Geoscience Reference
In-Depth Information
5.2.4 Molecular fossils
The usual fate of any dead organism is to decay to such a
degree that nothing remains of either the life morphology or the
traces it left as a result of its activities. However, even in these
cases it may leave behind its chemical signature. In the best
circumstances whole, or partial, DNA molecules may be
preserved, but usually only remnants of molecules
characteristic of a particular group of organisms survive. When
these are found we can infer something about evolution and
even environments. An example is the occurrence of
cyclopentane rings derived from the membrane lipids of marine
plankton. The number of such rings correlates with sea-surface
temperature (average annual or perhaps seasonal) and forms the
basis of the TEX
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palaeotemperature proxy. When collecting
such samples (Chapter 13) extra care needs to be taken not to
cross-contaminate between samples or to introduce
contaminants from the sampling and wrapping material.
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5.3 Fossil distribution and where to
fi nd them
Initially the best way to look for fossils is to examine loose pieces
of rock material ('fl oat') derived from the exposure. Float has
the advantage of exposing a large number of bedding surfaces
that can be examined for fossils. If fossils are visible the next
step is to try and relate the lithology of the fossil-bearing fl oat
to lithologies that are in place, for example, in the adjacent cliff
face. This lithology can then be examined for
in situ
fossil
material. If fossils are not obvious on the fl oat bedding surfaces,
expose further bedding planes by splitting the blocks parallel
to the bedding using a chisel or the chisel end of a hammer.
Preservation of organic body fossils is most likely to take place
in sediments deposited in low oxygen conditions where
dispersed organic matter is also easily preserved. This usually
means in dark coloured fi ne-grained sedimentary deposits,
typically mudstones. However, they are also preserved where
the sedimentation rate is high, the conditions are conducive to
rapid mineralization, or the chemistry limits dissolution of
shells or bones (Section 5.2.2). Here are some other features to
look out for.
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Carbonate fossils:
Body fossils composed of carbonate tests
or shells, as well as bones, are most likely to be found in
carbonate-rich rocks such as limestones and chalks. They
can also be found in siliciclastic rocks provided that they
have not been in acidic conditions during deposition and
subsequent burial.
