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 Preservation

Based in part on the previous version of this eLS article ‘ and . 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 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 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 (Selden and Nudds, 2004). preserved by rapid burial and incorporation into siderite These fine-grained shaly and platy carbonates carry a wide (iron carbonate) concretions. array of fossil organisms, including varied leaves, flowers, Offshore shallow sea settings are by far the most com- insects, birds, bats, crocodiles and fishes. The fully articu- mon settings for fossil preservation and much of what is lated remains of fishes apparently represent mass mortal- said below pertains to these types of occurrences. Beds of ities during water column overturn and mixing. These fossils are preserved most commonly by the winnowing and fossils are sold in rock shops around the world. An even burial effects of storm waves and currents. Shallow high- more spectacular example, of nearly the same age, in energy shoal deposits may consist of the broken and cor- Europe is the Messel Shale of Germany (Schaal and Zieg- roded remains of countless organism skeletons, especially ler, 1992; Selden and Nudds, 2004). This is an oil shale, foraminiferan tests, shells of molluscs and brachiopods, representing a lake deposit, that has produced exquisite and plates of echinoderm skeletons – especially those of fossils, including insects with brilliantly coloured exo- crinoids or sea lilies. In high-energy settings, burrowing skeletons, frogs with skin impressions, turtles, crocodiles, organisms (infauna) and their traces may be preferentially snakes, birds with intact feathers and a variety of mammals preserved relative to those living directly on the seafloor with skin and fur (rodents, bats, anteaters and early horses (epifauna). Locally, reef build-ups may be preserved in – even pregnant females with embryos). Again mass

2 eLS & 2013, John Wiley & Sons, Ltd. www.els.net Fossils and Fossilisation mortalities played a key role; it is possible that toxic gases (Stock and Harris, 1992; Selden and Nudds, 2004). These (including CO2) emanating from the lake may have killed natural petroleum seeps were apparently covered by pools both lake-dwellers and nearby terrestrial/aerial organisms. of water at times during late Pleistocene time (  10 000– Rapid entombment in anoxic muds further retarded decay. 12 000 years ago). Animals that came to drink at the pools, Much of the extremely detailed preservation of skin and such as horses, camels, giant ground sloths and mam- hair is actually a replacement by petrified bacteria that were moths, as well as predators (e.g. sabre-toothed cats and dire ‘frozen’ in their own metabolic wastes! See also: Lake wolves) and (e.g. condors) that were attracted Ecosystems; Messel by trapped prey, became mired and eventually foundered Swamps and oxbow lakes commonly accumulate into the tar; their skeletons were well preserved. organic-rich sediments that may preserve even rather Finally, spectacular examples of early Holocene to late delicate parts of plants that live in adjacent areas, together Pleistocene (410 000 years old) mammoths, woolly rhinos with some freshwater and terrestrial animals, including and other animals have been preserved by refrigeration insects and freshwater gastropods. The processes that (Kurte´n, 1986; Guthrie, 1990). Such organisms evidently transform plant matter or peat into coals tend to destroy fell through glacial ice or temporarily thawed ground and details of the plants. However, surrounding shaly or sandy were frozen rapidly. Even their flesh is preserved, and in sediments may preserve plant remains with excellent detail some cases is still edible. Deoxyribonucleic acid (DNA) is and, rarely, the development of calcium carbonate con- extracted from the tissues of mammoths and has been cretions within coals (‘coal balls’) prevent compaction and compared with that of modern elephants with the inter- preserves exquisite details of plant cellular structure (e.g. esting result that mammoths appear to be more closely Scott and Rex, 1985). related to Indian elephants than African elephants. Preservation of remains of organisms in terrestrial environments is, on the whole, a much less common and chancy process than in marine environments. Many ter- Taphonomy restrial organisms reside in nondepositional or erosional environments. Their remains can only be preserved if they Taphonomy is the study of processes that influence pre- are transported into another setting. Most soils are corro- servation of biotic remains as fossils (Efremov, 1940; Sei- sive environments in which shells and even bones of ani- lacher, 1973; Donovan, 1991; Martin, 1999). The field mals are rapidly destroyed; however, some alkaline (high- includes the disciplines of (a) biostratinomy, the study of pH) soils may preserve the more resistant portions of processes affecting organism remains/traces prior to their skeletons, such as teeth and land snail shells. Plant roots final burial and (b) fossil diagenesis, investigation of phe- may also leave distinctive traces (phytoturbation). See also: nomena affecting potential fossils after burial. Recently, Evolution of Ecosystems: Terrestrial; Terrestrial Eco- taphonomy has developed, both as a means of assessing systems in the Past 100 Million Years bias in the fossil record and as a critical tool for River point bars and channel-base gravels may preserve palaeoenvironmental analysis. Taphonomic analyses pro- logs, resistant bones; even carcasses of dinosaurs drowned vide valuable information on the parameters of ancient during floods may become stranded and then buried by the environments. The mechanical and chemical properties of layers of sediment accreted to the outside of the sand bars. skeletons of organism have probably been invariant Such a setting has been inferred for the famous Jurassic through geological time, despite the nonuniformity of sandstone ledge on display at Dinosaur National Monu- organisms. Certain taphonomic features (e.g. articulation ment, Utah. Disarticulated skeletons may undergo differ- of delicate skeletal elements) provide unambiguous evi- ential transport and sorting in stream currents (Dodson dence for episodes of very rapid sedimentation; conversely, et al., 1980). Experimental studies using bones of modern highly corroded fossil material provides a distinctive sig- animals show that teeth, jaws and some dense bones tend to nature of gradual accumulation of degraded material over accumulate near the source of the skeletons on bars, long spans of time. See also: Fossil Record: Quality; whereas light, easily transported parts, such as vertebrate, Palaeoenvironments tend to roll downstream and become abraded and/or des- troyed. See also: Dinosauria (Dinosaurs) Amber, the hardened resin of plants, usually coniferous Biostratinomy trees, may preserve delicate remains of plants and small animals, especially insects, including even soft parts, cells Death of organisms and remnants of DNA (e.g. Poinar, 1992). The resin apparently seals out most bacteria and prevents most The taphonomic history of organic remains generally decay. begins with the death of organisms, although in specific Rarely, remains with skin and hair of, for example, instances skeletal parts lost during the lifetime of organ- ground sloths, are preserved by mummification and des- isms, especially moulted exoskeletons in arthropods and iccation in caves. Another remarkable site of preservation cast-off leaves and reproductive structures of vegetation, for terrestrial animals consists of traps, notably the famed may become a part of the preserved record. Lagersta¨tten tar seeps of Rancho La Brea (now Los Angeles, California) involve mass mortality of organisms due to environmental

