After the sediments became rock and the bones had probably been replaced
by stone (fossilized), this part of the world, which lay near or below
sea level for millions of years, began to rise. Great forces acted upon
the earth’s crust. These forces created faults, or fractures, in the
rock crust along which movement occurred. And what had once been sea
bottom was moved upward and became lofty mountains. This titanic change
has been called the Laramide Revolution; it closed the Mesozoic Era with
the formation of the Rocky Mountains.
[Illustration: _Stegosaurus._ AN ARMORED DINOSAUR OF THE JURASSIC
PERIOD. (FROM A DRAWING BY CHARLES R. KNIGHT. COURTESY, AMERICAN
MUSEUM OF NATURAL HISTORY.)]
Although the effects of the Laramide Revolution were not as profound at
Dinosaur as they were east of it, they were quite important. The rocks
were lifted to form the southwest flank of Split Mountain—a small arch,
or anticline, on the south side of the Uinta Mountains. This mountain
building explains the pronounced southward tilt of the Dinosaur Ledge
and other rock layers visible in the quarry area. As the land rose,
streams flowed more rapidly, cutting deeper into the rocks and carrying
away the debris. Gradually thousands of feet of this debris—shale,
sandstone, and clay—were stripped away through erosion.
Finally all the material on top of the Morrison sandbar weathered away.
Some 140 million years after burial the fossil bones were exposed by the
agent that had buried them so long ago—running water! All that remained
was for them to be found, and that was the luckiest chance of all. Just
suppose they had been uncovered a million years ago—only a second in
geologic time. No one would have been present to discover them, and
through the years they could well have crumbled into dust and been blown
away.
Why Did Dinosaurs Become Extinct?
At Dinosaur National Monument only Morrison rocks of the upper Jurassic
Period contain the fossil bones of dinosaurs. After Morrison time, the
Cretaceous seas invaded this area. More than 5,000 feet of sandstone,
shale, and mudstone were formed from sediments deposited in these seas.
Elsewhere in North America and the rest of the world, the diversity and
numbers of dinosaurs actually increased. Entirely new groups evolved and
achieved success in the battle for survival. The climax of reptile
development seems to have come near the end of Cretaceous time in the
Mesozoic Era. As the dinosaurs ruled the continents, so did other
strange reptiles dominate the seas. Had you been able to see this
ancient world, you would surely have been convinced that the dinosaurs
and other reptiles would rule forever.
But it was not to be. The dinosaur hordes were wiped out and the
reptiles reduced to the position of relative insignificance they occupy
today. Such a profound and sudden change in the evolutionary trend of
life must have had a cause, and scientists have sought it. Several
theories have been proposed to explain extinction of dinosaurs, and they
are most interesting.
At the end of Cretaceous time, some of our great mountain ranges were
formed. It was a time of earthquakes and of volcanoes that belched forth
clouds of ash and rivers of molten rock. Some people would say these
catastrophic events killed all the dinosaurs. The scientist shakes his
head. If these events killed dinosaurs, why not the other animals that
lived with the dinosaurs. And what of those parts of the world that had
no volcanoes, what killed dinosaurs there?
Changes in environments, the drainage of lakes and swamps as young
mountains rose, changes in vegetation as new plants replaced old, and
sudden shifts of climate occurred. These conditions could explain local
extinction, but there were places where these changes did not occur and
yet all dinosaurs in all places died.
A one-time favorite theory suggested that increasing numbers of small
mammals ate dinosaur eggs, but there were many mammals eating dinosaur
eggs during all of Cretaceous time and the dinosaur hordes increased.
Many more mammals during succeeding ages have not killed off the
turtles, snakes, lizards, and crocodiles that lay eggs and exist in
great numbers today.
Some disease or combination of plagues may have swept the dinosaurs into
extinction. If so, no evidence has been found to date that confirms or
denies. However, most paleontologists do not accept this theory.
These are some of the theories that have been advanced to explain the
sudden extinction of dinosaurs throughout the world. Each theory will
explain the death of some dinosaurs in some places but attempts to apply
any of them, or combinations of them, to worldwide extinction have
failed.
This dinosaur story is like a mystery thriller with the last pages torn
out. A most important part is missing. That is true and the
paleontologist knows it. He also knows the riddle will probably never be
solved. He might point out, however, that no one has successfully
explained the extinction of the passenger pigeon which occurred quite
recently, nor do we know why some other species of wildlife are on the
brink of extinction today. The paleontologist is not the only one who
must say, “I don’t know.”