Biogeography in the Sub-Arctic. Группа авторов
taxa clearly have an earlier fossil record from North America and might have migrated to Europe from the west (Amentotaxus, Decodon, Koelreuteria, Mahonia and possibly Tilia). In rare cases, plant lineages may have reached Greenland but not crossed over to Europe or North America (Fagus is known from the Palaeocene of Greenland, the Eocene of North America, including Axel Heiberg Island, and Greenland, but has no Eocene fossil record in Europe). P. fraxinifolia may have been restricted to Europe and Scotland.
Neogene Links
It has traditionally been suggested that the NALB had become unavailable for animal and plant migration in the course of the Oligocene (McKenna 1983). However, both new subsidence models (e.g. Poore et al. 2006) and palaeobotanical data (Denk et al. 2011 and references in their Chapter 12) suggest that this link persisted much longer than previously thought, providing a functioning ‘land bridge’ particularly for plants (see also recent reviews by Tiffney and Manchester 2001; Tiffney 2000, 2008). The exceptionally rich Neogene record of Iceland allows distinguishing directions of transatlantic migration during the Miocene and Pliocene. The oldest floras of Iceland are characterized by taxa that had a markedly widespread northern hemispheric distribution during large parts of the Cenozoic (Glyprostrobus, Sequoia, Cercidiphyllum, Platanus, Liriodendron, Sassafras, etc.). These taxa may have migrated to Iceland either from the east or from the west. A similar pattern is seen in the early late Miocene floras, where less common taxa such as Rhododendron section Pontica have closely similar related taxa in the modern floras of eastern North America and western Eurasia. At the same time, a number of late Miocene taxa recovered from Iceland clearly migrated from Europe (Fagus gussonii Massalongo emend. Knobloch and Velitzelos, Trigonobalanopsis). Similarly, the pollen record suggests that Quercus sect. Quercus/Lobatae migrated to Iceland from the west as late as between 7 and 6 and 5.5 Ma (Denk et al. 2010a). Younger floras record a stepwise loss of ‘exotic’ taxa until the complete extinction of the (warm) temperate elements of the ancient Icelandic flora during the cold phases of the Pleistocene. Interglacial and postglacial plant colonization of Iceland occurred predominantly from the east.
Conclusion and Future Research
In this chapter we briefly summarized current knowledge about the Cenozoic floras of the sub‐arctic North Atlantic. From the biogeographic point of view, two issues are of major importance within this temporal and geographical frame. First, how timely is the ‘Arcto‐Tertiary element’ (biogeographic) hypothesis of Engler? And second, how long into the Neogene did the NALB provide a functioning link for plant migration across the North Atlantic?
The ‘Arcto‐Tertiary element’ Hypothesis
The present data demonstrate that several modern north temperate tree taxa did actually thrive in the sub‐arctic during the Paleogene. Also, there is unequivocal evidence for the presence of several ‘Arcto‐Tertiary elements’ in Greenland (for example Fagus). Some of these records are, however, not the oldest for these temperate taxa (for example Acer). Future palynological investigations of Palaeocene sediments are on their way and will provide final clues to solving whether the ‘Arcto‐Tertiary element’ represents a secondary radiation of taxa that originated in mid‐latitudes to high latitudes or whether they actually originated at high latitudes. Nevertheless, the present data demonstrate that this biogeographic concept is all open for discussion.
The North Atlantic Land Bridge
Most recent investigations of an exceptionally rich plant fossil record demonstrate that the NALB facilitated plant migration between North America and Europe until the late Miocene. This finding is in agreement with low molecular divergences found both for animal and plant lineages with a transatlantic distribution. Little genetic differentiation of these lineages strongly suggests that gene exchange via the NALB must have occurred long into the Neogene (Denk and Grimm 2010; Kornobis et al. 2011). The improved understanding of the history of the NALB is crucial for basic biogeographic assumptions. For example, Donoghue and Smith (2004) based on molecular divergence times between transatlantic sister lineages (erroneously) suggested that taxa with an inferred divergence time of less than 30 million years ago must have migrated from North America via the Bering Strait (and via East and Central Asia) to Europe. The quality of future biogeographic studies using modern plants and molecular differentiation patterns to infer historical biogeography will largely depend on revised fossil data in order to arrive at meaningful biogeographic scenarios.
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