Читать книгу Biogeography in the Sub-Arctic - Группа авторов - Страница 35

Palaeocene to Eocene plant‐bearing sedimentary rocks are exposed in the Disco–Nuussuaq–Svartenhuk Halvø area (at 69 to 72°N), West Greenland, and in the Kap Gustav Holm–Scoresby Sund–Shannon area (at 69 to 74°N), East Greenland (Figure 1). Extensive fossil plant collections exist from the Paleogene sediments of West Greenland (e.g. Heer 1868 et seq.; Seward and Conway 1935; Koch 1963), whereas material from East Greenland is sparse (e.g. Mathiesen 1932; Seward and Edwards 1941), rendering these floras less informative.

The Cretaceous–Cenozoic lithostratigraphy in the Disco–Nuussuaq–Svartenhuk Halvø area, West Greenland, including the plant‐bearing Paleogene sediments, has recently been revised by Dam et al. (2009). The strata of the Nuussuaq peninsula are divided into two major units, the Nuussuaq Group and the younger Western Greenland Basalt Group (WGBG). The Nuussuaq Group is composed of five Cretaceous formations (Kome, Slibestensfjeldet, Upernivik Næs, Atane, Itilli), one Cretaceous/Palaeocene formation (Kangilia), and four Palaeocene formations (Agatdal, Quikavsak, Eqalulik, Atanikerluk). Some formations extend over vast areas, whereas others are local and contemporaneous with formations in neighbouring regions (for details see Dam et al. 2009). The Agatdal Formation comprising the ‘Agatdalen flora’ of Koch (1963, 1972a, b) and the Quikavsak Formation (Nuuk Qiterleq Member) comprising the ‘Upper Atanikerluk A flora’ of Heer (1868, 1869, 1874, 1880, 1883) are believed to be partly contemporaneous and resulting from the same tectonic event prior to accumulation of the overlying so‐called Naajaat palaeo‐lake sedimentary rocks (e.g. Pedersen et al. 1998; Dam et al. 2009; Grímsson et al. 2016b). The Quikavsak Formation is followed by the Atanikerluk Formation (Naujât Member), which comprises the ‘Upper Atanikerluk B flora’ of Heer (1880, 1883). Radiometric ages of contemporaneous volcanic structures give an age between 62 and 60 Ma (Selandian age; Storey et al. 1998; Dam et al. 2009) for the Atanikerluk Formation (Upper Atanikerluk B flora; Figure 2). The older Agatdal Formation (Agatdalen flora) and Quikavsak Formation (Upper Atanikerluk A flora) are between 64 and 62 Ma (late Danian age; see Figure 7 in Grímsson et al. 2016b).

The Paleogene strata on the island of Hareø (west of Disco; Figure 1) are divided into the Maligât Formation and the younger Hareøen Formation, both of which are part of the WGBG. The Hareøen Formation is divided into two successive members, the Aamaruutissaa Member (clastic) and the Talerua Member (volcanic; Hald 1976, 1977; see Figure 2 in Grímsson et al. 2015). The Aamaruutissaa Member comprises the ‘Hareø flora’ of Heer (1883) and Nathorst (1885). Lavas of the overlying Talerua Member have been radiometrically dated at ca. 39 Ma (cf. Schmidt et al. 2005) and are of late Bartonian age (late Eocene). This suggests that the underlying sedimentary rocks comprising the Hareø flora are of late Lutetian to early Bartonian age, ca. 42–40 Ma old (Figure 2); this is also supported by palynological data (Grímsson et al. 2015).

The first comprehensive treatment of fossil plants from the Cenozoic of Greenland was by Heer (1868, 1869, 1874, 1883), who studied plant fossils from 20 localities in Greenland. Heer described 143 plant species from Upper Atanikerluk A and 78 from the younger Upper Atanikerluk B horizon; from Hareø, 53 plant species were described (Heer 1883). Taxa recovered from nearly all localities were Metasequoia [as Sequoia langsdorfii, Taxodium] and Trochodendroides [as Populus arctica]. From the Upper Atanikerluk A flora, 12 species of Fagaceae were identified, among which nine were Quercus. In addition, four species of (evergreen) Laurus were recognized by Heer in this horizon. The latter were missing in the Upper Atanikerluk B flora (Heer 1883). From Upper Atanikerluk B, 10 species of Fagaceae were recorded and two species of Ilex (Aquifoliaceae) indicating the presence of evergreen angiosperms in the lower Palaeocene. From Hareø, the most characteristic elements reported by Heer were Ginkgo, Cupressaceae (incl. Taxodiaceae), Pinus, Acer, Fagaceae (three species of Fagus and Castanea, four species of Quercus) and Platanus (Heer 1883). Some of Heer's original taxa have later been revised and lumped, resulting in many less species for the individual floras (cf. Koch 1963; Kvaček et al. 1994). Other of Heer's taxa are in need of revision (Figure 3A as Paliurus pusillus and 3D as Cissites steenstrupi Heer).

