Terrestrial Palaeoecology and Global Change

Chapter 4. Palaeoecogeography
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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-