The De Geer, Thulean and Beringia routes: key concepts for understanding early Cenozoic biogeography
I re-evaluate the specific biogeographical significance of each of the land bridges (Beringia, Thulean and De Geer) in the Northern Hemisphere during the latest Cretaceous–early Cenozoic, showing that the Thulean and De Geer routes did not operate contemporaneously.
Northern Hemisphere landmasses.
I review the recent climatic, sea-level, geotectonic, palaeofloristic, and marine and terrestrial faunal data that have emerged since the establishment in the 1980s of the biogeographical concepts of the early Cenozoic Northern Hemisphere land bridges and present a synthesis supporting a revised scenario for early Cenozoic biogeographical development.
Palaeogeographical and geotectonic data, supported by strong floral and faunal evidence, suggest that the palaeogeographical and chronological frames for the formation of all three land bridges are different from those originally proposed. Dispersal events via the causeways seem to have taken place during specific time intervals resulting from fluctuations in sea level and climate.
The De Geer and Thulean routes were not contemporaneous. The former existed during the latest Cretaceous to the early Palaeocene, joining North America with Eurasia. The Thulean route became established well after the interruption of the De Geer route, offering a southerly connection between western Europe and North America in at least two episodes: c. 57 Ma and c. 56 Ma. The Bering route functioned in two warm periods: 65.5 Ma (coinciding with the De Geer route) and c. 58 Ma, during the Palaeocene (possible Eocene exposures are not considered here). The formation of the De Geer route explains faunal similarities between the Puercan and Torrejonian North American land mammal ages (NALMAs) and the Shanghuan Asian land mammal age (ALMA). The Thulean route explains faunal similarities between the Clarkforkian (Cf1) and Wasatchian (Wa0, 1) NALMAs, and the Cernaysian and Neustrian (PE I, II) European land mammal ages. The Bering route explains faunal similarities between the Gashatan ALMA and the Tiffanian (Ti5) NALMA.