Terrestrial Palaeoecology and Global Change

Chapter 9. Crises
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Trochodendroides forests replacing the coniferous Sequoia–Parataxodium forests.
Taxonomic diversity of the latter increased considerably over the Maastrichtian owing to
the invasion of new angiosperm species at the pioneer–early successional stages. Parataxodium,
a highly polymorphous genus of taxodiaceous conifers, has since been lost as a
result of its splitting into the progenitorial Taxodium-like and Metasequia-like morphotypes,
while Seqouia has receded from its dominant status, with a single relict polyploid
species surviving to this day.
Of the replacing dominants, Metasequoia is distinguished by the decussate arrangement
of leaves and cone scales, a paedomorphic character in conifers. Trochodendroides
is an aspen-like leaf morphotype of a deciduous woody angiosperm with exceptionally
large panicles of follicular fruits producing great numbers of small winged seeds, a
pioneer dispersal habit. Both seem to have first appeared as deciduous components of
riparian vegetation with sporadic fossil records over the Late Cretaceous. Their ubiquitous
records above the KTB show broad ranges of morphological variation.
At the PTB, the vojnovskyalean (cordaitalean) climax forests, persistent over the
Permian, disappeared at the onset of trap volcanism while the species-rich pecopterid
marshes with helophytic lycopsids and arthrophytes were replaced by the monodominant
lycopsid marshes with Pleuromeia. Only the hydroseral conifer–peltasperm communities
survived into the Early Triassic as a source of Mesozoic gymnosperm radiations
(IV.3).
Palaeobotanical examples may include the Dinantian vegetation of Scotland, inhibited
from reaching the edaphic climax by seismic debris flows and frequent fires over the
volcanic landscapes (Bateman & Scott, 1990), and similar developments in the Miocene
(Cross & Taggart, 1983; Collinson & Scott, 1987a, 1987b).
In the Cretaceous epeiric seas, a climax of ecosystem developments seems to have
been reached with the spectacular morphological complexity of the larger carinate and
bicarinate foraminifers (Caron & Homewood, 1983), the exoskeletons in the epifaunal
serpulids and bryozoans, the excessive shell ornamentation in ammonites, the durophagous
adaptations in echinoderms and vertebrates (Surlyk & Birklund, 1977), and with the
hesperornithiform birds and mosasaurs appearing at the top of the trophic cascades.
Reefal successions typically started with bryozoans and advanced toward the rudist
climaxes (Kauffman, 1974). The KTB extinctions selectively affected the advanced
forms of the climax stages, while the pioneer stages were enhanced. Extinction of rudists
and a concomitant rise of bryozoans is a typical example. A short-term rise of small
foraminifers at the expense of the morphologically advanced globotruncanids definitely
falls into the same category.
Marine records also show a selective mass extinction of poor dispersers in spite of
their competitive superiority (Tilman et al., 1997). At an early stage of the KTB extinction,
local species disappeared before cosmopolitan forms (Keller et al., 1994). This
confirms that the loss of the γ component owing to cosmopolitanism of the early successional
fine-grain strategists could have contributed to the total decrease in biological