Biology Reference
In-Depth Information
early Eocene, it will return and form the now familiar Southern Hemi-
sphere forests of southern beech.
Along the trek from pole to pole at 100 Ma, the average temperatures
are from about 12°C-14°C in the Arctic, to 23°C-25°C across the equator,
to 12°C-14°C in southern South America (a range of 13°C). At present, the
Earth's surface temperatures range from
40°C to 40°C (a range of 80°C).
Particularly relevant for the warm climates to come in the late Paleocene
and early Eocene, and for the declines in the middle Eocene and in the
middle Miocene, is that at 40°C air holds 470 times more water than at
-
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40°C (Flannery 2005).
FIFTY-FIVE MILLION YEARS AGO
What a difference 45 million years can make. During the period between
100 and 55 Ma, the angiosperms replace the gymnosperms as the promi-
nent component in most plant communities. The New World ecosystems,
beginning with the equable conditions of the Middle Cretaceous, experi-
ence an overall cooling of 4°C, then begin what is initially a slow climb to
warmer climates. There is drainage of the epicontinental seas, an asteroid
lands in Mexico, and the dinosaurs disappear (with the exception of their
avian descendents), which facilitates the rise to prominence and the per-
sistence of the primates, although as small arboreal forms they may have
evolved successfully anyway. In the earliest Paleocene, ungulates (hooved
animals) appear. Approaching 55 Ma in the early Eocene, there is radiation
of rodent groups, artiodactyls, and perissodactyls (equids, rhinos, and tapir-
oids); signifi cant uplift of the western North American mountains; exten-
sive fl oral and faunal exchange with Europe, especially of tropical elements
in the early through middle Eocene; and the beginning of disruption of the
North Atlantic land bridge in the middle to late Eocene. The Brooks Range,
Alaska Range, and the Rocky Mountains reach half or more of their present
elevation; a collage of terranes forms most of western North America; the
rise of the Rocky Mountains continues the stretching and thinning of strata
to the west, resulting in further differential uplift and collapse to form the
Basin and Range Province; and the Sierra Nevada begin to appear.
At 55 Ma, the epicontinental sea is gone from most of interior North
America, although there are still shallow waters across much of the Yucatán
Peninsula and the Isthmus of Tehuantepec. Along the Atlantic Coastal Plain,
the shoreline extends inland to the foothills of the Appalachian Mountains,
around their southern end northward to about Cairo, Illinois, and then
southward to just east of Austin to Del Rio, Texas, and into Mexico follow-