eLS & 2013, John Wiley & Sons, Ltd. www.els.net 3 Fossils and Fossilisation crises, such as storms, seismic shock events, volcanic (e.g. Briggs, 2003) and may remain intact for many years. eruptions, overturn of the water column, biological poi- See also: Cellulose: Biogenesis and Biodegradation; Plant soning (such as ‘red tides’) and anoxia. However, only Cell Walls; Soils and mortality events associated with episodes of rapid burial The removal of oxygen does not prevent microbial will generally be recorded. Interestingly, this is often the activity (Figure 1). Anaerobic bacteria simply utilise a var- most enigmatic aspect of the postmortem history of a fossil, iety of alternative oxidants, such as nitrates, manganese as later events often strongly overprint and/or completely and iron oxides and sulfates for metabolism of organic destroy evidence of the mechanism responsible for death, if matter (Berner, 1980; Allison, 1988; Briggs, 2003). These that mechanism leaves any recognisable trace at all. microbial reactions are only two or three times slower than Modern studies are critical in this regard, in that they help aerobic decay. However, anoxia prevents scavenging. The elucidate common causes of death and their taphonomic microbial reactions that are involved in anaerobic decay signature. See also: ; Harmful Algal Blooms may also trigger precipitation of early diagenetic minerals, such as pyrite, carbonate and phosphorite, which may preserve the decaying tissues themselves (Berner, 1968; Soft-tissue decay Briggs, 2003). See also: Anaerobic Respiration Once an organism dies it becomes a sedimentary particle Not surprisingly, many cases of soft-part preservation and its tissues and skeleton move towards equilibrium are associated with dysoxic mudrock facies (Seilacher et al., (Figure 1). Aerobic bacterial decay proceeds rapidly after 1985; Bottjer et al., 2002), typically dark, slightly organic- death, with resultant loss of soft parts (Allison, 1988). The rich shales, such as the Burgess Shale (Middle Cambrian, most degradable organic compounds, including internal British Columbia). Others are found in fine-grained lime- organs and muscle tissues, are very rarely encountered as stones (e.g. Jurassic Solnhofen Limestone of Germany, fossils. In the rare cases where such labile tissues are pre- which yielded the famed early bird Archaeopteryx). Still served, very early diagenetic mineralisation has normally other soft-bodied fossils are in concretions, amber, tar or played a significant role. Ligaments and certain plant ice. In all cases there is an absence of oxygen, exclusion of materials, such as wood (lignin), are more resistant to decay scavengers and retardation of bacterial decay (Allison and Briggs, 1991). See also: Burgess Shale The hard skeletons of organisms are more chemically stable, and may, in some circumstances, be in equilibrium Living organisms (epifaunal) with their surroundings and are therefore more preservable.

Transport and orientation Living organisms (infaunal) Biostratinomy Organisms’ skeletons can be sensitive indicators of Palaeoecology Dead remains Reworked remains hydrodynamic conditions in the depositional environment (Figure 2). Under low-energy environments the remains of organisms may be buried in situ. Examples of in-place Immediate Bioturbation preservation include most trace fossils, skeletons of Mixed layer burial (biological organisms that are preserved in potentially unstable reworking and destruction) orientations or skeletons of encrusting organisms pre- served cemented directly to their substrates. Such life

Approximate scale: 1 cm = m Buried remains Ca2+ orientations provide evidence not only for study of the

HCO3= Diagenesis palaeobiology of organisms but also for that of sedi- mentological conditions. Another particularly sensitive Taphonomy Early diagenesis Concretion indicator is the occurrence of associated, disarticulated moult parts of trilobites or other arthropods. It is virtually impossible for separated portions of the skeleton to be Fossil record transported any distance and still remain associated (Brett Hist. Transition and Baird, 1986; Brett et al., 2012; see Figure 3 and Figure 4). See also: Invertebrate Skeletons; Palaeoclimatology; Tri- Figure 1 Schematic showing the processes of fossilisation in a shallow lobita (Trilobites) marine seafloor environment. Living community occupies the water, The argument that well-articulated, fragile fossils, such sediment–water interface and the upper sediments. Burrowers, such as clams, churn the sediment and mix skeletal remains both downward and as vertebrate skeletons, crinoids or trilobites, have not been upward in the mixed layer. Once-buried shells may be reworked transported must be made cautiously. There is experi- (disinterred) and damaged or destroyed. Eventually, some remains become mental evidence from tumbling barrel experiments that if buried in the ‘historical layer’ (hist.) and may become part of the organisms are transported within the first few hours fol- permanent geological record. Processes affecting organism remains up to lowing death, their remains may stay articulated (Allison, the time of final burial fall in the realm of biostratinomy; geochemical processes occurring in the sediments including those long after burial are 1986). However, if the skeleton has undergone partial dis- considered aspects of fossil diagenesis. Modified from Martin (1999). articulation, but with most parts remaining in close

4 eLS & 2013, John Wiley & Sons, Ltd. www.els.net Fossils and Fossilisation

Settling Rivers, tidal streams High drag (unstable) b

a Low drag (stable) Stable fall (b) Concavo-convex shells

Unstable fall Telescoping Preferred Concordant orientation (a) Extremely slow (c) Burial under Steady fall burial/Prefossilisation /Exhmation thin mud blanket n

