Biogeography in the Sub-Arctic. Группа авторов

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      The importance of the early Paleogene North Atlantic Land Bridge (NALB) for intercontinental flora and fauna exchange has been underscored by many authors (e.g. McKenna 1983; Tiffney 1985, 2008). In his classic paper from 1985, Tiffney mentions plant genera that are shared between the early Miocene Brandon Lignite Flora of eastern North America and Paleogene and Neogene floras of western Eurasia to illustrate the importance of the NALB. Subsequently, the shared North American–European fossil record of numerous additional plant genera suggest the same migration route (reviewed in Manchester 1999; Manchester et al. 2009; Amentotaxus, Cedrelospermum, Cercidiphyllum, Corylopsis, Gordonia, Koelreuteria, Mastixia, Phellodendron, Platycarya, Tapiscia, Tilia and Toricellia). Most recently, Paleogene disjunctions involving the NALB have been suggested for Decodon (Grímsson et al. 2012), Castanopsis (Sadowski et al. 2018, 2020), Eotrigonobalanus (Denk et al. 2012), Mahonia (Güner and Denk 2012) and Spirematospermum (Fischer et al. 2009). Notably, most of these taxa have not been recorded in the Paleogene fossil record of the sub‐arctic northern North Atlantic region and Svalbard. Until now it was assumed that the NALB played a crucial role for inter‐continental plant and animal migration via Greenland and the Faroe Islands (the so‐called Thulean route; McKenna 1983), but fossils from Greenland and the Faroe Islands that proved such a link were absent. Palynological data now provide direct evidence for a number of genera with a Paleogene transatlantic distribution to have thrived on Greenland, suggesting that they actually migrated via the NALB. For instance, Quercus sect. Quercus/Lobatae has previously been known from the middle Eocene of Axel Heiberg Island (McIntyre 1991, pollen; McIver and Basinger 1999, foliage) and Baltic amber deposits (Crepet 1989; Sadowski et al. 2020); Grímsson et al. (2015) provided unambiguous evidence for the presence of this type of Quercus in the Eocene of Greenland. Similarly, Fagus has previously been known from the middle Eocene of Axel Heiberg Island (McIntyre 1991, pollen). Recently, Grímsson et al. (2015, 2016a) provided the first record of Fagus pollen for the late Palaeocene (Agatdalen) and middle Eocene of West Greenland (Hareø; incl. leaves), complementing the records from Axel Heiberg Island (McIntyre 1991; Denk and Grimm 2009b). However, in view of the lack of Fagus in Eocene sediments of western Eurasia, there is currently no evidence for a transatlantic migration of Fagus during the Palaeocene or Eocene (Denk and Grimm 2009b).

Among extinct lineages of Fagaceae, foliage and pollen of the genus Eotrigonobalanus are reported from Palaeocene and Eocene sediments of West Greenland (Grímsson et al. 2016a; Figures 5 and 6). Eotrigonobalanus had a wide distribution in Eurasia (Kvaček and Walther 1989; Hofmann et al. 2011) from the Palaeocene/Eocene onwards and extending until the Miocene (Kvaček and Walther 1989). Foliage traditionally assigned to Dryophyllum from Paleogene sediments of the North American Gulf Coastal Plain (Jones and Dilcher 1988) co‐occurs with pollen closely similar to Eotrigonobalanus (Denk et al. 2012). The fossils from Europe, North America and Greenland may all belong to the same extinct lineage that had a wide distribution across the sub‐arctic North Atlantic in the Paleogene. Another extinct genus with a transatlantic Paleogene distribution is Platanites (Boulter and Kvaček 1989, Isle of Mull; McIver and Basinger 1999, Canada). Another member of Platanaceae, Platanus subgenus Glandulosa, is represented with two species in the BIP floras. Boulter and Kvaček (1989) reported P. fraxinifolia (Johnson and Gilmore) Walther from the Palaeocene of Mull, whereas P. bella is known from the Palaeocene of Greenland (Koch 1963; Kvaček et al. 2001). This has interesting biogeographic implications, as P. fraxinifolia essentially is a Central European species. However, Boulter and Kvaček (1989) also speculated that the specimens from Mull could be conspecific with P. bella but because of the insufficient informative characters in the infructescences of the Mull material did not assign the specimens from Mull to P. bella.

      In summary, these examples demonstrate a variety of possible migration routes during the Paleogene. Widespread taxa, such as the lineage comprising Eotrigonobalanus, might have migrated over the North Atlantic from both directions and migration from North America to Eurasia or vice versa may also have involved the Bering Land Bridge (see also, Tiffney and Manchester 2001). Taxa that migrated across the NALB either from the west or from the east are Cedrelospermum, Cercidiphyllum, Corylopsis, Mastixia, Platanites, Quercus and possibly Spirematospermum (Figure 6M and N) among others. A few 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’ СКАЧАТЬ