The partial skeletons of more than 20 individual dinosaurs and the
scattered bones of about 300 more have been discovered in the Dinosaur
Quarry. Many of the best specimens may be seen today at museums of
natural history in the larger cities of the United States and Canada.
The quarry is easily the largest and best preserved deposit of Jurassic
dinosaurs known today.
How and why did so many dinosaur skeletons accumulate here? How were
they preserved? These are among the common questions asked of park
rangers and naturalists at Dinosaur. The answer is a combination of
circumstances and luck.
Many people get the impression from the mass of bones in the quarry wall
that some catastrophe such as a volcanic explosion or a sudden flood
killed a whole herd of dinosaurs in this area. True enough this could
have happened, but it probably did not. The main reasons for thinking
otherwise are the scattered bones and the thickness of the deposit. In
other deposits where the animals were thought to have died together, the
skeletons were usually complete and often all the bones were in their
proper positions, or articulated. In a mass killing the bones would have
been deposited on the stream or lake bottom together at the same level,
but in this deposit the bones occur throughout a zone of sandstone about
12 feet thick. The mixture of swamp dwellers and dry-land types also
seems to indicate that the deposit is a mixture derived from different
sources. Rounded fragments of fossil bone have been discovered in the
quarry—fragments that attained their pebblelike shape by rolling along
the stream bottom.
If the mass of bones was not the result of catastrophe what did happen?
The quarry area is a dinosaur graveyard, not a place where they died. A
majority of the remains probably floated down an eastward flowing river
until they were stranded on a shallow sandbar. Some of them, such as the
stegosaurs, may have come from far-away dryland areas to the west.
Perhaps they drowned trying to ford a tributary stream or were washed
away during floods. Some of the swamp dwellers may have mired down on
the very sandbar that became their grave while others may have floated
for miles before being stranded.
Even today similar events take place. When floods come in the spring,
sheep, cattle, and deer are often trapped by rising waters and
frequently drown. Their bloated carcasses float downstream until the
flood recedes and leaves them stranded on a bar or shore where they lie,
frequently half buried in the sand, until they decompose. Early
travelers on the Missouri River reported that shores and bars were
frequently lined with the decomposing bodies of bison that had perished
during spring floods.
In Dinosaur National Monument, the positions in which partial skeletons
of the dinosaurs lie suggest that they decomposed on a sandbar. The
bones on the underside of a skeleton are often arranged as they were
when the animal was alive, while those on the upper or exposed side may
be scattered. Such scattering would be expected as the ligaments and
muscles holding the bones together decomposed; stream currents and
scavengers could then disperse them. Stream currents are suggested by
the position of the long, flexible tails and necks of the large plant
feeders. These, like streaming water plants in a river, trail downstream
to the east.
[Illustration: _Camptosaurus_—AN ARNITHISCHIAN PLANT-EATER. (DRAWN
BY J. G. GERMAN. COURTESY, AMERICAN MUSEUM OF NATURAL HISTORY.)]