≥ o (a) (1–b/a) 0.4 i t c e s Perpendicular - Wave (oscillation) Current s s Mytilus ro C Clustering: n Pla Oblique

Imbrication Turritella Edgewise (b) Slow burial/ (d) Burial under Turritella Disarticulation/Assemblage-mixing thick mud blanket Unimodal

Stacking 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 (c) (d) (e) Bimodal Nesting brachiopod shells are disarticulated to somewhat fragmented; small corals and bryozoans are fragmented to somewhat corroded. (c) Mass mortality Figure 2 Aspects of orientation of skeletal materials. (a) Shows response of and rapid burial beneath a thin mud layer; delicate fossils such as the shells to free settling. (b) Response to current flipping. (c) Wave or crinoid (in middle) are partially articulated but somewhat scrambled by oscillatory currents orient elongate shells bimodally with the long axis burrowers. (d) Mass mortality and rapid burial (obrution) beneath a thicker parallel to wave propagation. (d) Unidirectional currents align conical or mud layer preserving delicate crinoid (upper left) and trilobites intact; ellipsoidal objects parallel to the current and with the apex upcurrent; black brachiopods (far right) and bryozoans (lower left) are buried in life position. silhouettes represent rose diagrams or circular histograms of compass Adapted from Brett and Baird (1997). 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 The preferred orientation of elongate skeletal particles Allen (1990) and Kidwell et al. (1986). has been the subject of numerous observational and experimental studies. Elongate objects that do not roll, such as wood fragments and shell fragments or anchored association, postmortem transport is unlikely to have crinoid (‘sea lily’) columns, will normally become oriented occurred (Figure 4). parallel to prevailing current directions. Similarly, conical Many skeletal elements, including the valves of most shells, such as those of many nautiloid cephalopods and brachiopod, ostracod and clamshells, and trilobite cranidia gastropods, will become aligned parallel to the current and (heads) and pygidia (tails), have approximately concavo- with apex or pointed end pointing up-current (Allen, 1990). convex dish-shapes. These skeletal parts may occur in Hence, depending upon the type of fossil, it may be possible random orientations with respect to sedimentary bedding, to determine not only the line of current action but also the primarily in heavily bioturbated sediments, but more direction of propagation. Shells of similar size displaying commonly show preferred convex-up or convex-down two peaks of opposite orientation (Figure 2) are aligned orientations. perpendicular to the propagation direction of oscillatory Shells in a preferred convex-up position are probably currents or waves, sometimes in minor ripple troughs. most typical of concentrated shell beds. Numerous studies See also: Mollusca (Molluscs) have shown that even gentle currents will affect shells resting Stacks of edgewise or shingled shells apparently occur on the seafloor in such a way that they flip to a hydro- where densely packed shells were affected by oscillatory, dynamically stable, convex-up orientation. Hence, the storm-generated waves or currents and provide another occurrence of abundant shells in convex-upward positions indication of deposition well within the depth range of on bedding planes may provide evidence for reworking of storm wave effects (Seilacher, 1973). shells under current-agitated conditions (Figure 2). Beds of preferentially concave-upward shells are not as common. These will occur primarily where the shells are Disarticulation, fragmentation and allowed to resettle from suspension (e.g. Allen, 1990). corrosion Concave-upward settling may occur in areas close to storm wave-base in which the rather gentle storm-generated Skeletal elements of organisms are variably affected by waves lift shells temporarily off the bottom and allow free- biotic and abiotic destructive agents postmortem. The fall back to the substrate (Figure 2). degree to which the skeletons are affected is a function not

eLS & 2013, John Wiley & Sons, Ltd. www.els.net 5 Fossils and Fossilisation

elements. However, even under conditions of anoxia, bacterial decay of ligaments is rapid and the slightest cur- rents will serve to disarticulate the skeleton (Figure 3). Once elements are disarticulated, or even before this happens, they may be acted upon by various hydrodynamic processes that serve to sort, fragment and abrade them. Hydrodynamic size sorting is a result of turbulent events that suspend skeletal particles and sediment and allow them to resettle. Lateral or downslope transport of particles is possible, and the relative size of a fossil element may deter- mine the distance from the living site of an organism that certain portions of it may eventually become buried. As a general rule, smaller or lower density particles can be transported farther than larger or denser particles; however, an important exception to the expected size distribution of transported particles occurs when skeletal elements are composed of unusually high- or low-density materials. For example, the plates of echinoderms, although often the approximate size of sand grains, are composed of a complex, microscopic meshwork that imparts low density (and ease of transport) relative to more solid particles of a similar size. Thus, fresh echinoderm plates behave hydrodynamically similar to particles of a much smaller size. This relationship is further complicated by the potential of transport after some interval of burial or early mineralisation (see below), or if the particles have been encrusted by epibionts. In these instances, the skeletal elements will be denser than fresh particles and, consequently, will behave hydrodynamically similar to particles of larger sizes or resist transport entirely Figure 4 Moult ensemble of the trilobite Phacops saberensis, Lower (Seilacher, 1973). Devonian, Morocco, showing disarticulated cephalon and articulated Biased ratios of skeletal parts may provide evidence for thorax-pygidium; such associated moult parts provide evidence for a lack of seafloor disturbance at the time of burial and thereby provide strong sorting or of preferential destruction. For example, it is a evidence for life activities in the environment. Reproduced from Brett et al. common observation that certain portions of trilobite (2012). Reprinted by permission of Society for Sedimentary Geology. skeletons and one of the two differently shaped valves of many brachiopods are preferentially preserved, whereas other parts tend to be fragmented. Many brachiopod shell only of the delicacy of their original construction but also beds show many times as many pedicle as brachial valves. of time before burial (Figure 3). Moreover, the brachial valves display no hint of abrasion Organisms with multielement skeletons composed of or corrosion, whereas many of the pedicle valves are several parts, including arthropods, echinoderms (particu- strongly abraded and/or corroded (Brett and Bordeaux, larly forms such as starfish or asteroids) and most ver- 1990). This indicates that the pedicle valves, which possess tebrates, essentially cannot be exhumed and reworked as strong internal structures, were better able to withstand intact specimens (Brett and Baird, 1986). The occurrence of destructive processes. Most brachial valves were frag- completely articulated remains thus provides dramatic evi- mented into unrecognisable small pieces before they could dence for a population of organisms that was abruptly wiped become strongly corroded or abraded. Such biased ratios out. Modern experimental studies indicate that the initial of parts may provide an important indicator for the extent degradation of connective tissues occurs within a period of a of reworking in a particular shelly deposit. Beds with few hours after death, whereas destruction of ligaments and approximately one-to-one ratios of valves may indicate complete disarticulation ensues in weeks to months. See also: rather rapid and intact burial, whereas those that show a Arthropoda (Arthropods); Echinodermata strongly biased valve ratio probably represent time-aver- Most well-preserved benthic fossils are preserved aged accumulations. During prolonged exposure on the sea approximately in their living sites, so bottom-water anoxia bottom, intense storms or even biotic disturbances, frag- can be ruled out as a preservational factor. However, some mented brachial valves preferentially remain. See also: beds of intact pelagic fossils, such as fishes, squids or Brachiopoda marine reptiles, may represent carcases that settled onto Fragmentation of shells may occur via biotic or abiotic anoxic seafloors (Seilacher et al., 1985; Allison, 1988; means. Much fragmentation is thought to reflect physical Bottjer et al., 2002). Inhibition of scavenging in these impact of shells with one another. However, some breakage environments may prolong the association of skeletal may be the work of predators. Crabs, for example, produce

