Friday, 11 November 2016
When is a fossil a fossil...
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’.