Terrestrial Palaeoecology and Global Change

Chapter 9. Crises
361
of biological diversity. Mass extinction is a rough regulation of biological diversity via
population strategies adjusted to long-lasting environmental disturbances.
IX.5. Replacement of ecological dominants
The idea of biotic turnovers over the critical boundaries has originated from the replacements
of charismatic groups, in particular, of dinosaurs by mammals. This latter
replacement signifies the end of the Mesozoic and the beginning of the new era. Opponents
of the idea have argued that, since dinosaurs constituted but a minor part of biological
diversity, their elimination (typically considered in this line of thought as the result of
a competitive replacement by mammals) does not signify a global biotic turnover of
whatever, if any, scientific meaning. If we instead focus on the replacement of gymnosperms
by angiosperms, then the boundary of geological eras (the Mesophytic/Cenophytic)
must be placed in the middle of the Cretaceous. A choice of one or another
option for the boundary is therefore a matter of convention.
Such considerations have led many evolutionists to believe that stratigraphic levels of
accelerated turnover rates can be objectively defined for particular lineages rather than
for biota as a whole. The eras of life need not be taken literally as distinct stages of
organic evolution. Some lineages do show high turnover rates over the selected boundaries,
others do not. Plants have evolved under a different beat than animals while the
dinosaur extinctions on land at approximately the same time level as the ammonite extinctions
in the sea is a mere coincidence.
However, with the rise of biospheric ecology, this logic no longer seems faultless.
Ecological niches of the larger dinosaur herbivores seem to have remained vacant for at
least 5 m.y. after their extinction, hence competitive elimination is unlikely. Some 150
m.y. long coexistence of dinosaurs and mammals only means that the dinosaurian communities
and their contemporaneous mammalian communities were fairly well-equilibrated,
with minimal niche overlaps, while the replacement means that this long-standing
ecological equilibrium was eventually disrupted. Adaptive radiation of modern mammals
postdates, rather than predates, the decline of dinosaurs. Archaic mammals perished
simultaneously with dinosaurs, although over a somewhat longer time interval. The buildup
of mammalian diversity is due to new types of vegetation, such as grassland, as well
as new trophic niches, such as frugivory.
The gymnosperm/angiosperm story is fairly consistent with the above version of
the dinosaur/mammal story (Krassilov, 1997a and elsewhere). Both only make sense
in the context of biospheric evolution. Angiosperms first appeared in the mid-Neocomian,
concomitantly with the therian mammals (Aegialodon dawsoni and subsequent
records of the metatherian–eutherian grade: Lillegraven, 1969; Lillegraven et al., 1979;
Fox, 1980) and the true birds, jointly signifying a major innovation of terrestrial plant
communities.