Terrestrial Palaeoecology and Global Change

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Chapter 4. Palaeoecogeography 115 1997c). The boundary markers are Phoenicopsis, a deciduous tree dominant in the northern realm (Siberia, northern Kazakhstan, the Urals, Scandinavia, western Canada, Greenland) and Cycadeoidea, a pachycaul bennettite characteristic of the southern realm as far as the Black Hills, Dakota, British Weald, Middle East, Mongolia, and northern Sikhote Alin. Cycadeoidea associates with pinnate bennettitalean leaves of the Ptilophyllum–Zamites–Otozamites morphotypes, neither of which was ever found in the northern realm except at a few ecotonal localities (e.g., in Mongolia or Scotland: Krassilov, 1982; Van der Burgh & Van Konnijnenburg-Van Cittert, 1984). The Phoenicopsis/Cycadeoidea boundary thus marks the northern limit of the Ptilophylletea, as well as of the matoniaceous and schizaeaceous ferns Nathorstia, Klukia, Ruffordia, Stachypteris, Weichselia, the peltasperms Pachypteris and allied genera as well as the araucariaceous conifers. At the same time it divided the ranges of giant (stem diameters over 3 cm) and slender horsetails, Todites and Osmunda among the dominant osmunadaceous ferns, aphlebial and non-aphlebial Coniopteris among the treeferns and their dwarf varieties (Krassilov, 1978d, 1987). These latter distinctions reflect aspective differences of the northern and southern fern-marsh communities (IV.2.6). No less conspicuous are the ecogeographic differences in the zonal distribution of scale-leaved coniferoids (including the hirmerellaceous gnetophytes), both the abundance and diversity of which greatly increased across the boundary giving the southern vegetation a sclerophyllous aspect. However, in northern Asia, Classopollis, a palynological marker of brachyphyllous vegetation, reached to 60°N, justifying a division of the temperate zone into the warm-temperate and cool-temperate subzones. The angiosperm palynological markers, such as Proteacidites, indicate a similar differentiation in the Late Cretaceous (Herngreen & Chlonova, 1981). Deciduousness of the northern Arcto-Mesozoic (Phoenicopsis) vegetation is amply documented by the taphonomy of leaf mats with leafy spur-shoots while their association with coal beds suggests a summer-wet climate. In contrast, leaf mats are rather uncommon in the southern zone. Here the gypsiferous redbeds and lacustrine carbonates are the commonest lithologies while coal accumulation was patchy and insignificant. Both sedimentological and palaeovegetational features suggest a Mediterraneantype climate (Francis, 1984). Dry season is marked by false rings of fossil wood, as well as by narrow sclerites on the fossil fish scales (Krassilov, 1983a, VII.1.1). The distinctness of the boundary is scarcely related to temperature differences alone, the more so that the equatorial to polar temperature gradient was much gentler than now, with the Cretaceous equatorial means nearly as at present against about 0°C at the poles (Sellwood et al., 1994). Seasonality of precipitation might have been critical, with the boundary marking a transition from summer-wet to summer-dry climates. Tethys seas would have sustained a low latitude anticyclone that made the Meso-Mediterranean zone drier than its modern equivalent. On the weight of all available evidence, the Phoenicopsis/Cycadeoidea boundary can be considered as the first order ecogeographic divide separating the nemoral Arcto-
116 Valentin A. Krassilov. Terrestrial Palaeoecology Mesozoic realm from the xerophytic Meso-Mediterranean one (Figs. 53 – 56). Ecotonal assemblages are known from northern Sikhote Alin (the northernmost occurrence of Cycadeoidea in Asia: Krassilov & Shorokhova, 1989), the Tyrma River northernmost Fig. 53. A scheme of the Late Jurassic to Early Cretaceous climatic zonation based on plant macrofossil and palynological records: (1) A cool temperate zone lacking Classopollis, (2) temperate zone of deciduous Phoenicopsis forests, (3) summer-dry subtropical zone with thick-stemmed cycadeoids, Ptilophyllum, and diverse brachyphyllous coniferoids, (4) equatorial zone with brachyphyll coniferoids (gnetaleans) and elaterbearing pollen morphotypes, (5) southern temperate zone with diverse ginkgophytes, small-leaved Ptilophyllum and with fossil woods showing seasonal growth increments (after Krassilov, 1985). Fig. 54. the northernmost occurrence of Cycadeoidea, a pachycaul bennettite in the Albian of northern Sikhote Alin, about 50°N (Krassilov & Shorokhova, 1989).
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Terrestrial Palaeoecology and Global Change

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