Biology Reference
In-Depth Information
is considerably reduced in extent in Patagonian Chile and Argentina. South
America is still connected through Antarctica to Australia, and Araucaria
and Casuarina are widespread.
At 55 Ma, the ecosystems encountered from pole to pole are beach/
strand/dune; freshwater herbaceous bog/marsh/swamp; aquatic, present
since the Late Cretaceous; lowland neotropical rain forest, present since
the Paleocene; lower to upper montane broad-leaved forest, including a de-
ciduous forest association in the far-northern lowlands and in highlands far-
ther south; the beginnings of a coniferous forest (montane coniferous forest
in the northern Rocky Mountains, spreading into the lowlands northward
as the boreal coniferous forest, and into mountainous regions to the south
as montane coniferous forest); versions of shrubland/chaparral-woodland-
savanna (without extensive grasses) on drier sites, and mangrove (without
Rhizophora ) along the tropical shores. At about 45 Ma in the middle Eo-
cene, ecosystems and lineages will take on a decidedly more modern aspect.
There are still no extensive modern (i.e., angiosperm-dominated) deserts,
grasslands, alpine tundra (páramo), or tundra.
SEVENTEEN MILLION YEARS AGO
The physical confi guration of North America at 17 Ma consists of the west-
ern cordillera approaching modern elevations by uplift and accumulation of
volcanic materials in the Brooks and Alaska ranges, Rocky Mountains, and
the Sierra Madres of Mexico, and by erosion down to near-modern levels
in the Appalachian Mountains. Lava fl ows and ejection of ash in the west
provide ideal conditions for the preservation of extensive Miocene fl oras
and faunas that will leave an extraordinary record of lineage and ecosystem
history. In particular, the fl ood basalts of the Columbia River region will
eventually cover 63,000 mi 2 and accumulate to over 6000 feet in thickness.
There will be over 300 individual fl ows between 17 and 6 Ma. To the south-
west, the Colorado Plateau in the Miocene extends over 130,000 mi 2 at the
junction of Colorado, New Mexico, Utah, and Arizona, and it is between
5000 and 11,000 feet thick. Between about 20 and 24 Ma, the plateau had
undergone a period of major uplift that raised it more than 3 km, and 1 km
above the adjacent Basin and Range Province. At 17 Ma, the Sierra Nevada
Mountains are beginning to cast a rain shadow, creating increasingly dry
conditions at low to middle elevations across the Colorado Plateau and the
Basin and Range Province.
In the middle Miocene, the Florida Peninsula south of about 27°N is
mostly submerged, shallow water still covers much of the Isthmus of Te-
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