6 eLS & 2013, John Wiley & Sons, Ltd. www.els.net Fossils and Fossilisation distinctive breaks in gastropod shells as they peel back the fragmentation, valve ratio biasing, corrosion and/or shell to get at the organism’s soft parts (e.g. Vermeij, 1983). abrasion (Brett and Baird, 1986). Recent research shows It is important to distinguish between shells that have that darkened shells also correlate with these character- been physically abraded and those that have been corroded istics indicating increased exposure time on the seafloor by biogeochemical processes. Abrasion of skeletons is (Kolbe et al., 2011). Conversely, deposits that accumulate indicative of high-energy environments, such as the surf as sudden events will display higher proportions of zone where shells may be fragmented, faceted and even articulated material or little or no shell breakage, including polished by effects of ‘sandblasting’. intact, delicate forms such as branching bryozoans, little or Chemical corrosion and bioerosion tend to predominate no biasing of valve ratios, and few, if any, fossils that are over physical abrasion in low-energy, offshore settings. heavily corroded, abraded or otherwise altered in their Bioerosion is particularly effective in degrading carbonate surfaces. With slower sedimentation, the organismal skeletons, such as corals and shells, in shallow, tropical remains become increasingly degraded by disarticulation, environments. Shells tend to become etched or micritised breakage and varying degrees of corrosion/abrasion and by the action of microboring algae in the photic zone. biasing. Ratios of articulated to disarticulated shells Moreover, certain animals, notably clionid sponges and among bivalves or brachiopods may be an important endolithic bivalves, tend to riddle shells with a series of pits indicator of the duration of time and extent of mixing in a or galleries of boreholes. These may cause as much as 25% deposit. weight loss in coral skeletons (Bromley, 1992). Under exceedingly rapid burial, even soft parts may be Even in life, skeletons of marine animals may become preserved. Certain fossils, especially those with skeletons encrusted with epibiontic organisms, such as corals and composed of articulated elements, are sensitive indicators bryozoans. Postmortem shells may be encrusted both of rapid burial. Articulated and closed examples of bivalves externally and internally. Internal encrustation provides an may provide evidence for burial that is rapid enough to excellent indication that shells have laid disarticulated for a preclude rotting of adductor muscles and a natural ten- period of time on an oxygenated sea bottom. In some dency for the shell to splay open at the hinge. The occur- instances, skeletons that have been corroded or abraded are rence of closed shells filled with calcite spar, as opposed to subsequently encrusted and this provides another indicator sediment, may indicate pulses of burial that covered the of prolonged residence time of skeletons on the sea bottom. shells whereas interiors were still occupied by soft parts that subsequently decayed, leaving a void space that is sec- Burial ondarily filled by minerals. Deposits containing very high proportions of articulated Burial of remains of organisms is critical to fossil preser- specimens will have accumulated over short intervals of vation (Figure 3). Even the most robust skeletal parts cannot time. Such groupings of fossils may provide near-instant- last indefinitely without burial and most are destroyed in aneous population census samples. periods of years. Slow burial also permits the mixture of Storms are probably the most effective agents producing different generations of skeletal material, a phenomenon both mortality and burial in shallow marine areas. Many of known as time-averaging (Walker and Bambach, 1971; the spectacular obrution, or rapid sediment ‘smothering’, Kidwell, 1993). The scale of time-averaging for certain events represent the distal deposition of sediments modern environments has been calibrated using dating suspended by storms and transported by resultant, basin- techniques such as carbon-14 and amino acid racemisation, ward-flowing gradient currents. For terrestrial settings, with the result that some settings such as tidal flats in the flood-related sedimentation events, including mudslides, Gulf of California showing shells differing in age by as probably play the same role. Turbidity currents are critical much as several thousand years being deposited in the same to preserving deeper water assemblages of the continental sedimentary units (Kowalewski et al., 1998). slope and deep basin, including grazing and farming traces Various agents of sedimentation may produce burial. of the Nereites ichnofacies. These include settling of heavy carcases into loose sedi- ments, settlement of suspended sediments or volcanic ash, encroachment of sand ripples or dunes and inmixing of Fossil Diagenesis skeletal remains by burrowing organisms. Under more unusual conditions, organism’s remains may become Early diagenetic phenomena may provide information engulfed in unusually sterile media that prevent decay. regarding the geochemistry of bottom waters and the upper Examples include mammoths refrigerated for thousands of ‘taphonomically active zone’ of the sediment column years in permafrost, desiccated slot carcasses in caves and (Figure 1). Diagenetic features of note include evidence for organism remains impregnated by tar and fossil insects in early dissolution, compaction and mineralisation of fossils. amber (Selden and Nudds, 2004 and references therein). See also: Sedimentation Skeletal minerals and their stability Skeletal condition is valuable for recognising qualita- tively the burial rates (Figure 3). Condensed skeletal accu- Organisms secrete a variety of skeletal minerals, including mulations will display high frequencies of disarticulation, calcite, aragonite, silica and phosphate (apatite), as well as

