoic, and most of its endemic early Cenozoic mammal fauna seems to be derived from at least two sweepstakes dispersal events, an earlier one (no later than early Paleocene) from North America, and a later event (late Eocene) from Africa. Close proximity or a possible land connection between Patagonia and the Antarctic Peninsula is implied by the discovery in Antarctica (Seymour Island) of a small number of typically Patagonian taxa. The late middle Eocene age of the assemblage (Bartonian, or in the gap between the early and late Casamayoran) suggests that these were relict taxa that were isolated from the early Paleogene Patagonian fauna (Reguero et al., 2002). Nevertheless, the presence in Antarctica of marsupials believed to lie near the base of the Australian radiation supports the hypothesis that therian mammals reached Australia through Antarctica by the early Eocene, and probably before then (Woodburne and Case, 1996).

Known Early Cenozoic faunas from Africa are largely confined to a few areas of the northern Sahara, with an important exception from the middle Eocene of Tanzania (see Chapter 10). Although many groups appear to be endemic, there are hints of affinities with European faunas, which might have dispersed between present-day Spain and Morocco.

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