The geologists who attempt to reconstruct the geography and climate of
the Jurassic Period first gather all possible facts and try to fit them
together to form a logical pattern. The results are then examined for
weak points and an attempt is made to find field evidence throwing light
on these weak points. The final result represents the sum of our
knowledge at the time but is subject to change as new facts are
obtained. Thus the following outline represents present thinking that
may be changed somewhat by future studies.
The land for miles around the Dinosaur Quarry was a low-lying desert in
early Jurassic time. The mountains you see now had not yet been formed,
and the whole desert area lay close to sea level. Great restless sand
dunes drifted across this level land to form a blanket 700 feet thick.
As the earth’s crust sank, these dunes were covered by a long arm of an
arctic sea that extended southward along the present trend of the Rocky
Mountains across Canada, Montana, Wyoming, and Utah. Millions of years
later, in late Jurassic time, when the Morrison formation was deposited,
the area rose again and the stage was set for the dinosaurs.
Imagine if you can, the vast plains extending from Mexico to Canada and
from central Utah to the Mississippi River. To the west were high
mountains in the Great Basin region of Nevada and western Utah. From
these highlands flowed great sluggish streams that carried large amounts
of sand and silt. Since the plains were almost flat, swamps and small
lakes were probably numerous. The streams may have changed their courses
from time to time as they were not confined to deep valleys. When the
whole region emerged from the sea the climate became more humid.
Volcanoes were active far to the west; the winds carried clouds of
ejected dust eastward and deposited them on the plains. Semi-tropical
conditions probably existed throughout the United States and in parts of
Canada. These deposits are called the Morrison formation.
The warm humid climate provided ideal conditions for plant growth. Great
forests of lush vegetation covered the land. Many of these plants have
since disappeared, but some of their related species may be found today
in the tropics. Most of the plants of our Temperate Zone had not yet
evolved. However, there were tall stands of a type of pine, and other
evergreens. There were also gingkos and curious tree ferns.
Various herb ferns formed a ground cover as thick and lush as grass on a
well-watered prairie. Palmlike ferns resembling today’s cycads were
common, while along the river grew horsetail rushes like those living
today. Flowering plants of the Recent Epoch of geologic time (in which
we are now living) had not yet made their appearance. Thus the hardwood,
broadleaf forests of oak, elm, beech, maple, and similar trees were
absent. So too were the flowering shrubs familiar to the Temperate Zone.
Even the grasses were missing.
However, if you could picnic in this strange plant world you would soon
be slapping mosquitos and cursing the ants. For even in such ancient
times these insects were present; and so were a great variety of other
insects as is known from the more than 1,000 species that have been
discovered in Jurassic rocks. Among them were representatives of most
modern orders such as grasshoppers, beetles, moths, ants, and flies.
Jurassic insects probably looked much like those of today.
Among the most interesting of the strange reptiles were the pterosaurs
that dominated the skies. They resembled the modern bats in some ways
but their leathery wings were supported on each side by one finger
instead of four, and their skins were scaly or bare instead of hairy.
Some forms had long tails that were flattened at the tip and helped them
balance in flight, but others were tailless. Some pterosaurs were no
larger than sparrows while others had wing spans of 3 to 4 feet.
Crocodiles sunned themselves on the banks of sluggish streams and lakes.
They probably looked a good deal like those that live in modern swamps
and their habits were similar. Many a small dinosaur fell victim to
their stealthy attack and disappeared beneath the waters of some ancient
stream.
Birds have been found in Upper Jurassic rocks of Germany and may have
lived here too. Their fossil remains would probably have been classed as
reptiles had not feather imprints been a part of them. About the size of
crows, these reptile-like birds had small conical teeth, three-clawed
fingers on each wing, and a long tail instead of the fan of feathers
seen on modern birds.
Small mammals were also living at the time the Morrison formation was
being deposited and their remains have been found in the dinosaur quarry
at Como Bluff, Wyo. The largest ones were about the size of a house cat
but the majority were much smaller, probably about the size of today’s
mice.
We do not know much about the habits of these early mammals but they
were probably rather shy and retiring. This would be expected in the
world of giants where they lived. Some of them lived in trees and there
was one group whose skull characteristics resemble those of the rodents.
It is likely that these primitive mammals lived a life similar to that
of the rodents millions of years later.
This, then, was the setting, the stage upon which the dinosaurs played
their leading roles. Although we have restricted our discussion to
Morrison time in northeastern Utah, the same or similar animals lived
all over the world. Worldwide humid and mild climates produced a
similarity of plant and animal life during most of the Mesozoic Era
whose like has not been seen in the last 60 million years. It was a
strange world and ruled by strange animals, but it must have been an
interesting one.