A few studies briefly summarized the Palaeocene floras of (West) Greenland (Pedersen 1976; Mai 1995; Kvaček 2010) but did not provide new data or comprehensive revisions of the Greenland floras. However, Mai (1995) pointed to the presence of the extinct Fagaceae Eotrigonobalanus in the early Palaeocene flora of Atanikerluk, along with several other extinct types of Fagaceae. This observation has recently been confirmed by both pollen and leaf fossils from Agatdalen (Grímsson et al. 2016a, 2016b).

Koch (1963) collected and described plant fossils from the late Palaeocene Agatdalen Formation, Nuussuaq Peninsula. Most common macrofossil elements of the Agatdalen flora are Metasequoia (locally), Trochodendroides [as Cercidiphyllum], Macclintockia, and Platanus bella (Heer) Kvaček, Manchester and Guo [as Dicotylophyllum bellum (Heer) Seward and Conway]. Koch (1963) recognized the extant genus Liriodendron (Figure 3B) but placed leaf imprints similar to modern Sassafras (Figure 3C) within the extinct genus Lauraceaephyllum based on subtle differences in venation between the fossil and the modern genus. Revision of the Agatdalen macroflora by Grímsson et al. (2016a) reduces the ca. 38 taxa by Koch (1963) down to ca. 32. Still, the newly studied palynoflora from the same formation is relatively rich, comprising at least 145 angiosperms, 25 gymnosperms and around 30 different spore types (Grímsson et al. 2016a, 2016b). Overall, the palaeovegetation of the Agatdalen area appears to have been dominated by riparian elements (ferns, Taxodioideae, Trochodendroides, Platanus, Figure 4C and D), and Fagaceae, Betulaceae, Hamamelidaceae and Vitis (Figure 4A) as elements of the well‐drained hinterland. The enigmatic plant Macclintockia also played a significant role in the palaeofloral assemblages.

Figure 3 Palaeocene fossil leaves from West Greenland (Agatdalen, Atanikerluk) and the Faroe Islands (Mykines). (A) Small acrodromous leaf from Upper Atanikerluk B, MGUH 6443 [as Paliurus pusillus Heer in Heer (1883; Pl. LXXXI, figs 9 and 10)]. (B) Liriodendron sp., from Agatdalen, MGUH 10393 [as cfr. Liriodendron sp. in Koch (1963; Pl. 16, fig. 1)]. (C) Sassafras sp., from Agatdalen, MGUH 10420 [as Lauraceaephyllum stenolobatus Koch in Koch (1963; Pl. 30, fig. 2)]. (D) Large actinodromous leaf from Upper Atanikerluk B, MGUH 6435 [as Cissites steenstrupi Heer in Heer (1883; Pl. LXXXI, fig. 1)]. (E) Metasequoia occidentalis (Newb.) Chaney, from Mykines (Beinisvørð Formation), S134471. (See colour plate section for colour representation of this figure.)

Figure 4 Palaeocene leaves and Eocene pollen from West Greenland (Agatdalen, Hareø) and the Miocene of Iceland (Selárdalur, Botn). (A) Vitis sp., from Agatdalen, MGUH 10421 [as cfr. Vitis olriki Heer in Koch (1963; Pl. 30, fig. 3)]. (B) Tilia selardalense Grímsson, Denk & Símonarson, from Selárdalur. (C) Platanus sp., from Agatdalen, MGUH 10412 [as Platanus sp. cfr. aceroides Goepp. in Koch (1963; Pl. 26, fig. 1)]. (D) Platanus sp., from Hareø. (E) Platanus sp., from Botn. (F) Platanus leucophylla (Unger) Knobloch, from Selárdalur, IMNH 302.

