Page 71 - Introduction to Paleobiology and The Fossil Record

58 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
The paleontological study of taphonomy,
which includes all the processes that occur
after the death of an organism and before its
fossilization in the rock, may seem ghoulish.
In fact, many of the analytic approaches used
by taphonomists are also used by forensic
scientists. A crime scene investigator who is
called to inspect a corpse may be asked how
long ago the body was buried. The forensic
scientist looks at the state of decay – is there
any flesh remaining, do the bones still contain
fat, what do the remnants of hair and fi nger
nails look like? But now there is a whole
armory of analytic techniques. For example,
measurement of the chemistry of the bone
and, in particular the assessment of the rare
earth elements (scandium, yttrium and the 15
lanthanides), can help pinpoint the time of
death. These forensic science methods are
used by archeologists and, stepping back
farther in time, also by paleontologists.
A related issue is the quality of the fossil
record. Following the decay and loss of fossils,
what is actually left? Can paleontologists trust
the rock record and use their patchy fossil
finds to somehow understand large-scale pat-
terns of evolution? Critics are right to point
out that paleontologists should be careful
when they attempt to reconstruct a whole
plant or animal, and try to understand its
biomechanics, when they have just a few
bones or bits of twigs. Care is required
also in seeking to understand patterns of
diversity change and evolution when many
fossil species are missing. There is a heated
debate about this issue, with some scientists
claiming that the fossil record is desperately
bad and next to useless, while others claim
that the fossils do, in fact, tell us the history
of life. We will look at taphonomy fi rst, and
the changes that have occurred in typical
fossils since they were living organisms, and
then consider the wider implications for
paleobiology.
FOSSIL PRESERVATION
Fossilization
When a plant or an animal dies, it is likely
that it will not end up as a fossil. For those
that do, there are several stages that normally
occur in the transition from a dead body to a
fossil (Fig. 3.1):
1 Decay of the soft tissues of the plant or
animal.
2 Transport and breakage of hard tissues.
3 Burial and modification of the hard
tissues.
In rare cases, soft parts may be preserved, and
these examples of exceptional preservation
are crucially important in reconstructing past
life.
There are two kinds of fossil, body fossils,
the partial or complete remains of plants or
animals, and trace fossils, the remains of the
activity of ancient organisms, such as burrows
and tracks. In most of the book, “fossil” is
used to mean “body fossil”, which is the usual
practice. Trace fossils are treated separately in
Chapter 19.
Hard parts and soft parts
Fossils are typically the hard parts – shells,
bones, woody tissues – of previously existing
plants and animals. In many cases these skel-
etons, materials used in supporting the bodies
of the animals and plants when they were
alive, are all that is preserved. Skeletons may
nonetheless give useful information about the
appearance of an extinct animal because they
can show the overall body outline and may
show the location of muscles, and woody
tissues of plants may allow whole tree trunks
and leaves to be preserved in some detail. The
fossil record is biased in favor of organisms
that have hard parts. Soft-bodied organisms
today can make up 60% of the animals
in certain marine settings, and these would
all be lost under normal conditions of
fossilization.
There are a variety of hard materials in
plants and animals that contribute to their
preservation (Table 3.1). These include inor-
ganic mineralized materials, such as forms of
calcium carbonate, silica, phosphates and iron
oxides. Calcium carbonate (CaCO 3 ) makes
up the shells of foraminifera, some sponges,
corals, bryozoans, brachiopods, mollusks,
many arthropods and echinoderms. Silica
(SiO 2 ) forms the skeletons of radiolarians and
most sponges, while phosphate, usually in the
form of apatite (CaPO 4 ), is typical of verte-
brate bone, conodonts and certain brachio-
pods and worms. There are also organic hard
tissues, such as lignin, cellulose, sporopollenin