Encyclopedia of Evolution.pdf - Online Reading Center

fossils and fossilization
The grain of the wood is still visible in a petrified trunk of a tree that
lived about 00 million years ago, now in Petrified Forest National Park,
Arizona. (Photograph by Stanley A. Rice)
derived. The chemical differences between the fossil and its
matrix can sometimes be of help in extracting the fossil. For
example, shells of invertebrates can sometimes be isolated if
the matrix is dissolved in concentrated hydrofluoric acid.
Sediments are the major, but not the only, place to find
fossils. Many insects, stumbling into conifer sap, became
entangled and engulfed; the sap became amber, and the
insects were preserved in exquisite detail. This happened frequently
enough that there is a worldwide market for insects
preserved in amber.
Ancient wood other than petrified wood, human bodies
from a bog, and mummies preserved in Andean caves are not
usually considered fossils, nor are bodies preserved by human
activity (such as Egyptian mummies). Fossils retain the structure
of the original organisms, sometimes with considerable
internal detail; therefore coal and oil are not considered fossils,
since they have been transformed into uniform organic
materials (even though they are called fossil fuels). These
remains can be as valuable as fossils in scientific research.
Leaves that have been preserved for 20 million years in cold,
wet, anoxic conditions in deposits near Clarkia, Idaho, are
still green but oxidize into black films upon exposure to air.
The leaves were so chemically intact that scientists could
extract chloroplast DNA from them (see DNA [evidence
for evolution]).
The hard parts of organisms (such as the calcium carbonate
shells of invertebrates, and the calcium phosphate bones
of vertebrates) are most easily preserved and constitute the
vast majority of fossils. Teeth are particularly popular fossils
to study, not only because they are hard (and abundantly preserved)
but also because they can reveal what kind of diet the
animal had—grinding teeth for grazers, sharp teeth for carnivores.
The study of fossil bones can also reveal more than
just the anatomical structure of the animal. Study of tissue
layers inside of bones reveals the growth rate of the animal
and its age at death. This is how scientists have determined
that Tyrannosaurus dinosaurs had a high metabolic rate and
grew rapidly, up to 880 pounds (400 kg) by the age of 20
years (see dinosaurs).
Under conditions of very slow decomposition and
very thorough burial, soft parts of organisms, and entire
organisms without hard parts, can be preserved. A famous
example is the Burgess shale in Canada. Impressions of
soft-bodied animals, many parts of plants, and even singlecelled
microbes have frequently been preserved. Since fossilization
almost always requires burial in sediment, most
fossils are of the aquatic organisms that already live in or
near the sediments. Aquatic invertebrates therefore constitute
the best fossil assemblages. Terrestrial organisms are
fossilized when they fall into or are washed into the sediments,
as when primitive birds fell into low-oxygen pools
that later became the limestone deposits of Solnhofen limestone
in Germany (see arcHaeopteryx). The odds are
very much against the preservation of fossils from terrestrial
organisms: It is estimated that only one bone in a billion is
preserved as a fossil. If all 290 million Americans, most of
whom have 206 bones, were to die, only about 60 bones
would be fossilized under normal conditions. The odds
are not even good for organisms of shallow oceans, where
only 40 percent of the organisms have even the possibility
of being fossilized. Rarely, massive graveyards of fossils
(Lagerstätten) such as the Burgess shale or the Solnhofen
limestone have resulted from conditions that were remarkably
good for preservation.
An intimate knowledge of biology is necessary in the
study of fossils. This allows a great deal of information about
the living animal to be reconstructed from the fossils of just
its bones. Sites of muscle attachment leave traces on the
bones, therefore the musclature of an extinct animal can be
reconstructed from bone fossils. Blood vessels inside the living
bones may have left traces in the fossilized bones, allowing
scientists to determine the extent of blood vessel formation,
and hence whether the animal was warm-blooded. Occasional
lucky finds reveal even more about the animal’s life.
Gastroliths are smooth stones found associated with dinosaur
skeletons. Like modern birds (which are classified with
the dinosaurs; see birds, evolution of), many dinosaurs
swallowed stones which helped to grind plant food in their
stomachs. The grinding action caused the stones to become
smooth. Coprolites (fossilized feces) often allow scientists to
reconstruct the diet of the animal.