Biology Reference
In-Depth Information
green gymnosperms like spruce, fi r, hemlock, along with birch, have been
assembling in the northern Rocky Mountains; and after the LPTM/EECL, a
boreal coniferous forest association will begin to spread into the lowlands,
replacing the deciduous angiosperms and gymnosperms as the prominent
community. Ice is also present at least seasonally and locally in the High
Arctic in the late Eocene.
Toward the south, in the western United States in the early to middle
Eocene, the vegetation is a subtropical forest, locally modifi ed by slope,
exposure, and soils into a seasonally dry forest (shrubland/chaparral-
woodland-savanna, especially east of the Rocky Mountains); grasses will
not appear in North America until the early to middle Eocene. Character-
istic genera include Cedrela (Mexican “mahogany'), Cinnamomum (cinna-
mon), and Oreopanax . This vegetation grades into even more tropical forest
and tropical rain forest along the oxbow lakes and river channels in the
lowlands of southeastern United States, with deciduous elements mostly in
the Appalachian uplands. The estimated MAT in the southeast in the early
Eocene is about 25°C-27°C.
In Mexico, Central America, and the Antilles, the early Eocene vegeta-
tion is not known, and middle to late Eocene fl oras—Cuenca de Burgos,
Ixtapa, Mexico; Gatuncillo, Panama; Saramaguacan, Cuba; Guys Hill,
Jamaica—only provide general views of a warm-temperate to tropical veg-
etation, with variations controlled primarily by physiography and soils.
In South America in the early Eocene, there are moderate uplands in
the Northern Andes with an inlet through the present-day Maracaibo re-
gion. There are ancient uplands in the Guiana and Brazilian shields, and
another inlet occurs in the south through the Paraguay-Uruguay-Paraná
lowlands. The rest of the continent is mostly low-lying, and the Amazon
Basin is swamp with mangrove ferns, Nypa , other palms, and Pelliceria . The
proto-Amazon and Orinoco rivers are fl owing westward, and from a suit-
able vantage point in Amazonia, the Pacifi c Ocean can be seen through
low places in the early Northern Andes Mountains. The recently discov-
ered giant boid snake from Colombia suggests that by the late Paleocene
the MAT may have reached 34°C, causing the evolution and extinction of
some tropical forms and forcing the northward and southward migration of
others into more temperature-compatible zones. In short, the tropical rain
forest is changing even in the equatorial latitudes at the height of Paleogene
warmth and moisture.
Tropical rain forest grows in the lowlands of southern South America,
and elements extend to the southern tip of South America, as well as north-
ward all the way to the Arctic Circle. The temperate deciduous Nothofagus
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