Landscape History - Mountains and Plains: The Ecology of Wyoming Landscapes - page 13

from the older rocks and washed into the newly defined
inter mountain basins.
the Mesozoic would have been especially exciting
for biologists, as dinosaurs were abundant in the still-
tropical climate. About forty different kinds are recog-
nized in the fossil record from Wyoming alone. 5 Some
were huge, suggesting that the vegetation was abundant
enough to provide large amounts of food. others were
small, less than a few feet long. these reptiles, along
with other nondinosaurian reptiles like the flying ptero-
saurs, were important residents of earth for more than
100 million years. Fossils of some of the most famous
dinosaurs have been found at como Bluff, between the
towns of Medicine Bow and Rock River. they include
Diplodocus, Apatosaurus, Stegosaurus, Allosaurus, and
Dryosaurus. 6 Bison, pronghorn, and other familiar
mammals would not be present for another 200 million
years.
coexisting with the dinosaurs were flowering plants
(angiosperms), such as magnolia, palm, fig, breadfruit,
sassafras, cinnamon, sweetgum, and willow trees, along
with tree ferns, giant horsetails, clubmosses, and vari-
ous kinds of gymnosperms. 7 each of these types of
plants had various species, some adapted to the low-
lands and others to the uplands. Some coal deposits
began to form during this time, coincident with the
dinosaurs, but most coal in the Powder River Basin did
not begin forming until the Paleocene, about 60 million
years ago, long after the large reptiles became extinct.
the Mesozoic ended with a cataclysmic event that
many believe led to the worldwide extinction of dino-
saurs and numerous other forms of life. Known as the
cretaceous-Paleocene extinction event, 8 its cause is
still debated, but there is little doubt that it was dra-
matic, possibly lasting only a few hours. A widely
accepted explanation associates the extinction with a
large asteroid impact on what is now the north shore of
the Yucatan Peninsula. this occurred about 65 million
years ago, spewing huge quantities of dust into space,
greatly reducing the amount of sunlight reaching earth,
which created cold temperatures that many species
could not tolerate. 9
But recently, an alternative explanation associates
the extinction with heat rather than cold. 10 According
to this hypothesis, earth's surface was heated by the air-
borne material resulting from the asteroid's impact. the

WYOMING

equator

Fig. 2.2. Approximate location near the equator, about 240
million years ago, of the land area now known as Wyoming.
Adapted from Lageson and Spearing (1988).

were uplifted and essentially leveled by erosion within
tens of millions of years.
By the early Mesozoic Era , about 240 million years
ago, most of Wyoming was slightly above sea level—
and still near the equator (fig. 2.2). Fluctuating coastal
environments in the triassic Period led to formation of
the colorful sandstones and shales of the chugwater
and Spearfish formations, both rich in iron oxides and
known as redbeds. Subsequently, in the Jurassic Period,
extensive sand dunes were consolidated into the nug-
get Sandstone. Various episodes of volcanism occurred
toward the end of the Mesozoic, during the cretaceous
Period, leaving deposits of fine ash across the landscape
and in the sea. 3 Much of this ash became bentonite, a
sodium-rich clay of considerable economic and ecologi-
cal importance.
Also during the cretaceous, thrust faulting created
the Hoback, Salt River, Sublette, and Wyoming moun-
tain ranges, now located near the Wyoming-idaho
border. 4 these mountains, known collectively as the
overthrust Belt and now an important source of oil
and gas, are composed of sedimentary rock. Another
round of mountain building began about 75 million
years ago, during the late cretaceous, with folding
and faulting of earth's crust throughout the region.
For about 30 million years, Precambrian granite and
other igneous and metamorphic rocks, together with
overlying sedimentary strata, were thrust upward. this
period of mountain building is known worldwide as
the Laramide orogeny. the overlying Paleozoic and
Mesozoic sedimentary strata gradually were eroded