Figure 5 Palaeocene fossil leaves from Atanikerluk, West Greenland. (A) Fagopsiphyllum groenlandicum (Heer) Manchester, from Upper Atanikerluk A, MGUH 6894 [as Fagus castaneaefolia Ung. in Heer (1868; Pl. XLVI, fig. 3a)]. (B) Fagopsiphyllum groenlandicum (Heer) Manchester, from Upper Atanikerluk A, MGUH 6269 [as Castanea ungeri Heer in Heer (1883; Pl. LXIX, fig. 3)]. (C) Fagopsiphyllum groenlandicum (Heer) Manchester, from Upper Atanikerluk A, MGUH 6270 [as Quercus grönlandica Heer in Heer (1883; Pl. LXIX, fig. 4)]. (D) Eotrigonobalanus sp., from Upper Atanikerluk B, MGUH 6349 [as Quercus laharpii Gaud. in Heer (1883; Pl. LXXIV, fig. 2)]. (E) Eotrigonobalanus sp., from Upper Atanikerluk B, MGUH 6372 [as Pterocarya denticulata Web in Heer (1883; Pl. LXXVI, fig. 1)]. (F) Eotrigonobalanus sp., from Upper Atanikerluk B, MGUH 6390 [as Laurus reussii Ettingsh. in Heer (1883; Pl. LXXVII, fig. 7)]. (G) Eotrigonobalanus sp., from Upper Atanikerluk B, S109006 [as Laurus primigenia Ung. in Heer (1880; Pl. III, fig. 8a)]. (H) Eotrigonobalanus sp., from Upper Atanikerluk B, S109107. (See colour plate section for colour representation of this figure.)

Macrofossils of the Upper Atanikerluk A flora comprise a large number of leaves resembling various modern genera of Fagaceae. Among these, Fagopsiphyllum is fairly abundant (Figure 5A to C, see Plate section). Figure 5C shows a typical leaf of Fagopsiphyllum; in contrast, the leaves shown in Figure 5A and B, although falling into the morphological variability of Fagopsiphyllum, clearly are reminiscent of leaf types encountered in modern taxa of Quercus section Cyclobalanopsis, and of the castaneoid genera Lithocarpus and Castanopsis (see illustrations in Camus 1929 et seq.). Another distinct leaf type is morphologically similar to both leaves from the Paleogene of North America (Dryophyllum‐like leaves; Jones and Dilcher 1988) and to Eotrigonobalanus from Europe (Kvaček and Walther 1989; Figure 5D to H). The pollen spectrum of the Upper Atanikerluk A flora has not yet been studied with high resolution scanning electron microscopy (SEM). However, the contemporaneous Agatdalen flora and the middle Eocene Hareø flora have recently been investigated and yield a remarkably high diversity of Fagaceae pollen (Grímsson et al. 2015, 2016a). Pollen unambiguously referable to Fagus, along with macrofossils of this genus, provide reliable evidence of this genus in the Cenozoic of Greenland (Figure 6A and B and Figure 7A). In addition, pollen of various Castaneoideae (according to Ørsted; see Praglowski 1984) is documented (Figure 6C and D). Numerous types of Quercus pollen represent various sections within the genus: Sect. Lobatae (Figure 6I and J), Lobatae/Quercus (white or red oaks; Figure 6K and L) and sect. Protobalanus (see Grímsson et al. 2015). Apart from this, additional Fagaceae pollen types represent extinct or ancestral lineages, Eotrigonobalanus (Figure 6G and H), Paraquercus (see pl. 12, Figures 1 to 5 in Grímsson et al. 2016b), and the pollen shown in Figure 6E and F. The latter is indistinguishable from pollen of the modern Quercus sect. Ilex (according to Denk and Grimm 2010; Denk et al. 2017; synonymous with subgenus Heterobalanus according to Menitsky 1984, 2005). This type of pollen has been suggested to be plesiomorphic within oaks (Denk and Grimm 2009a). In the Neogene of Europe, pollen of this type co‐occurs with leaves of Quercus drymeja Unger and Q. mediterranea Unger belonging to Quercus sect. Ilex (Denk et al. 2010b). Based on the modern distribution of this group of oaks from the Mediterranean area to south of the Himalayas and Southeast Asia (Menitsky 1984; Denk and Grimm 2010) and of the absence of foliage indicative of this group, the pollen encountered in the Eocene Hareø flora of Greenland may originate from an extinct and ancestral member of Quercus that had retained the plesiomorphic pollen. This would be in accordance with the presence of Fagaceae foliage with unclear generic/infra‐generic affinities in the Palaeocene to Eocene of Greenland.

The presence of Aponogeton in the Eocene of West Greenland (Grímsson et al. 2014) is recorded both in the palynological (Figure 6O and P) and macrofossil records of Hareø (Figure 7B and C). The foliage associated with the pollen partly recalls foliage from the Eocene/Oligocene Renardodden Formation of Svalbard ascribed to the extinct genus Haemantophyllum by Budantsev and Golovneva (2009, plate 88, Figures 5 and 6) and originally described as Alsima macrophylla by Heer (1876). The fossil pollen from Hareø clearly suggests closer affinities to Aponogeton than to any other members of the Alismatales (Grímsson et al. 2014).