Terrestrial Palaeoecology and Global Change

Chapter 9. Crises
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spore-pollen assemblage and a macrofossil assemblage with small-leaved Trochodendroides,
Liriophyllum and occasional Czekanowskia came from the uppermost dinosaur
beds (Markevich, 1994; Bugdaeva, 2001). The Upper Tsagajan member consists of thick
conglomerates and gravels of a major erosion cycle thinning upward to sandstones/siltstones
of a meandering river system. There are 23 fossil plant beds of various riparian
facies. In the coarse-grained member with lenticular clays, the poorly sorted fan deposits
repeatedly prograded over the flood-plain. The oxbow clays imbedded in the debris-flow
gravels contain abundant (hypo)autochthonous remains of few aquatic and wetland species,
with both their foliar and reproductive organs preserved on the same bedding plane, as
well as allochthonous remains of occasional leaves and samaras. The coarsely laminated
crevasse-splay and the thin-bedded levee siltstones of avulsion cycles contain leaf mats of
a single or few broadleaved species. The following assemblages are recognized:
(1) Trochodendroidion, represented in all facies types by leaf–fruit assemblages, in
which Trochodendroides arctica is a single or a single dominant species reconstructed
as a small to medium-sized deciduous tree, heteroblastic, with aspen-like polymorphous
leaves and with large panicles of follicular fruits producing a great number of small
winged seeds. The Trochodendridion includes two distinct variants:
(1a) Trochodendroidetum arcticae, represented by the single-species leaf mats on
bedding planes of channel sandstones, with Trochodendrocarpus panicles and Trochodendrospermum
seeds belonging to the Trochodendroides plant.
(1b) Platano-Trochodendroidetum arcticae, represented by the two- to severalspecies
leaf mats in the levee and oxbow facies (Fig. 149), with Trochodendroides and
platanoids in nearly equal proportions. The relatively diverse assemblages of oxbow
clays also contain Taxodium while the biserrate morphotypes Viburniphyllum and Tiliaephyllum
occur in occasional leaf-mat patches. A single locality contains Protophyllum
and Gleichenites, a residual Cretaceous fern–platanophyll community (above) conceivably
survived in few patches of the riparian mosaic.
(2) Tiliaephylletum tsagajanicae, represented by the single-species leaf mats of
levee siltstones, with overlapping Tiliaephyllum, a cordate biserrate broadleaved morphotype
(Fig. 36) representing a patch of shrubby vegetation, perhaps seral, reflecting
gaps in the forest canopy.
(3) Mixed conifer–broadleaved upland vegetation represented in the oxbow and levee
facies by occasional samaras, cone-scales, fragmentary leaves and dispersed leaf cuticles
that are preservationally different from the bulk of plant fossils. The coarser levee
facies contain abundant plant debris but determinable remains are infrequent. Notable
among the latter are occasional leaf fragments and dispersed leaf cuticles of Ginkgo as
well as scale-leaves of Araucarites pojarkovae. Since fossiliferous lenses are imbedded
in the debris-flow piedmont deposits, the allochthonous plant remains may conceivably
represent an upslope vegetation of a single or several altitudinal belts. The conifers
are fairly diverse, comprising both “northern” (Metasequoia, Sequoia, Cupressinocladus,
Pseudolarix, Pinus) and “southern” (Araucarites, Podocarpus) genera. Alnites,