Geoscience Reference
In-Depth Information
5.2 Fossil types and preservation
If you don't know the names
of the fossils you fi nd, give
them temporary names (type
1, 2, 3, etc.). If these are
accompanied by detailed
descriptions, sketches and/or
photographs they can be
identifi ed later by reference
to specialist sources such as
topics and museum
collections.
5.2.1 Body fossil classifi cation
The chances are that in the fi eld you will not be able to
identify every fossil you fi nd. Actually you don't need to. A lot
can be deduced about the organism in the fi eld by careful
observation and you can make up a working classifi cation
based on simple observation of, for example, symmetry (Figure
A5.1). Even if we know nothing of the taxonomic identity of a
fossil (which is often the case with newly discovered species)
we can infer a lot about its mode of life and palaeoenvironment
by examination of its architecture and context within the
entombing rock (e.g. Worked Example 5.1, p. 84).
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Plants invariably fossilize as dispersed organs (leaves, fruits/
seeds, pollen/spores, trunks and roots) and most of the time we
don't know which parts belong together to make a specifi c
extinct species. For this reason each organ has a Latin name of
its own. Try to record as accurately as possible the morphology
of each plant part including shape, size and ornamentation; in
the case of leaves, margin details and venation are also
important (Figure 5.1).
Microscopic body fossils (microfossils) can rarely be identifi ed
in the fi eld and collecting is often carried out without knowing
if the sample will contain them. Section 5.3 provides notes on
maximizing the likelihood that samples are fossiliferous.
5.2.2 Body fossil preservation
The way that a body fossil is preserved indicates a great deal
about its transport, deposition and burial history as well as the
chemical and tectonic history of the entombing sediment.
Recognizing and recording the mode of preservation is
important to understanding the processes during deposition
and burial. (See Figure 5.3, p. 85, and Appendix A5, Table A5.1
for common preservation modes.)
Preservation modes are not mutually exclusive and all of them
can be affected by compression over time. If the body parts are
inherently resistant to compression and/or are permineralized/
petrifi ed soon after burial, 'fl attening' may be minimal. The
extent of fl attening relative to the compaction of the entombing
sediment indicates the timing of mineralization relative to
burial and sediment loading. More lateral distortions are
invariably due to tectonics and can be useful in understanding
the structural history of an area. In conjunction with
observations on the phases of mineralization a history of the
changing chemical environment over time can be
reconstructed.
