Terrestrial Palaeoecology and Global Change

Chapter 3. Palaeoecology
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Before the advent of angiopserms, the taxodiaceous rainforests (e.g., the Late Jurassic–Early
Cretaceous Elatididion) formed a single-stratum overstorey, with a ground
vegetation of bryophytes, small horstails and stunted tree ferns derived from the open
fern–horsetail marshes. The cycadophytes were poorly represented, perhaps reflecting
an ephemeral seral stage rapidly overshadowed by the coniferous canopy. Angiosperms
filled in the vacant niche of shade-tolerant understorey developing a broadleaved, “platanophyllous”
habit in the process.
The xeromorphic cycadophyte–brachyphyllous shrublands widespread in the mid-
Triassic to mid-Cretaceous Meso-Mediterranean realm might have derived from the
symphyllogenetically related sclerophyllous helophytic communities. Their expansion over
the Mesozoic drylands coincided chronologically with the appearance of the giant dinosaurian
herbivores and was probably enhanced by overbrowsing and deforestation. The
thermophilic cycadophyte species were winnowed by the Albian cooling, rendering the
xeromorphic shrublands undersaturated, open for the xeromorphic varieties of early angiosperms.
The Mesozoic freshwater macrophyte communities consisted of charophytes, mosses
and quillwarts, all submerged. A new structural level was added with floating ferns
that converged on their antecedent lycopsids by acquiring heterospory with adaptations
to long-distance dispersal by water-borne spores (amphisporions in the case of Heroleandra:
Krassilov & Golovneva, 1999, 2000). These free-sporing colonizers were soon
joined and partly eliminated by floating angiosperms forming a later seral stage. An
increasing complexity of floating vegetation reduced light available for submerged plants
inflicting a turnover in the rooted macrophyte community that was in effect invaded by
aquatic angiosperms.
Thus, the ecological and morphological diversity of the newly arising group was rapidly
built up with its spread up and down the catenic belts. It came to dominance with
further transformation of catenic belts to the end of the Cretaceous. In particular, the
appearance of broadleaved riparian vegetation in the Early Palaeocene amounted to a
transformation of the Cretaceous lowland forests by a cut-off of the conifer overstorey.
The resulting low diversity Platano-Trochodendroidetum is an example of an undersaturated
community, the short-term stability of which might have been owing to a highly
unstable environments over the Cretaceous/Tertiary transition. Simultaneous innovations
at the upper end of the catena are vitnessed by differentiation of the taiga/birch altitudinal
belts in the montane areas of the Pacific coast (IV.2.3; IX.9).
Most prominent catenic transformations over the Tertiary were owing to aridization
of either lowlands or uplands in response to the global climate changes. Sequoiadendron,
for instance, was part of a lowland vegetation until an aridization of the lowlands
drove it to the slopes of Sierra Nevada (Axelrod, 1986). Similar developments took place
in southern Australia where the highland rainforests, with Nothofagus emerging from
the understorey (Specht et al. 1992), have appeared in conjunction with the xeric lowland
vegetation. In both cases, xeromorphic plant communities might have derived from