Fossils and Fossilisation Introductory article Carlton E Brett, University of Cincinnati, Cincinnati, Ohio, USA Article Contents . Introduction James R Thomka, University of Cincinnati, Cincinnati, Ohio, USA . Depositional Settings of Fossil Preservation Based in part on the previous version of this eLS article ‘Fossils and . Taphonomy Fossilisation’ (2001) by Carlton E Brett. Biostratinomy . Fossil Diagenesis . Taphofacies Online posting date: 15th February 2013 Fossils are the recognisable remains or traces of activity of objects that spuriously resemble organic remains. Post- prehistoric life, typically defined as 410 000 years old. Pleistocene (510 000 years) organic remains are generally Pseudofossils are nonorganic objects that bear false termed subfossils. However, the definition includes two resemblance to organism remains. The fossil record is fundamentally different categories: (a) body fossils – remains, typically skeletons, of organism’s bodies; and (b) strongly biased toward organisms with hard parts, such as trace fossils – traces of organism behaviour, such as bur- mineralised skeletons of calcite, aragonite, phosphate, rows, borings, tracks and trails. In addition, a newly silica or refractory organic materials such as wood, that developing field deals with biomarkers, or certain types of live in areas prone to pulses of sediment accumulation. what can be considered ‘molecular fossils’. These are Hence, preservation is not only particularly favoured in compounds, mostly lipids, that result from the degradation shallow offshore, storm-affected and marine environ- and sulfidisation of complex organic compounds (de ments but also to a lesser extent, in the deep sea, lakes and Leeuw et al., 1995). Although these are not the original river point bars. Occasionally, rapid burial in anoxic set- molecules formed by organisms, some of them are none- ting coupled with early mineralisation leads to extra- theless specific to particular microbes or even higher ordinarily preserved fossil Lagersta¨tten. The study of eukaryotic organisms (Farrimond and Eglinton, 1990). fossil preservation – taphonomy – is subdivided into They can be extracted from fine-grained sediments and identified using techniques such as chromatography and biostratinomy and fossil diagenesis. Biostratinomic pro- compound specific mass spectrometry. Their occurrence cesses affect potential fossil remains between death and provides indirect evidence for the presence of particular final burial, including decay of organic parts, disarticu- groups of organism, including many otherwise undetect- lation, fragmentation, abrasion, bioerosion and dis- able primary producers, in ancient environments. Finally, solution. Fossil diagenesis constitutes processes that it should be noted that certain ancient objects, mainly affect organic remains subsequent to burial such as dis- microscopic in scale, particularly in the Archaean and early solution, compaction and early and late mineralisation. Proterozoic Eons, remain ambiguous as to their organic or Taphonomy reveals biases of the fossil record and also inorganic origin; these are sometimes termed dubiofossils. provides insights into depositional rates and processes. See also: Fossil Record; Speciation and the Fossil Record Palaeontology, the scientific study of the fossil record, provides primary documentation of biotic evolutionary history. The fossil record, however, is notoriously incom- Introduction plete. A majority of organisms lack skeletal hard parts and are rapidly degraded after death, providing little oppor- The term fossil is derived from the Latin fossa (ditch), an tunity for preservation; others live in settings wherein there allusion to the old notion of fossils as mystical objects dug is little or no chance for burial in sediments, and thus no from the ground (Rudwick, 1976). As presently defined, fossil record. Of some 35 recognised phyla (major groups) fossils comprise recognisable remains or traces of activity of animals only about nine have substantial fossil records. of prehistoric life; ‘prehistoric’ is operationally defined as Under extraordinary circumstances of nearly instant- greater than 10 000 years. This definition excludes strictly aneous burial, especially in anoxic sediments, articulated inorganic objects, specifically pseudofossils, mineralised multielement skeletons and even vestiges of soft tissues may be preserved, forming so-called Lagersta¨tten, such as the eLS subject area: Evolution & Diversity of Life famed Burgess Shale of Canada (Seilacher et al., 1985; Bottjer et al., 2002; Selden and Nudds, 2004). These fossil How to cite: ‘mother lodes’ provide exceptional insights into ancient life Brett, Carlton E; and Thomka, James R (February 2013) Fossils and and understandably