Terrestrial Palaeoecology and Global Change

354 Valentin A. Krassilov. Terrestrial Palaeoecology
Dinosaurs, etc. Temperization leads to afforestation of mid-latitudes and a spread
of deciduousness (VII.5). Since deciduous forests are inferior to open evergreen vegetation
in resilience and in nutritional quality of the leaf mass, afforestation means a drastic
reduction of trophic resources for the larger herbivores.
Dinosaur extinction is commonly perceived as enigmatic, implying an exotic explanation.
Yet dinosaurs were the larger herbivores dependent on phytomass production,
and predators dependent on the herbivore biomass. The early diverging insectivorous
line supposedly gave rise to the birds. The later mammalivorous branch might perish
with multituberculates and other archaic mammals. In the history of dinosaurs there
were at least two episodes of semiextinction at the Triassic/Jurassic boundary and in
the mid-Cretaceous (the ornithopod/sauropod replacement), both related to vegetational
changes.
Afforestation over the KTB affected the habitats of the richest dinosaur faunas in
North America, Amurland, Mongolia, northern China and elsewhere at the mid-latitudes.
The end-Permian afforestation involved the same areas, with the Eurangarian
vegetation extending to northern China. On taphonomic evidence, most of the dominant
arboreal forms were shoot-droppers. This kind of deciduousness primarily affects the
larger browsers that prune leafy shoots at high browsing levels (VIII.I.3).
The residual dinosaur communities of forested areas might have foraged on the residual
patches of cycadophyte thickets (as suggested by association of dinosaur remains
with Encephalartites in the Maastrichtian of Koryak Highlands: Krassilov et al., 1990).
Judged by the aquatic morphological adaptations and the gut contents, the Maastrichtian
duck-bills consumed a low nutritional aquatic phytomass or gyttja (Krassilov, 1981a;
Krassilov & Makulbekov, 1995).
The trophic factor might have inflicted a turnover of tetrapod communities at the
PTB as well. Most of the Permian terrestrial tetrapods became extinct towards the end
of this period. The end-Permian to Early Triassic lystrosaurids were ecologically equivalent
to the Maastrichtian hadrosaurids, both switching from browsing to aquatic grazing
in response to afforestation and the growing aquatic production (eutrophication, above).
Later in the history, afforestation over the shrub-tundra and savannoid “tundra-steppe”
landscapes drove to extinction the larger browsers of the Late Pleistocene mammalian
faunas (Ukraintseva, 1986).
Iridium. The cosmic impact model of the KTB extinction (Alvarez et al., 1980)
implies a giant crater, for which the 250 km wide Chicxulub is considered as the most
plausible candidate. It is a multiring basin with a central peak ring and a prominent Moho
rise beneath (Sharpton et al., 1992; Christeson et al., 2001). The latter feature seems to
better agree with magmatic origins. The rock melts and brecciation by allogenic impacts
(reviewed in Dressler & Reimold, 2001) are scarcely different from those caused by
explosive volcanic eruptions. Whatever the origins of Chicxulub, its correlation with the
KTB is unlikely because no evidence of an impact-generated (or a volcanic eruption
generated, as it might be) tsunami has been found in the transboundary sequences of the