FOSSIL MAMMALS AND EARLY EOCENE NORTH ATLANTIC LAND CONTINUITY MIALCOLM C. IMcKENNA' ABSTRACT Until recently American vertebrate paleontologists, particularly students of fossil maiials, have not generally accepted the concept of a former continuons land area around the north end of the Atlantic, connecting western Europe with North Aimerica. C. G. Simpson developed biological arguments based on fossil mammals supporting the existence of a corridor (Simpson, 1953 and references cited there) topologically connecting western Europe with North America in the early Eocene, but Simpson was influenced by the stabilistic geologic rationale of the tines when he located the position of the corridor in Asia because of supposed permanence of the Atlantic oceanic barrier during all of Tertiary time. Ile did not take into account the epicontinental Turgai Straits sea barrier in Asia that lay athwart his corridor in the early Tertiary. The plate tectonic geophysical synthesis of the history of the North Atlantic and Arctic Oceans is in accord with the mammalian timing evidence that a former Euramerican landmass as well as a biota was severed about 49 m.y. ago and that Holarctic land dispersai since that time has been via Asia alone, becoming possible again with Europe in the mid-Tertiary. Earlier, starting about 70 m.y. ago, a continental collision whose site is now within northeastern Siberia created land continuity between what were then Asia and North America, and by the Oligocene the Turgai Straits had finally dried, giving the Ilolarctic corridor essentially its present configuration. Shallow epicontinental waters have on several occasions crossed Beringia, as at present. Thus the land surface of Holarctica has been rearranged substantially since 70 m.y. ago, North America as a land surface having shifted its allegiance from Europe to Asia. Recently published geological and geophysical information also suggests that, in additionto early Eocene land continuity in the Greenland-Barents Shelf area, a subaerial dispersal routecrossing the volcanic Wyville Thompson Ridge from southeastern Greenland to the Faeroes andthen to Great Britain and Ireland may also have been possible for a time in the early Tertiary.This latter route is the long familiar but hypothetical Thulean Bridge, now given a new leaseon life by geophysical studies of "hot spots." Aside from the time-honored and romantic concept of Atlantis, a rationale fora former North Atlantic land area connecting western Europe ail the way to theNorth American mainland can be traced back at least to the 1850's. The conceptreached a high level of credibility among biogeographers such as Scharff (1907,1909, 1911) and geologists such as Arldt (1917), and the connection was usuallythought of as operating up until rather late in the Cenozoic. It should be recalledthat until Nansen's historie voyage in the Fram in 1893-1896 it was generallybelieved that the Arctic Ocean was shallow and that a significant amount ofunknown land still ]urked in those waters-land that might somehow have beena terrestrial dispersal route in pre-glacial times. In its extreme form, the idea ofa transatlantic late Cenozoic land bridge in the north is still with us (e.g., Strauch,1970), although the postulated locale is regarded as Iceland rather than fartherpoleward. On the other hand various authors, for instance Simpson (1953, 1965),Schwarzbach & Pflug (1957), and Schwarzbach (1959), have claimed that noconnection between Iceland and the British Isles existed after the Eocene or thatnone existed at ail during the whole Cenozoic Era. Still other authors have sug-Department of Vertebrate Paleontology, The American Museum of Natural History, andDepartment of Geological Sciences, Columbia University.ANN. MIssouRm BOT. CARD. 62: 335-353. 1975.
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