Terrestrial Palaeoecology and Global Change

84 Valentin A. Krassilov. Terrestrial Palaeoecology
Other plant communities for which light poses a major problem thrive in the highlatitude
areas of winter darkness. It is commonly taken for granted that a high latitude
fossil plant of rainforest affinities must be of a tropical derivation, but it might have been
the other way round: polar life forms, preadapted to shading due to their tolerance of
prolonged darkness, might find their way under the canopies of primordial rainforests.
IV.2.2. Temperate deciduous vegetation
Seasonal deciduousness might have appeared in leafless plants that shed green
branchlets, as extant Ephedra species do. The Devonian archaeopterids were shedding
leaf-like syntelomic systems that sometimes formed mat-type accumulations (Fig.37)
indicating seasonal deciduousness, as in the Late Devonian Tanaitis, a leafless heterosporangial
archaeopterid genus akin to Svalbardia, the northernmost (present-day
latitudes) representative of the group. The branch-mats occur on bedding planes of a
levee-type detrital siltstone containing also abundant fossil wood fragments (Krassilov et
al., 1988).
The modern-type deciduousness appeared with the Permian Angarian and Gondwana
floras of the broadleaved vojnovskyan “cordaites” and glossopterids respectively,
forming mats of individually shed leaves and leafy spur-shoots. In the Late Permian,
a dominant morphotype of ribbon-shaped leaves with a callous abscission scar was
joined by the newly appearing flabellate Ginkgo-like (Rhipidopsis) and aciculate coniferoid
morphotypes that became more prominent in the Mesozoic. The mid-Triassic
to Early Cretaceous taphofloras of northern Asia were dominated by Czekanowskia,
Phoenicopsis, Pseudotorellia, Ginkgoites, Pityophyllum and Nilssonia forming
extensive leaf-mats. All these plants were heteroblastic, shedding spur-shoots with
leaves (Fig. 38).
Shoot dropping was inherited by deciduous conifers, such as Parataxodium, dominant
over northern Asia in the Late Cretaceous. They formed mixed leaf mats with
broadleaved platanoids and hamamelids (Trochodendroides) that grew as understorey
and seral communities of the Platano-Parataxodietum (Krassilov, 1979). With a decline
of the latter over the Cretaceous/Tertiary boundary, new broadleaved deciduous
forest types were formed on the basis of the seral Platano-Trochodendroidetum, incorporating
the deciduous forms taxodiaceous conifers derived from Parataxodium
(Metasequoia, Taxodium, Glyptostrobus). They constituted the core of the Arcto-
Tertiary biome mixed forest types, with biserrate betuloid components first recorded as
dominant elements from intermontane depressions of circum-Pacific coastal ranges
(Krassilov, 1989c), supposedly representing an upland differentiation of the early hamamelid
complex.
Summer-dry deciduousness is poorly recorded because leaves are shed in a season
unfavorable for preservation scarcely forming leaf mats. It might have first appeared in