Terrestrial Palaeoecology and Global Change

246 Valentin A. Krassilov. Terrestrial Palaeoecology
and frequently polydominant. Sedimentologically this zone is dominated by redbeds while
leaf mats and leaf coals are almost exclusively confined to the Phoenicopsis zone.
The Phoenicopsis/Cycadeoidea boundary marks the northern limit of bennettitalean
morphotypes Ptilophyllum–Zamites–Otozamites (the Ptilophyllion of syntaxonomic classification).
Stem diameters of Equisetum (Equisetites) appreciably decrease north of the
boundary (from over 25 mm to less than 10 mm). The thermophilic matoniaceous, schizaeaceous
and cyatheaceous ferns Nathorstia, Klukia, Ruffordia, Stachypteris, Cyathea are
restricted to the southern zone, rarely penetrating the ecotonal assemblages, as do also the
peltasperms (Pachypteris) and araucariaceous conifers. Among the dominant osmundaceous
genera, Todites is replaced by Osmunda in the northern realm. Coniopteris species
with aphlebial pinnae occur south, those with decurrent pinnae north, of the boundary.
The deciduous czekanowskialean-ginkgoalean-coniferous vegetation (the Phoenicopsion)
dominated the mesotemperate Arcto-Mesozoic realm from mid-Triassic to
mid-Cretaceous when it was replaced by the deciduous conifer rainforest (the Parataxodietum).
The deciduous hamamelid broadleaves, Trochodendroides, platanophylls and
betuloid serratophylls, supposedly developing as seral stages and understorey of Parataxodium
forests, emerged as dominants of a progenitorial Arcto-Tertiary vegetation over
the Cretaceous/Tertiary boundary. In the Palaeocene, the Trochodendroidion extended
as far south as southern Gobi at about 43°N (Makulbekov, 1988), but retreated to the
north with the Eocene warming.
The thermophilic Eocene flora with laurophyllous evergreens spread to 60°N over the
mainland Eurasia, with marginal extensions to 70°N (VII.5). A reversal of climatic trend
with the rise of the Tibet – Himalayas and the central African plateaux in the Late Eocene
brought with it a stepwise displacement of the evergreen/deciduous boundary to the south,
continued, with fluctuations, over the Oligocene and Neogene. Since the late Neogene, the
boundary has remained close to its present-day position over the southern limits of the
Euxinian beech-oak forests and the Mandschurian oak forests at about 35°N.
VII.4.2. Climatic interpretation
Displacements of vegetation zones might have been the result of polar wander, continental
drift or climate change. The maximal amplitudes of displacement are recorded
for the Early Palaeogene, with the deciduous/evergreen boundary shifting from about
43°N in the Early Palaeocene to 60°N in the Early Eocene and back again in the Late
Eocene to Oligocene. Neither polar wander nor continental drift of comparable magnitudes
has ever been inferred for this time interval. Likewise, the pre-Cenozoic shifts of
phyogeographical zonation do not correlate with the alleged directions and magnitudes of
polar wander or continental drift. All over the Late Palaeozoic and Mesozoic, the boundaries
remained roughly parallel to the modern latitudes, which makes the idea of their
displacement by continental drift (rotation on sphere) unlikely.