A next step in the analysis of bryophyte distribution patterns is to understand how these distributions were achieved. In the context of Wegener's continental drift theory, the floristic differences between Southern (Holantarctic) and Northern (Holarctic) Hemisphere flora are assumed to be a reflection of the opening of the Tethys sea some 180 mya, which divided the Pangea supercontinent into two large land masses, Laurasia and Gondwana (Fig. 6.6) (Raven & Axelrod 1974). Fragmentation of Gondwana started c. 180 mya and was completed between 80 mya and 53 mya. The very ancient split of Gondwana, with subsequent isolation of southern South America, Australia and New Zealand, may be one of the main reasons why the flora of these areas nowadays exhibit the highest endemic rates worldwide (Fig. 6.5).
By contrast, Laurasia was assembled comparatively late. The opening of the North Atlantic was completed about 60 mya, but land bridges across the Faroes and Greenland still persisted until 40-30 mya, while Eurasia and western North America remained connected via the Bering bridge until 5.5-5.4 mya. These bridges offered an opportunity for migration between Eurasia and North America. For example, amphi-Atlantic ranges (Fig. 6.7), exhibited by 8% of the European and 7% of the North American moss flora and 15% and 12% of the European and North American liverwort flora, respectively, have traditionally been interpreted as a result of the historic connections through the North Atlantic bridge (Schofield 1988). The floras of North America and Europe thus appear strikingly similar, with 68 of 252 moss genera represented by the same species in North America and Europe and interpreted as an old common stock of Laurasian taxa (Frahm & Vitt 1993). As a consequence, rates of endemism in the Holarctic, with, for example, no endemic liverwort genus in northern Asia, one genus (1%) in Europe and three genera (2.5%) in North America, are much lower than in former Gondwanic splinters (Fig. 6.5).
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