Biogeography in the Sub-Arctic. Группа авторов
Vitis along with a flower of unclear taxonomic relationships (Calycites).
Similar floras are known from Isle of Skye, Scotland (Poulter et al. 2008, 2010) and Ballypalady (County Antrim, Ireland; Mai 1995). In the latter flora, cones of Pinus plutonis Baily are abundant and biogeographically interesting. According to Mai (1995), P. plutonis belongs to section Sylvestres subsection Resinosae and provides another potential link between Europe and North America.
Neogene Floras and Vegetation
Iceland
Iceland is a volcanic island with several intrabasaltic plant‐bearing sedimentary rock formations. The oldest sedimentary rocks are found in Northwest Iceland (ca. 15 Ma) and East Iceland (ca. 14 Ma) and are of Miocene age (Figures 1 and 2). In a simplified version the strata become progressively younger towards the volcanic zones crossing the centre of the island from southwest to northeast (for a recent review of the geology of Iceland see contributions in Sigmundsson et al. 2008). Sedimentary rock formations occur in Miocene to Pleistocene strata and often contain plant fossils. The palaeo‐floras of Iceland have been comprehensively studied in recent years (e.g. Denk et al. 2011; see also Denk et al. 2005; Grímsson et al. 2005, 2008; Grímsson and Símonarson 2006, 2008a, 2008b).
The middle Miocene plant assemblages of Iceland (15–12 Ma) record vegetation thriving under a warm and moist climate. Wetlands and riparian vegetation of the lowlands was characterized by warmth‐loving taxa, such as taxodiaceous Cupressaceae, Magnoliaceae (Magnolia, Liriodendron), Lauraceae (Sassafras), Platanaceae (Figure 4E and F) and others, whereas the well‐drained vegetation of the hinterland comprised forests dominated by Fagus with evergreen trees and shrubs in the understorey (Rhododendron, Ilex). The endemic linden tree, Tilia selardalense Grímsson, Denk and Símonarson (Figure 4B) was confined to these forests (Grímsson and Denk 2005; Grímsson et al. 2007a, 2007b; Denk et al. 2011). The oldest floras of Iceland share a few taxa with the Paleogene floras of Greenland and/or Svalbard: Glyptostrobus, Platanus, Fagus, and Tilia.
A major change is seen in the early Tortonian (10 Ma) floras of Iceland, both in the palynological and the macrofossil record. Whereas herbaceous taxa did not play a significant role in the older floras, they amount to 30% of all recorded plant taxa in the 10 Ma floras. This increase in herbaceous plants is accompanied by the first occurrence of small‐leaved Ericaceae typical of the modern tundra vegetation in Iceland (Vaccinium, Arctostaphylos) and boreal conifers such as Larix. Nevertheless, several warmth‐loving elements persisted and new elements are recorded (Ginkgo; Denk et al. 2005, 2011). Floras preserved in strata between 10 and 3.6 Ma reflect stepwise cooling; Fagus persisted until 7–6 Ma and Quercus until 5.5 Ma, whereas the evergreen, large‐leaved Rhododendron aff. ponticum L. ranges from the oldest to the 3.8–3.6 Ma floras. At 4.4–3.6 Ma, small‐leaved Salicaceae occur for the first time. The second major reorganization of the vegetation is recorded in floras from the Pliocene–Pleistocene transition. Temperate woody elements are not found in any of the Pleistocene floras, which are essentially similar to the modern flora of Iceland (Denk et al. 2011).
Biogeographic Implications
Paleogene Links
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 (