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The Fossil Remains of Dinosaurs
If dinosaurs died out 66 million years ago, how do we know as much about
them as we do? Nearly all of what we know about dinosaurs is based upon
the story that fossils tell.
Fossils are the remains of long-dead plants or animals. When an animal
dies, its body can be eaten by other animals, it can slowly rot away until
nothing is left, or it can be buried by sand or mud. If is buried by mud
or sand, its harder parts, generally teeth and bones, may be preserved.
As years pass, more mud or sand covers the remains, pressing down with
greater and greater pressure. Eventually, the sand and mud changes into
rock. Moisture in the rock seeps into the remains, gradually hardening
them and turning them into rock, too. Bone becomes rock, but rock that
appears different from that which surrounds it.
If the animal's body is covered quickly, under special, quiet circumstances,
some of the softer parts may be preserved. The animal's skin may form an
impression in the mud or moist sand before it rots, leaving a vacant space
that is later filled with minerals. This is called a natural cast, and
when it occurs, we have a record of what the texture of an animal's skin
was like.
The footprints of dinosaurs walking through mud along the shores of ancient
waterways have also been fossilized. These trackways give us valuable clues
about the way in which the dinosaurs moved and whether they traveled in
groups. Sometimes the footprints of theropods appear on top of those of
herbivores, suggesting that the herbivores were being stalked.
Because the Earth has changed dramatically in 66 million years, dinosaur
fossils are found in all sorts of places. A fossilized seashore with dinosaur
tracks, for example, may have been tossed by geologic forces to form the
nearly-vertical side of a cliff, making it look as if the dinosaur had
been running up the cliff. The fossil of a mammal dating from 55 million
years ago was found in an oil well drill core drawn from a depth of 2,460
feet (750 meters) beneath the surface of Louisiana; how much deeper might
dinosaur fossils lie?
Fossils are exposed naturally by erosion, accidentally by the digging that
man does in road building and other excavations, or quite intentionally
by paleontologists. Paleontologists never begin to dig blindly: the Earth
is too large, and fossils are too small. They know which locations are
likely to conceal the types of fossils they wish to uncover, and they know
which strata (layers of earth) in which to search.
The recovery of fossils is done in progressively finer stages. At first,
power tools may be used to remove large amounts of non-fossil-bearing earth.
Then shovels and pickaxes remove the earth closer to the fossils. Hand-spades,
hammers, and chisels come next, and finally, dental picks, “shovels” the
size of spoons, awls, and whiskbrooms.
As they are uncovered, fossils must be treated very carefully. Exposure
to air begins to destroy them, so paleontologists must cover them with
various coatings, such as plaster-soaked bandages.
As important as anything else is to record carefully where each fossil
was found and its relationship to all the others around it before it is
moved. This is done in the field by photography and sketching. Each fossil
is numbered as it is removed from the earth. Numbering each specimen and
recording its position helps when scientists begin to sort out the fossils
later in a laboratory, grouping them, and reassembling them.
One of the first fossil collectors we know of was not a scientist but a
nameless woman who lived some 30,000 years ago and apparently appreciated
the intrinsic beauty of fossils. In her grave, archaeologists found a necklace
composed of fossil seashells.
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