eLS & 2013, John Wiley & Sons, Ltd. www.els.net 7 Fossils and Fossilisation organic complexes such as the chitin–phosphate of lingulid Pyrite (iron disulfide) is common in many marine brachiopods. Carbonate skeletons are commonly pre- mudrocks because of the availability of iron in terrigenous served, although those composed of aragonite are more muds and the ubiquitous abundance of dissolved sulfate in prone to dissolution; bone apatite (especially enamel and marine (but not in fresh) water. Under anaerobic con- dentine) is also rather resistant in some environments. ditions, iron reduction can generate ferrous ions, which Siliceous skeletons are relatively rare and are restricted to may react with H2S generated by sulfate reduction to sponges and some microfossils. produce pyrite. Sediment will eventually become anoxic Different skeletal mineralogies have varying stabilities. with burial and pyrite can form either very early or rela- Under most normal marine environments the ortho- tively late in the burial history of a sediment (e.g. Berner, rhombic calcium carbonate polymorph aragonite is less 1980). stable than rhombohedral calcite, and is more prone to Within anoxic marine sediments low in organic matter, dissolution. The biocrystals of echinoderm plates are pyritisation tends to occur locally in the vicinity of anae- exceptionally resistant to dissolution because of their large robically decaying organisms. As a result of local sulfate size and may be the only skeletal carbonates remaining reduction, sulfide is liberated around this decomposing after prolonged exposure (Seilacher, 1973; Donovan, organic material (Berner, 1968, 1980; Brett and Baird, 1991). 1986; Canfield and Raiswell, 1991). Dissolved iron will Calcium hydroxyapatite is the major component of react at the site of sulfate reduction. Black, organic-rich vertebrate bones and teeth and occurs in some inarticulate sediments have abundant disseminated pyrite but poorly brachiopods. Organic phosphate is not initially a highly preserved fossils. Conversely, pyritised fossils tend to occur stable material. However, in certain environments, par- in bioturbated, grey mudstones, indicating environments ticularly dysoxic, organic-rich sediments, bone apatite with limited bottom-water oxygenation, with anoxic sul- (organic fluorophosphate) may be replaced by the far more fide-rich microenvironments near decaying organisms. stable form calcium carbonate fluoroapatite. Fluor- Many anaerobic decay processes generate bicarbonate oapatite is stable over wider pH ranges than is calcite, and is (HCO3), which may foster the precipitation of calcite or indeed one of the most stable biogenically associated siderite concretions around decaying organic matter (Ber- components. Once bones, teeth, conodonts or internal ner, 1980; Canfield and Raiswell, 1991). When fossils are fillings of phosphatic fossils have been replaced by calcium enclosed within carbonate concretions they are typically fluoroapatite, they are resistant to further dissolution. Such uncrushed, even if the surrounding sediments are strongly materials may form lag deposits, which remain stable on compacted. This proves that the concretions formed before the sea bottom for periods of hundreds of thousands to skeletons could be crushed by overburden pressure. Some perhaps millions of years and are common components of of the best-preserved fossils, such as those from the Penn- bone beds. See also: Bone Homeostasis: Extracellular sylvanian Mazon Creek area, occur entombed within early Calcium Levels and Their Control diagenetic concretions in mudstone. The bicarbonate In fossils that are preserved as moulds, the relative timing build-up around the decaying carcase served to nucleate of skeletal dissolution is commonly recorded. If skeletons the precipitation. It is also likely that permeability around are dissolved prior to compaction, they may still be the carcass was higher and this too may have promoted recorded as plastically deformed moulds. Such preser- later precipitation. vation would indicate undersaturation with respect to It is now recognised, however, that much of the car- aragonite in the upper sediments and possibly low-pH bonate within concretions was generated by the decay of conditions (Canfield and Raiswell, 1991). Conversely, disseminated organic matter within the sediment. The many fossil moulds display clear mosaic fracture patterns development of large concretions may thus require that the on their surfaces. Such shells remained undissolved until sediment surrounding a local nucleation centre remain after early phases of compaction, which caused brittle within the zone of sulfate reduction (i.e. in the upper few fracture of the shells. However, even skeletons composed of decimetres of the sediment column) for prolonged periods, readily dissolved aragonite may be indirectly preserved if probably at least a few thousand years (e.g. Raiswell and encrusted by other organisms with more resistant, calcitic Fisher, 2004). Hence, horizons of fossil-bearing con- skeletons, for example, preservation of excellent moulds of cretions may reflect episodes of rapid burial that were then clams and gastropods on the basal surfaces of bryozoans, a followed by sediment-starved intervals. Concretion beds process termed bioimmuration (Taylor, 1990). See also: underlie recognised diastems and concretions may also Fossils in Phylogenetic Reconstruction become reworked into erosion lag beds, bored and encrusted by organisms (Figure 1). Early diagenetic mineralisation Phosphatic nodules form more rarely, because phos- phorus is present in only very small quantities in seawater. Early diagenetic minerals, such as siderite, calcite and Decay of organic matter and reduction of phosphate- pyrite generally form as a result of the action of anaerobic bearing iron hydroxides liberate phosphate-bearing com- bacteria and are partly composed of their respiratory pounds into solution. Dissolved phosphate may be released byproducts. They may provide valuable information on back to the water column if anoxia persists to the sedi- sediment geochemistry and rates of burial. ment–water interface (Briggs and Wilby, 1996). However,

