The most common form of fossilization produces remains that
clearly record the biomineralised skeletons of prehistoric animals, the “hard
parts” of these creatures, since the soft tissues usually decay and
disintegrate soon after death. Dinosaur bones and teeth are such biomineralised
skeletal features, as are the shells of molluscs, the external skeletons
(exoskeletons) of insects, and the carapaces of crabs, to name only a few
examples. Even biomineralised hard parts can be fragile and subject to decay or
mechanical breakdown when they are small in size, which is why we are not up to
our necks in fossils. The thin and often hollow bones of flying vertebrates
such as pterosaurs, birds, and bats are notoriously fragile since they are
built so lightly, and they are therefore comparatively scarce in the fossil
record. Small mammals likewise have tiny bones that are easily fractured or
otherwise broken down and lost to time. The only remains of many species,
especially mammals, is often only their teeth. As with many animals, teeth are
the hardest parts of their bodies and therefore the most resistant to
post-mortem destruction. Many individual animals, especially the early record
of mammals and some entire groups, are known only through finds of their
fossilized teeth.
To extend our understanding of creatures known only through
fossil hard parts, we use comparisons with modern life forms that appear to be
structurally similar. This is the science of comparative anatomy, and it has
been successfully applied by biologists and palaeontologists for centuries. We
can also bracket extinct species with living species that are related to
ancestral and descendant members of the extinct group, this is known as the
extant phylogenetic bracket (EPB) and was developed by Larry Witmer (Ohio
University, USA). In many cases we find additional evidence, such as the scars
of muscle attachment on fossil dinosaur bones, which show that the EPB data
appear to be appropriate. Combining such evidence and EPB with a liberal
application of comparative anatomy, we can build an understanding of the
complete prehistoric animal, as it was with its soft tissues intact. While
evidence supports certain aspects of such extrapolation, in other respects this
work is necessarily speculative.
Fossilisation is a rare phenomenon that occurs only to a
tiny fraction of a community’s population at any given place and period, but in
most cases, no trace is left behind.. Nonetheless, if we consider the
fossilization of skeletal elements as the standard, then the fossilization of
soft-tissue structures is much rarer still. When these unique discoveries are
made, this type of fossilisation literally “fleshes out” our understanding of
the fossil record in many crucial ways. Even a single example of soft-tissue
preservation can be of tremendous value in the interpretation of fossil animal
types. These discoveries are of such special interest that it is worth
reviewing some of the classic examples of this phenomenon. In each case,
special circumstances prevented the ordinary loss of soft tissues.
It is clear that some organic remains of life have been
altered to a point that it is difficult to identify them as being a specific
organism, however their organic origin is not brought into question. Once such
group of organic compounds are hydrocarbons. A suite of economically important
‘minerals’ that have been altered so far, but still retain vestiges of organic
compounds that reveals their origin. The body fossils (bone, shell, etc.) of
dinosaurs and other vertebrates have both recognizable organic chemistry and
morphology, they have not been processed to the point that they may be
considered ‘products’ from the original organism.
Fossils are indeed partially composed of chemistry that
directly links them to the organisms from which the fossils remains came. They
really cannot be considered minerals (a solid naturally occurring inorganic
substance), but are truly ‘geobiological’ composites of both inorganic and
organic molecules that were constructed through biological and post-burial
processes that preserve the fossil through deep time. The alteration that
occurs to the biological tissue through subsequent mineralisation rarely
overprints the organic composition of an organism completely. Our team at the College of Charleston, University of Manchester and also at the Stanford Synchrotron Radiation
Lightsource (Stanford University, USA) have been chemically mapping fossils
using multiple imaging techniques to elucidate these geobiological composites
we commonly know as fossils.
The carbon cycle is remarkably efficient at recycling
organic material, but under certain preservational circumstances, some of the
chemical building blocks of an organism make it through this taphonomic filter.
In exceptionally preserved fossils, it is possible that remnants of structural
proteins and associated organic molecules survive and can be mapped to help
resolve original biological structures. The potential for new techniques to
compositionally or spatially resolve such ‘chemical fossils’ is being realized with the recognition of elemental and organic residues
that once comprised living tissue.
Until the advent of techniques sensitive enough to resolve trace amounts of
organic compounds and organically bound elements, it was difficult to untangle potential
material transfer from microbes, geochemical fluids and the contamination from
sampling/conservation techniques applied to a sample. However, the advent of
synchrotron-based imaging and infrared spectroscopy has revolutionized sample
analysis, enabling high-resolution scans that
spatially resolve reaction aureoles, precipitates, etc. The suite of de
novo techniques available to paleontology is completely changing our
understanding of what constitutes a ‘fossil’.
