Biology Reference
In-Depth Information
all of Mesozoic and Cenozoic time, rising 5°C-6°C, and possibly averaged
at least 15°C in polar regions. After that they started to decline, fl uctuated
in a transition interval between the late Eocene through the early Miocene,
and then fell further into the ice age world of the late Pliocene and Pleisto-
cene (fi g. 3.4). The trend is often described as moving from the hothouse
climates of the Paleogene to the icehouse climates of the Neogene. The ex-
pected consequence for the biota would be widespread tropical communi-
ties through about the early Eocene giving way to more temperate and sea-
sonal communities later in the Paleogene, and to increasingly widespread
dry forest and grasslands in the Neogene.
At 100 Ma, the angiosperms that defi ne most modern terrestrial plant
communities, other than the coniferous forest, were in the early stages of di-
versifi cation and radiation (e.g., Wang et al. 2009). Originating at about 135
Ma, early on they probably occupied mostly aquatic (Coiffard et al. 2007),
shallow-water to streamside, marsh, and other seasonally variable habitats
where deciduousness would impart a competitive advantage. Taxonomically
angiosperms were just emerging at the level of modern families and genera,
and ecologically just differentiating into forms with broadly similar habitat
requirements, so ferns and gymnosperms were still prominent components
of the vegetation. From this early interval, modernization at the ecosystem
level progressed from those with individual elements increasingly similar to
those of the present, to coalescence of these elements into early versions, and
fi nally greater differentiation into modern counterparts of the current com-
munities. Except for the coniferous forest, “modern” means angiosperm-
prominent communities. This tripartite progression is a convenient way to
envision the development of any ecosystem through time. However, in the
Cretaceous and early Paleocene, the environments and plant formations
were notably different from the present, so their classifi cation, as discussed
below, must refl ect the uniqueness of these ancient ecosystems.
NORTH AMERICA (NORTH OF MEXICO)
The Cretaceous climate at the high northern latitudes is described as mi-
crothermal, with some seasonality and within an overall climate distinctly
warmer than at present. This temperature classifi cation derives from fol-
lowing system:
Microthermal MAT
13°C
Mesothermal MAT 13°C-20°C
Megathermal MAT
<
>
20°C
