Biology Reference
In-Depth Information
colpate, columellate pollen), fl owering plants diversifi ed and radiated until
by the end of the Cretaceous some lineages were similar morphologically
and ecologically to modern families and genera, and some communities
resembled or were shadowy versions of modern ecosystems. Several events
then followed that elicited evolutionary/ecological responses that consti-
tuted early phases in the modernization of particular taxonomic groups
and assemblages—their golden moment in geologic time. The fi rst of these
within the time frame considered here was at the end of the Cretaceous
with the asteroid impact. For many plants, the dark and cooling three-year
winter night probably provided a further nudge toward deciduousness that
began as selection for unpredictable habitats and polar light regimes. This
put deciduous organisms in good stead to cope later with temperate cli-
mates as temperatures fell and dryness increased in middle Eocene and
later times.
The second event occurred around 55 Ma, and it provided impetus for
the global distribution, essentially from pole-to-pole, and early diversifi ca-
tion of a tropical vegetation that had been forming in the lower latitudes
during the Cretaceous and early Paleocene since at least 58 Ma. The emis-
sion of 1500 gigatons of methane and CO
2
from openings in the sea fl oor
between Greenland and Europe during separation of the plates raised MATs
by 5°C-6°C over an already warm world. The result was essentially a global
expanse of rain forest and rain forest elements. As the rain forest expanded
north and south from the equatorial latitudes, it encountered somewhat
different climates, moderately different physical landscapes, soils, and
competitors, and very different light regimens. It likely incorporated such
Laurasian elements as the Bombacaceae and Melastomataceae, which later
became prominent tropical Gondwana families (II, chap. 8). It is further
likely that the equatorial tropical ecosystems themselves were undergoing
change. Rather than existing under a MAP of 24°C-28°C and maintain-
ing a rather a stable biota, late Paleocene and early Eocene temperatures
may have averaged 34°C-36°C, with a number of lineages becoming extinct
or moving out. The composition of ecosystems was changing because the
environment may have been too warm for some component elements. For
many early-to-intermediate stem groups of tropical plants, this was a major
evolutionary period, and it is refl ected in the change from mostly ancient to
mostly modern genera between the early and late Eocene.
The next event was a decline in global MATs into the transition time
of the late Eocene, Oligocene, and early Miocene. During this interval,
adjustments were taking place in the composition and distribution of eco-
systems. This period has not received due recognition, but it was none-
