Terrestrial Palaeoecology and Global Change

Chapter 9. Crises
341
The Late Cretaceous climatic curve conforms to the concomitant sea-level curve,
the warming trend progressing, with transgression, from mid-Cenomanian to Campanian,
interrupted by a brief cooling event in the Coniacian (Krassilov, 1975a). This
climatic trend was reversed with temperization and a spread of deciduous vegetation
in the late Maastrichtian and over the KTB (Krassilov, 1975a and elsewhere;
Golovneva, 1994).
In the Late Permian, the Kazanian (Wardian) age was the warmest, with thermophilic
life forms prominent both in marine and terrestrial records. A turn to temperization
in the Tatarian is suggested by the phytogeographic changes, with the Angarian/
Subangarian floristic elements spreading south to northern Cathaysia. To the end of
the Permian, the northern Cathaysia lost its floristic distinctness while the tropical
vegetation survived in the southern Cathaysia alone (Krassilov, 2000b; Krassilov &
Naugolnykh, 2001; IV.3). A cooling over the latest Permian (Changhsingian) to the
basal Triassic (early Griesbachian) has been inferred also from conodont provinciality
(Mei & Henderson, 2001).
Monstrosity, diminution and prolific reproduction in a few generalist species as well
as the loss of smaller populations are the biotic signals of approaching biospheric crisis.
The loss of β and γ diversities is commonly ascribed to a floral/faunal mixing over new
migration routes, in particular over the emerging land bridges. However, the homogeneous
Early Triassic vegetation has spread with a transgression of epeiric seaways. The
fine-grain strategy and cosmopolitanism of the survivors might have been of a more
general significance, obliterating both ecological differentiation and provinciality. The
associated δ 13 C fluctuations indicate a release of the lighter carbon from the biota attesting
to a decrease in the total biomass with a drop of diversity.
IX.2. Boundary events
Traditionally, the KTB has been defined by the last appearances of ammonoids in the
marine facies and of dinosaurs in the terrestrial realm. In the North American stratigraphic
practice, the boundary has traditionally been drawn at the base of the first coal
seam above the last dinosaur bone, that is, at some distance above the dinosaur extinction
level. The iridium spike in non-marine sequences coincides with this conventional
boundary (Orth et al., 1980). A definition based on planktic foraminifers, nannoplankton
and palynological zonation places the KTB above the macrofossil turnover. The presently
widely held theory of extraterrestrial impact gives priority to the “boundary clay” with
shocked quartz, microspherules and a high iridium concentration, allegedly a deposition
of the impact ejecta, as a universal KTB marker.
The extraterrestrial impact story started at Gubbio, Italy (Alvarez et al., 1981), where
the iridium spike hits at a montmorillonitic clay 1 cm thick at the base of the lowermost
Palaeocene foraminiferal Globigerina eugubbina zone. The section comprises several