8 eLS & 2013, John Wiley & Sons, Ltd. www.els.net Fossils and Fossilisation if a micro-oxidised zone exists in the upper sediment then cast of the shell may be produced; the cast will have the the phosphates may be reprecipitated, especially around same topology as the original shell. phosphatic skeletal nuclei, such as bones and arthropod Finally, in the case of organic skeletons, such as plant carapaces. Phosphatic coatings are associated with the remains, preservation typically may involve carbonisation highest fidelity soft-tissue preservation (Briggs and Wilby, or coalification, residues of relatively inert hydrocarbon 1996; Briggs, 2003). material or carbon granules. If the sedimentation rate is low, phosphatic material will be concentrated at one layer in the sediment. Phosphatic fossil moulds or concretions are nearly always an indicator Taphofacies of low rates of sedimentation. Phosphatic nodule horizons are important indicators of sediment starvation and com- Taphofacies, a term originally introduced by Brett and monly indicate times of rising sea level. Baird (1986), are bodies of sedimentary rock that are defined Early diagenesis also plays a large role in the preser- by the taphonomic properties of enclosed fossils. In this vation of trace fossils. Preferential mineralisation of sense, taphofacies are analogous to lithofacies and biofacies, organic-rich, mucous-lined or pelleted burrow walls com- which define sedimentary bodies on the basis of physical monly leads to preservation of recognisable trace fossils rock properties and fossil content, respectively. Import- even in settings where discrete biogenic sedimentary antly, taphofacies are reflections of the physical, biological structures are rare (such as environments where biological and chemical processes operating on organismal remains, reworking of sediment – bioturbation – is intense and a and are therefore very strongly linked to lithofacies and homogenised, uniform sedimentary fabric is generated). biofacies, as the physical controls on the distribution of Further, if open burrow systems are emplaced in firm, sediment, sedimentary structures, and organism ecology compacted sediment, then material that passively falls into and behaviour will also exert a significant influence on fossil and fills in the empty burrow will be characterised by preservation. Many of these controls are summarised in the greater porosity and/or permeability than that of the sur- section on preservation and depositional environments rounding material. The increased capacity of burrow-fill- (above). In general, differences in rate of sediment input (and ing sediment to interact with fluids often leads to early consequent burial of organism remains), geochemical diagenetic mineralisation of burrows; this mineralisation properties of bottom water or porewater (and consequent often ends sharply at burrow walls, particularly in mud- diagenetic effects), and character of background- and event- dominated settings. deposited sediment, among other factors, result in the development of distinctive packages that vary predictably in taphonomic properties. Late diagenesis An important consideration in the classification and interpretation of taphofacies is the significance of both Mineralised skeletons such as calcite shells and vertebrate background and event sedimentation. Fossiliferous sedi- teeth may be preserved with little or no alteration for many ments generally preserve evidence for varying degrees of million years after burial and compaction. Rarely, even both modes of sediment input, resulting in units that con- coloration may be preserved. However, in many cases, tain contrasting taphonomic signatures that reflect rapid changes that are collectively termed petrification may affect burial events separated by longer intervals of exposure of the preservation of fossils. Commonly, the crystal structure skeletons or slow burial (Speyer and Brett, 1991). Dis- of the shell may be altered without any loss or gain tinguishing between both styles of preservation is critical to of chemical components; this is recrystallisation. For accurately interpret depositional or ecological dynamics, example, originally aragonitic mollusc shells may be and recognising subtle differences between background recrystallised to calcite. Porous skeletons such as wood and and/or event deposits in different taphofacies leads to bone may be impregnated with minerals such as calcite or increased resolution of paleoenvironmental parameters silica, a process termed permineralisation. In more extreme (Brett and Baird, 1986; Speyer and Brett, 1986, 1991). cases, the skeleton may undergo molecular replacement of Recently, for example, Thomka et al. (2012) used subtle original mineralogy by a different material. Common differences in the taphonomic state of crinoid plates to replacements include silica (silicification), calcite (calcifi- recognise taphofacies in an otherwise lithologically and cation) and pyrite (pyritisation). See also: Algal Calcifi- biotically homogeneous interval of mudstone. Though cation and Silification; Silica little insight could be gained through traditional sedi- In other cases, the dissolution of shell material after mentological analyses, taphofacies analysis revealed lithification of the surrounding sediments leaves open approximately decimetre-scale alternations between low- spaces separating internal and external moulds or impres- sedimentation intervals that were punctuated by episodic, sions of the skeletal part. Such moulds show a negative relatively low-energy, rapid burial events characterised by relief of the original shell (i.e. convex features of the shell thin burial layers, and intervals of higher sedimentation are concave in the mould and vice versa). In the very rare interrupted by higher energy rapid burial events charac- cases in which the void space left by dissolution is filled terised by thicker burial layers (Figure 5). This study, among secondarily with sediment or mineral material, a natural others, exemplifies the value of taphofacies, even subtle

eLS & 2013, John Wiley & Sons, Ltd. www.els.net 9 Fossils and Fossilisation

Taphonomy of crinoid bioclasts (specimens / kg sample)