have commanded considerable Fossilisation. In: eLS. John Wiley & Sons, Ltd: Chichester. attention. But, in a sense, all fossils are the result of rare DOI: 10.1002/9780470015902.a0001621.pub2 accidents of preservation. Even among those organisms eLS & 2013, John Wiley & Sons, Ltd. www.els.net 1 Fossils and Fossilisation that have readily preservable skeletons, estimates of the place by the growth and binding–cementing action of preserved proportion of those that once lived (based on algae, corals, sponges and other organisms (e.g. James and present diversities and assumed rates of evolutionary Bourque, 1992). In slightly deeper areas, below the effects overturn) are less than 10%. Observational studies of of waves, the washoff of muds, derived from storm surges, biostratinomy, pioneered by German researchers such as may blanket the sea bottom and preserve the skeletons of Richter (1928), Weigelt (1927) and Sha¨fer (1972) in the organisms intact. The deeper portions of inland sea basins mid-twentieth century and recently supplemented by frequently became oxygen depleted and the consequent experimental studies in actual marine environments (see absence of scavenging may aid in intact preservation of the Parsons-Hubbard et al., 2011), demonstrate the rapidity of bodies of organisms, especially of swimming cephalopods, degradation of mineralised skeletons in many depositional fish and larger marine vertebrates. Excellent examples environments and emphasise the notion that fossil preser- include the dark shales of the Lower Jurassic ( 190–200 vation is a rare event. See also: Burgess Shale; Exceptional million years old) of England and Germany, which pre- Preservation; History of Palaeontology serve whole bodies of fishes and ichthyosaurs (marine reptiles) (Seilacher et al., 1985). See also: Foraminifera; Mollusca (Molluscs); Shallow Seas Ecosystems Deep oceanic deposits may preserve microfossil assem- Depositional Settings of Fossil blages, mainly tests of foraminifera, radiolarians and other Preservation tiny planktonic organisms. However, low rates of depos- ition of deep-sea clays, together with generally oxidising A majority of fossils in the geological record are preserved and commonly corrosive conditions in colder water, limit in marine settings. The intertidal zone tends to be a rather the preservation of the skeletons of larger organisms. A destructive environment, especially in wave-swept coast- unique fauna dominated by molluscs, worms and ver- lines. Hence, few fossils are found in beach sands or rocky tebrates such as hagfish and sharks can develop around intertidal settings. Nonetheless, some ancient rocky whale falls; these large carcasses represent local ‘islands’ shoreline fossils (including encrusting barnacles and oys- that fuel the establishment of unique communities that can ters) have been buried and preserved fortuitously in place sometimes be recognised in the fossil record (e.g. Danise (Lescinsky et al., 1991). More commonly, such high energy et al., 2009). See also: Deep Ocean Ecosystems settings are characterised by only trace fossils, generally the Freshwater lakes provide unique opportunities for fossil small, ubiquitous boring known as Trypanites (Jia-yu and preservation that are similar in some ways to those of Johnson, 1995). marine settings. However, fresh water supports a much Lower-energy tidal mud/sand flats and adjacent estu- lower diversity and abundance of scavenging, burrowing aries, bays and lagoons may provide opportunities for and skeleton-boring organisms. Consequently, preser- preservation. In particular, some estuarine areas are sub- vation may be favoured in these settings. The bottoms of ject to very rapid deposition of tidally laminated sediments; some large, deep lakes also become density stratified with these have produced some extraordinary fossil assem- warm, near-surface waters isolated from the cool, typically blages. An excellent example is the famous Mazon Creek oxygen-depleted lower water column (hypolimnion). In occurrence from the Upper Carboniferous (Pennsylvanian; such cases, the deep, dysoxic to anoxic sediments may 300 million years old) of northern Illinois (Shabica and preserve remains of organisms exceptionally well. There Hay, 1997). Remains of more than 100 species of marine are many examples of spectacularly preserved fossils in organisms including jellyfishes, squids, worms, lobe-finned ancient lake deposits, especially from Cenozoic times (the fishes and sharks are preserved, together with plants (4350 last 65 million years). Perhaps the best-known of these is species), insects (4140 species), myriapods, spiders and the Eocene (45–50 million years old) Green River oil ‘shale’ amphibians derived from nearby lands. These fossils were of Wyoming and Colorado