The purpose of this document is to familiarize you with some of the major groups of fossils which have proved important in determining relative time (in allowing us to establish a sequence of events for the history of the Earth).
Commonly, paleontology is divided into three areas:
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1) vertebrate paleontology, the study
of animals with backbones;
2) invertebrate paleontology concerns itself with animals without backbones, and
3) paleobotany, the study of fossil plants.
Vertebrate paleontology is significant because it provides information on the evolution, distribution and habits of the group of organisms to which we belong; and because vertebrates have been the most successful large* animals to adapt to land.
Paleobotany.
Despite occasionally spectacular finds, the fossil record of plants is not
as well known as that of animals, and in general, plant fossils have not
yet turned out to be as useful geological tools as animal fossils.
Palynology, the study of plant pollen has been extremely useful in the
study of past ecosystems.
(* The Phylum Arthropoda with its millions of species is by far the most
successful group to adapt to land)
Fossilization
There are many different ways in which an
organism can leave evidence of its presence.
Direct evidence means that we have
some part of the animal or plant itself. Rarely, we find preserved
the remains of the entire animal. In one famous case, a baby
mammoth (an extinct relative of elephants) was found frozen in the
permafrost (permanently frozen ground) of Siberia, complete with the
mouthful of buttercups he was chewing on when he fell to his death.
Usually, however, we are not that lucky. Most often, but not always,
the soft tissues decay; the picture is that of actual mammoth hair.
Usually we find hard parts such as shells, bones or teeth. Often,
these hard parts are embedded in solid rock and have themselves turned to
rock, or petrified. Petrification can occur in several ways. If the
original material is still there, but all the voids have been filled by
some mineral, we speak of permineralization. If the original
material has been replaced by another, we call it replacement.
Sometimes, only a thin film of organic matter remains, as we often see in
the case of leaves, graptolites or the remains of some fishes. This is carbonization.
Not
all evidence includes the actual remains. Indirect
evidence includes molds, casts, imprints, coprolites and
tracks. Molds are the hollows left in the surrounding material (the
matrix) after the remains of the organism have been dissolved or removed.
Some of the most famous historical molds were discovered in Pompei where
the remains of fleeing Romans were entombed in the vulcanic ash of the
79AD eruption of Mt. Vesuvius. Archeologists were able to fill these molds
with plaster and these casts provide us with a glimpse of the
people as they fell and died 
from
suffocation. This process occurs when a different mineral fills in molds
and preserves a replica of the original. Leaves and other fossils often
leave their imprints on rocks and, in a way, tracks are
nothing but a special kind of imprint. In some cases we only know that an
organism existed because we have found its tracks. Finally coprolites,
(fossilized feces) have given us much information about the digestive
systems and dietary habits of many an extinct animal, as well as for early
man.