0 20 40 60 80 100120 140 160 180 200

45

40

35

30

25

20 Pennsylvanian Barnsdall formation 15

Encrustation 10 Breakage

5 Minor articulation Main bed Bed 0 Bed 1 Bed 2 Bed 3 Bed 4

0

Figure 5 Distribution of selected taphonomic features on crinoid plates from the Copan crinoid Lagersta¨tte (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. (2012). ones, in identifying depositional processes in seemingly A closely related concept is that of the taphonomic grade homogeneous or inscrutable sections. (Brandt, 1989). Taphonomic grade systems are used to Detection of lateral changes in taphofacies, particularly semi-quantitatively compare and contrast fossil preser- when studied in conjunction with analysis of lithofacies vation, and are therefore very helpful in distinguishing and and biofacies, can provide much insight into palaeo- defining taphofacies. Taphonomic grade systems are geography, including the position of ancient shorelines and established by placing specimens of selected organisms into deep basins, the direction of former storm tracts and the categories that span a spectrum from ‘well preserved’, in presence of subtle topographic features. Vertical changes in whatever way such a concept is defined (articulated, com- taphofacies indicate evolving conditions through time, plete, containing organic matter, etc.), to ‘poorly pre- which is particularly important in interpreting changes in served’. The number of intermediate grades may vary relative sea-level occurring at scales that are more refined depending on the morphology of the subject organisms and than those that are commonly resolvable using purely the desired level of resolution. Hence, every specimen sedimentological methods. At a much coarser scale, oper- recovered from a deposit can be classified with regard to ating at the level of evolutionary processes, the appearance taphonomic state; eventually, the proportion of specimens or disappearance of certain taphofacies indicates eco- representing each taphonomic grade can be compared logical or evolutionary innovations (e.g. increased bur- between different localities or between different horizons at rowing, elevated predation or scavenging, shifts in skeletal the same locality. Further, comparisons can be made mineralogy) or major, secular changes in environmental between different types of organisms recovered from each conditions (e.g. seafloor or atmospheric oxygenation, shifts sampling horizon. In this way, the criteria used to define in ocean chemistry, migration of landmasses into different distinct taphofacies can be applied in a more numerical way climatic regimes). than by qualitative observations.

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Continued work on taphofacies is directed toward Danise S, Dominici S and Betocchi U (2009) Mollusk species at a identifying more precise diagnostic criteria for recognising Pleistocene shelf whale fall (Orciano Pisano, Tuscany). Palaios the taphonomic signature of particular depositional 25: 449–456. environments. This has exciting potential for incorpor- Dodson TM, Bakker RT, Behrensmeyer AK and McIntosh JS ation into multidisciplinary investigations into strati- (1980) Taphonomy and paleoecology of the dinosaur graphic and geochemical cycles. beds of the Jurassic Morrison Formation. Paleobiology 6: 208–232. Donovan SK (ed.) (1991) The Processes of Fossilization. New References York: Columbia University Press. Efremov JA (1940) Taphonomy: a new branch of paleontology. Allen JRL (1990) Transport hydrodynamics: shells. In: Briggs Pan-American Geologist 74: 81–93. DEG and Crowther PR (eds) Palaeobiology: A Synthesis, pp. Farrimond P and Eglinton G (1990) The record of organic com- 227–230. Oxford: Blackwell Scientific. ponents and the nature of source rocks. In: Briggs DEG and Allison PA (1986) Soft-bodied animals in the fossil record: the role Crowther PR (eds) Palaeobiology: A Synthesis, pp. 217–222. of decay in fragmentation during transport. Geology 14: Oxford: Blackwell Scientific. 979–981. Guthrie RD (1990) Frozen Fauna of the Mammoth Steppe. Chi- Allison PA (1988) Konservat-Lagersta¨tten: cause and classifi- cago: University of Chicago Press. cation. Paleobiology 14: 331–344. James NP and Bourque A-P (1992) Reefs and mounds. In: Walker Allison PA and Briggs DEG (1991) Taphonomy of non- RG and James NP (eds) Facies Models: Response to Sea Level mineralized tissues. In: Allison PA and Briggs DEG (eds) Change, pp. 323–347. St. John’s, Newfoundland: Geological Taphonomy: Releasing the Data Locked in the Fossil Record, pp. Association of Canada Press. 25–70. New York: Plenum Press. Jia-yu R and Johnson ME (1995) A stepped karst unconformity as Berner RA (1968) Calcium carbonate concretions formed by the an Early Silurian rocky shoreline in Guizhou Province (South decomposition of organic matter. Science 159: 195–197. China). Palaeogeography, Palaeoclimatology, Palaeoecology Berner RA (1980) Early Diagenesis: A Theoretical Approach. 121: 115–129. Princeton: Princeton University Press. Kidwell SM (1993) Patterns of time-averaging in the shallow Bottjer DJ, Etter W, Hagadorn JW and Tang CM (eds) (2002) marine fossil record. In: Kidwell SM and Behrensmeyer AK Exceptional Fossil Preservation: A Unique View on the Evolution (eds) Taphonomic Approaches to Time Resolution in Fossil of Marine Life. New York: Columbia University Press. Assemblages, pp. 275–300. Denver: Paleontological Society Brandt DS (1989) Taphonomic grades as a classification for fos- Short Courses in Paleontology 6. siliferous assemblages. Palaios 4: 303–309. Kidwell SM, Fu¨risch FT and Aigner T (1986) Conceptual Brett CE and Baird GC (1986) Comparative taphonomy: a key to framework for analysis and classification of fossil concen- paleoenvironmental interpretation based on fossil preservation. trations. Palaios 1: 228–238. Palaios 1: 207–227. Kolbe SE, Zambito JJ IV, Brett CE, Wise JL and Wilson RD Brett CE and Baird GC (eds) (1997) Paleontological Events: (2011) Brachiopod shell discoloration as an indicator of Stratigraphic, Ecological, and Evolutionary Implications. New taphonomic alteration in the deep-time fossil record. Palaios 26: York: Columbia University Press. 682–692. Brett CE and Bordeaux YL (1990) Taphonomy of brachiopods Kowalewski M, Goodfriend GA and Flessa KW (1998) High- from a Middle Devonian shell bed: implications for the genesis resolution estimates of temporal mixing within shell beds: the of skeletal accumulations. In: MacKinnon L and Campbell L evils and virtues of time-averaging. Paleobiology 24: 287–304. (eds) Brachiopods Through Time, pp. 219–226. Dunedin, New Kurte´n B (1986) How to Deep-Freeze a Mammoth. New York: Zealand: Balkema Press. Columbia University Press. Brett CE, Zambito JJ IV, Hunda BR and Schindler E (2012) Mid- de Leeuw JW, Frewin NL, Van Bergen PF, Sinninghe Damste´JS Paleozoic trilobite Lagersta¨tten: models of diagenetically and Collinson ME (1995) Organic carbon as a palaeoenviron- enhanced obrution deposits. Palaios 27: 326–353. mental indicator in the marine realm. In: Bosence DWJ and Briggs DEG (2003) The role of decay and mineralization in the Allison PA (eds) Marine Palaeoenvironmental Analysis from preservation of soft-bodied fossils. Annual Review of Earth and Fossils, pp. 43–71. London: Geological Society Special Publi- Planetary Sciences 31: 275–301. cation 83. Briggs DEG and Wilby PR (1996) The role of calcium carbonate- Lescinsky HL, Ledesma-Va´squez J and Johnson ME (1991) calcium phosphate switch in the mineralization of soft-bodied Dynamics of Late Cretaceous rocky shores (Rosario For- fossils. Journal of the Geological Society of London 153: mation) from Baja California, Mexico. Palaios 6: 126–141. 665–668. Martin RE (1999) Taphonomy: A Process Approach. Cambridge: Bromley RG (1992) Bioerosion: eating rocks for fun and profit. Cambridge University Press. In: Maples CG and West RR (eds) Trace Fossils, pp. 121–129. Parsons-Hubbard K, Walker SE and Brett CE (eds) (2011) The Paleontological Society Short Courses in Paleontology 5. Shelf and Slope Experimental Taphonomy Initiative (SSETI): Boulder: Paleontological Society Press. Thirteen years of taphonomic observations on carbonate and Canfield DE and Raiswell R (1991) Carbonate precipitation and wood in the Bahamas and Gulf of Mexico. Palaeogeography, dissolution: its relevance to fossil preservation. In: Allison PA Palaeoclimatology, Palaeoecology 312: 195–379. and Briggs DEG (eds) Taphonomy: Releasing the Data Locked Poinar GO Jr (1992) Life in Amber. Stanford: Stanford University in the Fossil Record, pp. 411–463. New York: Plenum Press. Press.

