Environmental Engineering Reference
In-Depth Information
reconstructions, and speculations that help understand the dynamic nature of the biotic
communities of
La Frontera
in their deep historical and biogeographical context.
6.2 Out of the Tropics
Long before the deserts of North America evolved, the climate and vegetation of
La Frontera
were tropical. In the Paleocene, geological time period that began with the extinction of
the dinosaurs 65 million years ago (mya), temperate evergreen and tropical rainforests
were widespread across the continent with little regional differentiation.
7
The climates of
North America were warm and humid with forests with strong Asian affinities; primi-
tive ferns (
Anemia
), cycads (
Dion, Zamia
), and palms occurred as far north as Alaska and
70°N Lat. in Greenland. Eocene fossils of several palms (
Phoenicites, Sabalites
) were found
in Alaska.
7, 8
An alligator, a soft-shell turtle, a primitive tortoise, a primitive monitor lizard,
and a ground boa were reported from Eocene sediments at 78°N Lat. on Ellesmere Island,
Canada.
9,10
Through the Eocene (54-35 mya), deciduous trees became increasingly more common,
providing the first evidence of a dry season and the presence of tropical deciduous for-
ests.
11-13
There were evolutionary radiations in many plants and animals as they adapted
to new heat and moisture regimes as more sunlight penetrated the forest canopy to the
ground. Many of the adaptations to aridity in modern desert plants and animals evolved
in the dry seasons of these early Tertiary tropical forests.
6.3 Miocene Revolution
6.3.1 Mountain Building
From the late Oligocene to the middle Miocene (about 30-15 mya), a series of enormous vol-
canic eruptions changed the climates and established the modern biogeographic provinces
of North America.
14
The Rocky Mountains were uplifted at least 5003 ft near Florissant,
Colorado, while more than 7004 ft of volcanic rocks were deposited in the Jackson Hole
area of west-central Wyoming.
15
During the same period, a kilometer-thick layer of rhyo-
litic ash settled in the Sierra Madre Occidental of western Mexico—overlaying a kilometer
of early Tertiary andesites!
16
With the uplift of the mountains, there were profound climatic and biotic consequences.
The upper flow of the atmosphere was blocked for the first time, preventing tropical mois-
ture from both the Pacific Ocean and the Gulf of Mexico from reaching the mid-continent,
thereby drying out the modern Great Plains and Mexican Plateau. Harsher climates initi-
ated evolutionary radiations in the modern successful plant and animal groups as well
as segregated drought- and cold-tolerant species into new environmentally limited biotic
communities, or biomes, including tundra, conifer forests, and grasslands that were dis-
tributed along elevational and latitudinal environmental gradients.
13,17
Tropical forests
were restricted to ribbons along the lowlands of Mexico and central America. Thornscrub,
the dry vegetation found today at the lower, drier edges of tropical deciduous forest, was
likely the regional vegetation covering the drier areas in the present Chihuahuan and
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