Geoscience Reference
In-Depth Information
lofted dust and rocks fell to earth through the atmo-
sphere and were heated by friction to temperatures
higher than most unprotected animals could tolerate.
Fires would have burned across earth's surface wherever
there was sufficient fuel. Supporting this hypothesis is
the fossil record, which shows that, on land, only rela-
tively small animals survived—those sheltered from
the heat in burrows or rock crevices. they included a
variety of small mammals that had managed to coexist
with the dinosaurs for millions of years. Also, protected
by the water, aquatic life such as turtles and crocodiles
survived. could a comparable asteroid strike earth
again? over timespans of hundreds of millions of years,
the improbable becomes probable.
pine, redwood, spruce, sweet gum, sycamore, walnut,
and willow. others, such as breadfruit and palm, are
found today only in warmer climates. Some trees were
large, up to 75 feet tall and 5 feet in diameter, as indi-
cated by fossil logs in Yellowstone national Park and the
Shirley Basin south of casper (see chapter 15). 12
coastal swamps were widely scattered during this
time, with cypress and other wetland trees—similar
perhaps to the cypress swamps of southeastern north
America today. Sphagnum bogs around the edges of lakes
were also widespread. With a warm, subtropical environ-
ment year-round, and an estimated annual precipitation
of 100 inches of rain per year, plant growth was rapid.
Moreover, the anaerobic conditions of the swamps and
bogs enabled preservation of dead plant material. With
continued uplift and erosion from the adjacent moun-
tains during the Laramide orogeny, the accumulating
peat was overtopped with deep sediments here and there
that compressed the plant material into coal. today, most
coal is too deep to mine, except where further uplifting
and erosion occurred, leaving some of it near the sur-
face. that accounts for the abundant, easily accessible
coal in the Powder River Basin—one of the world's larg-
est deposits. Some of the seams are more than 200 feet
thick. Because this coal developed largely in a freshwater/
estuarine environment, its sulfur content is low. 13
Plant fossils in the Bighorn Basin have been studied
extensively. Scott Wing, a Smithsonian institution paleo-
botanist, concluded in 1981 that 54 to 38 million years
ago, during the eocene, the basin had a subtropical cli-
mate with a dry season. the characteristic vegetation was
an evergreen broad-leaved forest with some conifers. For-
ests dominated by conifers most likely were characteristic
of the mountains, but snowfall was probably a rare event,
and then only at high elevations. Many of the fossils are
of plants that are common in the region today, whereas
others now occur naturally only in Southeast Asia, for
example, ginkgo and dawn redwood. Wing concluded
that the different kinds of plants were segregated into
different communities along gradients of temperature,
water availability, and salinity. He estimated that the
annual precipitation in the Bighorn Basin at that time
was 60-100 inches, much higher than the 4-24 inches
that falls there now. However, the shrub that became
known as greasewood was present, suggesting semi-arid,
saline environments in some places. 14
The Age of Mammals
With the dinosaurs gone, mammals evolved to occupy
the vacated ecological niches. this marked the begin-
ning of the Cenozoic. early in this era, in the Paleocene
epoch, the mammals included condylarths, creodonts,
tillodonts, pantodonts, and multituberculates—for
which no common names can be found in the modern
fauna. Most were small, described by paleontologists
as shrew-like, rodent-like, weasel-like, or beaver-sized.
Some of the herbivores had long, sharp front teeth,
possibly to assist with digging roots and tubers. they
might have been omnivores. though condylarths were
small in size, a branch of the group included the ances-
tor of present-day hooved animals, such as horses, elk,
and pronghorn. the pantodonts included the very first
large mammals, which have been described as bear-
like or hippo-like herbivores. early primates, rodents,
and coyote-sized carnivores also existed at this time in
the area that would become Wyoming. After another
10 million years of evolution, by the beginning of the
eocene epoch, these forms, along with bats and oth-
ers, had evolved into the verifiable early ancestors of
the mammals we recognize today. 11
Flowering plants also became increasingly common
during the early cenozoic. Some types of trees, shrubs,
and herbs found during this time, about 60 million
years ago, are still living in parts of north America and
eurasia. the trees included ancestors of alder, beech,
birch, black locust, chestnut, cottonwood, cypress, dog-
wood, elm, fir, ginkgo, hickory, magnolia, maple, oak,
 
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