Biology Reference
In-Depth Information
decrease the exchange of life-forms, especially tropical organisms and those
with limited capacity for dispersal, between North America and Europe
across the North Atlantic land bridge after the Eocene, and to increase the
migrations of temperate organisms and those with greater capacity for dis-
persal, over the Bering land bridge between North America and Asia in the
later Tertiary and Quaternary. The result was a realignment in the affi nities
of the North American fauna, recognized in the biogeographic literature as
the “Grande Coupure.” There was a 60 percent change in the land mammal
fauna due to increased migration of rodents and large mammals from Asia.
The shift was from small mammals and arboreal forms of the forest, includ-
ing many primates, to large herding ungulates of more open habitats—the
beginning of the North American land mammal megafauna.
Another “coupure,” but less “grande,” was the contraction of tropical veg-
etation toward the middle latitudes, into the lowlands, and along the coasts,
with a corresponding expansion of temperate and deciduous vegetation from
polar and highland habitats. There was also a noticeable modernization of
the fl ora at the generic level. A large number of plant fossils, particularly
fossil pollen, of early Eocene and older ages are diffi cult to relate to extant
genera, hence the use of an artifi cial system of nomenclature. In the middle
Eocene, however, most of the fossil specimens are similar morphologically
to extant genera and to some species. When the purpose of a paleobotani-
cal (plant macrofossil) or paleopalynological (plant microfossil) study is
geological—zonation of formations, stratigraphic correlation—an artifi cial
system is still often used for middle Eocene and younger plant fossils. If the
intent is biological, however—lineage history, vegetation and environmental
reconstruction, biogeography—the long-term, immensely time-consuming
efforts at building a comprehensive, vouchered reference collection and es-
tablishing the biological affi nities of the specimens is both possible and nec-
essary if the full potential of the methods are to be realized (chap. 4).
Modern lineages make for modern communities, and in the middle
Eocene as older vegetation types were disappearing, new and familiar
ecosystems appeared, versions of others became more recognizable, and
individual elements evolved that would coalesce later in the Tertiary. The
middle Eocene was a pivotal time when development of the vegetation ac-
celerated from essentially ancient to essentially modern at the level of gen-
era, plant formations, and ecosystems.
In terms of global trends and new paradigms, the Oligocene and early
Miocene represent a 20-million-year period of adjustment between the
conditions of the Paleocene and early Eocene and the dramatically new en-
vironments that began to develop in the middle Miocene (15 Ma)—a time
