Fossils and Fossilisation

Abstract

Fossils are the recognisable remains or traces of activity of prehistoric life, typically defined as >10 000 years old. Pseudofossils are nonorganic objects that bear false resemblance to organism remains. The fossil record is strongly biased toward organisms with hard parts, such as mineralised skeletons of calcite, aragonite, phosphate, silica or refractory organic materials such as wood, that live in areas prone to pulses of sediment accumulation. Hence, preservation is not only particularly favoured in shallow offshore, storm‐affected and marine environments but also to a lesser extent, in the deep sea, lakes and river point bars. Occasionally, rapid burial in anoxic setting coupled with early mineralisation leads to extraordinarily preserved fossil Lagerstätten. The study of fossil preservation – taphonomy – is subdivided into biostratinomy and fossil diagenesis. Biostratinomic processes affect potential fossil remains between death and final burial, including decay of organic parts, disarticulation, fragmentation, abrasion, bioerosion and dissolution. Fossil diagenesis constitutes processes that affect organic remains subsequent to burial such as dissolution, compaction and early and late mineralisation. Taphonomy reveals biases of the fossil record and also provides insights into depositional rates and processes.

Key Concepts:

  • Fossils are prehistoric (>10 000 years), discrete remains or traces of behaviour of once‐living organisms.

  • Preservation in the fossil record is a rare event that generally requires organisms with ‘hard parts’ (mineralised or resistant organic skeletons) and entombment within sediments.

  • Extraordinary preservation of articulated skeletons and even soft parts requires very rapid burial, often associated with mass mortalities, in low oxygen sediments and various types of early diagenetic mineralisation.

  • A variety of settings may favour exceptional preservation, including storm‐influenced continental shelves, deeper marine environments and stagnant lagoons, freshwater lakes, including maars or volcanic lakes, caves, tar pits and permafrost.

  • The study of fossil preservation, taphonomy, is subdivided into biostratinomy and fossil diagenesis.

  • Biostratinomy comprises the study of all processes that affect potential fossils from the time of death, or production of a trace or sheddable structures (leaves, seed and exoskeletal parts), to final burial.

  • Biostratinomic processes include decay of soft parts, infilling by disarticulation of bivalved or multielement skeletons, breakage, bioerosion, abrasion, transport and chemical corrosion.

  • Fossil diagenesis includes compaction, early mineralisation of various sorts including infillings and/or replacements of pyrite, phosphate, and silica and late‐stage mineralisation including overgrowth of earlier‐formed minerals and major episodes of (sometimes selective) dissolution.

  • A combination of biostratinomic and diagenetic characteristics can aid in identifying taphonomically defined assemblages or taphofacies that may provide a ‘fingerprint’ of particular depositional environments.

Keywords: taphonomy; preservation; fossils; depositional environment; biostratinomy; burial; diagenesis; time‐averaging; palaeoecology

Figure 1.

Schematic showing the processes of fossilisation in a shallow marine seafloor environment. Living community occupies the water, sediment–water interface and the upper sediments. Burrowers, such as clams, churn the sediment and mix skeletal remains both downward and upward in the mixed layer. Once‐buried shells may be reworked (disinterred) and damaged or destroyed. Eventually, some remains become buried in the ‘historical layer’ (hist.) and may become part of the permanent geological record. Processes affecting organism remains up to the time of final burial fall in the realm of biostratinomy; geochemical processes occurring in the sediments including those long after burial are considered aspects of fossil diagenesis. Modified from Martin .

Figure 2.

Aspects of orientation of skeletal materials. (a) Shows response of shells to free settling. (b) Response to current flipping. (c) Wave or oscillatory currents orient elongate shells bimodally with the long axis parallel to wave propagation. (d) Unidirectional currents align conical or ellipsoidal objects parallel to the current and with the apex upcurrent; black silhouettes represent rose diagrams or circular histograms of compass orientations. (e) Various fabrics assumed by skeletons in response to currents (lower left) and waves; plan view of bedding surfaces; cross section shows fabrics of shells on the edges of beds of sediment. Adapted from Allen and Kidwell et al..

Figure 3.

Examples of fossil assemblages recording variable amounts of time before burial, based on samples from middle Devonian shales of western New York State. (a) Very low rate of deposition; note corroded and phosphatised (black) fossil debris. (b) Slow deposition; trilobites and brachiopod shells are disarticulated to somewhat fragmented; small corals and bryozoans are fragmented to somewhat corroded. (c) Mass mortality and rapid burial beneath a thin mud layer; delicate fossils such as the crinoid (in middle) are partially articulated but somewhat scrambled by burrowers. (d) Mass mortality and rapid burial (obrution) beneath a thicker mud layer preserving delicate crinoid (upper left) and trilobites intact; brachiopods (far right) and bryozoans (lower left) are buried in life position. Adapted from Brett and Baird .

Figure 4.

Moult ensemble of the trilobite Phacops saberensis, Lower Devonian, Morocco, showing disarticulated cephalon and articulated thorax‐pygidium; such associated moult parts provide evidence for a lack of seafloor disturbance at the time of burial and thereby provide strong evidence for life activities in the environment. Reproduced from Brett et al.. Reprinted by permission of Society for Sedimentary Geology.

Figure 5.

Distribution of selected taphonomic features on crinoid plates from the Copan crinoid Lagerstätte (Pennsylvanian) of Oklahoma. Note that encrustation and breakage show increases in thinner units, indicating intervals of slow sedimentation where disarticulated crinoid skeletons remained exposed in an oxygenated setting, allowing other organisms to utilise this material as substrata, and where breakage through microbial degradation or scavenging was enhanced. Interestingly, these units are also characterised by an increase in articulated crinoid crowns, indicating that the intervals of slow sedimentation were episodically interrupted by rapid, but thin and subtle, sedimentation events. This exemplifies the utility of taphofacies analysis, as the entire depicted interval is fairly uniform in terms of sediment character and presence/absence lists of organisms. Reproduced from Thomka et al..

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Further Reading

Allison PA and Bottjer DJ (eds) (2011) Taphonomy, Second Edition: Process and Bias Through Time. New York: Springer.

Briggs DEG and Crowther PR (eds) (2001) Palaeobiology II. Oxford: Blackwell Science.

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Speyer SE and Brett CE (1988) Taphofacies models for epeiric sea environments: Middle Paleozoic examples. Palaeogeography, Palaeoclimatology, Palaeoecology 63: 222–262.

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Brett, Carlton E, and Thomka, James R(Feb 2013) Fossils and Fossilisation. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001621.pub2]