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Raiswell R and Fisher QJ (2004) Rates of carbonate cementation Taylor PD (1990) Preservation of soft-bodied and other organ- associated with sulphate reduction in DSDP/ODP sediments: isms by bioimmuration – a review. Palaeontology 33: 1–13. implications for the formation of concretions. Chemical Geol- Thomka JR, Mosher D, Lewis RD and Pabian RK (2012) The ogy 211: 71–85. utility of isolated crinoid ossicles and fragmentary crinoid Richter R (1928) Aktuopala¨ontologie un Pala¨obiologie, eine remains in taphonomic and paleoenvironmental analysis: an Abgrenzung. Senckenbergiana 10: 285–292. example from the Upper Pennsylvanian of Oklahoma, United Rudwick MJS (1976) The Meaning of Fossils: Episodes in the States. Palaios 27: 465–480. History of Palaeontology, 2nd edn. New York: Science History Vermeij GJ (1983) Traces and trends of predation, with special Publications. reference to bivalved animals. Palaeontology 26: 455–465. Schaal S and Ziegler W (1992) Messel – An Insight into the History Walker KR and Bambach RK (1971) The significance of fossil of Life and the Earth. Oxford: Clarendon Press. assemblages from fine-grained sediments: time-averaged com- Scott AC and Rex G (1985) The formation and significance of munities. Geological Society of America Abstracts with Pro- Carboniferous coal balls. Philosophical Transactions of the grams 3: 783–784. Royal Society B: Biological Sciences 311: 123–137. Weigelt J (1927) Rezente Wirbeltierleichen und ihre Pala¨obiolo- Seilacher A (1973) Biostratinomy: the sedimentology of bio- gische Bedeutung. Leipzig: Verlag von Max Weg. logically standardized particles. In: Ginsburg RD (ed.) Evolving Concepts in Sedimentology, pp. 159–177. Baltimore: Johns Hopkins University Press. Further Reading Seilacher A, Reif WE and Westphal F (1985) Sedimentological, Allison PA and Bottjer DJ (eds) (2011) Taphonomy, Second Edi- ecological, and temporal patterns of fossil Lagersta¨tten. tion: Process and Bias Through Time. New York: Springer. Philosophical Transactions of the Royal Society B: Biological Briggs DEG and Crowther PR (eds) (2001) Palaeobiology II. Sciences 311: 5–23. Oxford: Blackwell Science. Selden P and Nudds J (2004) Evolution of Fossil Ecosystems. Kidwell SM and Behrensmeyer AK (eds) (1993) Taphonomic Chicago: University of Chicago Press. Approaches to Time Resolution in Fossil Assemblages. Shabica CW and Hay AA (eds) (1997) Richardson’s Guide to the Paleontological Society Short Courses in Paleontology 6. Fossil Fauna of Mazon Creek. Chicago: Northeastern Illinois Boulder: Paleontological Society Press. University Press. Nagle JS (1967) Wave and current orientation of shells. Journal of Sha¨fer W (1972) Ecology and Palaeoecology of Marine Environ- Sedimentary Petrology 37: 1124–1138. ments. Chicago: University of Chicago Press. Nudds JR and Selden PA (2008) Fossil Ecosystems of North Speyer SE and Brett CE (1986) Trilobite taphonomy and Middle America: A Guide to the Sites and their Extraordinary Biotas. Devonian taphofacies. Palaios 1: 312–327. Chicago: University of Chicago Press. Speyer SE and Brett CE (1991) Taphofacies controls: background Speyer SE and Brett CE (1988) Taphofacies models for epeiric sea and episodic processes in fossil assemblage preservation. In: environments: Middle Paleozoic examples. Palaeogeography, Allison PA and Briggs DEG (eds) Taphonomy: Releasing the Palaeoclimatology, Palaeoecology 63: 222–262. Data Locked in the Fossil Record, pp. 502–541. New York: Plenum Press. Stock C and Harris JM (1992) Rancho La Brea: a record of Pleistocene life in California. Natural History Museum of Los Angeles County, Science Series 37: 1–113.

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