taphonomy

Sponge borings and serpulid worm encrusters on a modern shell of the bivalve Mercenaria in North Carolina.

Taphonomy^{[note 1]} is the study of decaying organisms over time and how they become fossilized (if they do). The term taphonomy, (from the Greek taphos - τάφος meaning burial, and nomos - νόμος meaning law), was introduced to paleontology in 1940 by Russian scientist, Ivan Efremov, to describe the study of the transition of remains, parts, or products of organisms, from the biosphere, to the lithosphere, i.e. the creation of fossil assemblages.^[1][2]

Taphonomists study such phenomena as biostratinomy, decomposition, diagenesis, and encrustation and bioerosion by sclerobionts.^[3] (Sclerobionts are organisms which dwell on hard substrates such as shells or rocks.)

One motivation behind the study of taphonomy is to better understand biases present in the fossil record. Fossils are ubiquitous in sedimentary rocks, yet paleontologists cannot draw the most accurate conclusions about the lives and ecology of the fossilized organisms without knowing about the processes involved in their fossilization. For example, if a fossil assemblage contains more of one type of fossil than another, one can either infer that that organism was present in greater numbers, or that its remains were more resistant to decomposition.

During the late 20th century, taphonomic data began to be applied to other paleontological subfields such as paleobiology, paleoceanography, ichnology (the study of trace fossils) and biostratigraphy. By coming to understand the oceanographic and ethological implications of observed taphonomic patterns, paleontologists have been able to provide new and meaningful interpretations and correlations that would have otherwise remained obscure in the fossil record.

An articulated wombat skeleton in Imperial-Diamond cave (Jenolan Caves).

The La Brea tar pits represent an unusual depositional environment for their epoch (Pleistocene) and location (southern California).

Archaeologists study taphonomic processes in order to determine how plant and animal (as well as human) remains accumulate and differentially preserve within archaeological sites. This is critical to determining whether these remains are associated with human activity. In addition, taphonomic processes may alter biological remains after they are deposited at a site. Some remains survive better than others over time, and can therefore bias an excavated collection.

Experimental taphonomy testing usually consists of exposing the remains of organisms to various altering processes, and then examining the effects of the exposure.

Preservation of biopolymers

The taphonomic pathways involved in relatively inert substances such as calcite (and to a lesser extent bone) are relatively obvious, as such body parts are stable and change little through time. However, the preservation of "soft tissue" is more interesting, as it requires more peculiar conditions. While usually only biomineralised material survives fossilisation, the preservation of soft tissue is not as rare as sometimes thought.^[4]

Although chitin exoskeletons of arthropods are subject to decomposition, they often maintain shape during permineralization, especially if they are already somewhat mineralized.

Both DNA and proteins are unstable, and rarely survive more than hundreds of thousands of years before degrading.^[5] Polysaccharides also have low preservation potential, unless they are highly cross-linked;^[5] this interconnection is most common in structural tissues, and renders them resistant to chemical decay.^[5] Such tissues (resistant chemical in brackets) include wood (lignin), spores and pollen (sporopollenin), the cuticles of plants (cutan) and animals, the cell walls of algae (algaenan),^[5] and potentially the polysaccharide layer of some lichens.^{[citation needed]} This interconnectedness makes the chemicals less prone to chemical decay, and also means they are a poorer source of energy so less likely to be digested by scavenging organisms.^[5] After being subjected to heat and pressure, these cross-linked organic molecules typically 'cook' and become kerogen or short (<17 C atoms) aliphatic/aromatic carbon molecules.^[5] Other factors affect the likelihood of preservation; for instance scleritisation renders the jaws of polychaetes more readily preserved than the chemically equivalent but non-sclerotised body cuticle.^[5]

It was thought that only tough, cuticle type soft tissue could be preserved by Burgess shale type preservation,^[6] but an increasing number of organisms are being discovered that lack such cuticle, such as the probable chordate Pikaia and the shellless Odontogriphus.^[7]

It is a common misconception that anaerobic conditions are necessary for the preservation of soft tissue; indeed much decay is mediated by sulfate reducing bacteria which can only survive in anaerobic conditions.^[5] Anoxia does, however, reduce the probability that scavengers will disturb the dead organism, and the activity of other organisms is undoubtedly one of the leading causes of soft-tissue destruction.^[5]

Plant cuticle is more prone to preservation if it contains cutan, rather than cutin.^[5]

Plants and algae produce the most preservable compounds, which are listed according to their preservation potential by Tegellaar (see reference).^[8]

Notes

References

Further reading

• Emig, C. C. (2002). Death: a key information in marine palaeoecology. In: Current topics on taphonomy and fossilization, Valencia. Col.lecio Encontres, 5: 21-26.
• Greenwood, D. R. (1991), The taphonomy of plant macrofossils. In, Donovan, S. K. (Ed.), The processes of fossilisation, p.141-169. Belhaven Press.
• Lyman, R. L. (1994), Vertebrate Taphonomy. Cambridge University Press.
• Shipman, P. (1981), Life history of a fossil: An introduction to taphonomy and paleoecology. Harvard University Press.
• Taylor, P. D. and Wilson, M. A. (2003), Palaeoecology and evolution of marine hard substrate communities. Earth-Science Reviews 62: 1-103. [1]

External links

• The Shelf and Slope Experimental Taphonomy Initiative is the first long-term large-scale deployment and re-collection of organism remains on the sea floor.
• The Journal of Taphonomy
• Bioerosion Website at The College of Wooster[2]
• Comprehensive bioerosion bibliography compiled by Mark A. Wilson[3]
• Taphonomy