Dpl

ing carbon and oxygen to carbon dioxide and water, according to this equation:

Microbial decay can also take place in anaerobic conditions, that is, in the absence of oxygen, and in these cases nitrate, manganese dioxide, iron oxide or sulfate ions are necessary to allow the decay to occur.

The second set of factors controlling decay, temperature and pH, may be the most important. High temperatures promote rapid decay. Decay proceeds at normal high rates when the pH is neutral, as is the case in most sediments, because this creates ideal conditions for micro-bial respiration. Decay is slowed down by conditions of unusual pH, such as those found in peat swamps, which are acidic. Fossils preserved in peat or lignite (brown coal) may be tanned, like leather, and many of the soft tissues are preserved. Examples are the famous Neolithic and younger "bog bodies" of northern Europe, in which the skin and internal organs are preserved, and silicified fossils in the lignite of the Geiseltal deposit in Germany (Eocene) that show muscle fibers and skin.

Decay depends, thirdly, on the nature of the organic carbon, which varies from highly labile (likely to decay early) to highly decay-resistant. Most soft parts of animals are made from volatiles, forms of carbon that have molecular structures that break down readily. Other organic carbons, termed refractories, are much less liable to break down, and these include many plant tissues, such as cellulose.

The normal end result of scavenging and decay processes is a plant or animal carcass stripped of all soft parts. In rare cases, some of the soft tissues may survive, and these are examples of exceptional preservation.

Exceptional preservation_

There are many famous examples of exceptional preservation (Table 3.2). Certain fossil-bearing formations of different ages, termed Lagerstätten, have produced hundreds of remarkable fossil specimens, and in some cases soft parts are preserved. In the most spectacular cases, soft tissues such as muscle, which is composed of labile forms of organic carbon, may be preserved. Usually, however, only the rather more decay-resistant soft tissues, such as chitin and cellulose, are fossilized. Plant and animal tissues decay in a sequence that depends on their volatile content, and the process of decay can only be

Table 3.2 Some of the most famous fossil Lagerstätten (sites of exceptional preservation) in the world.

Lagerstätten

Location

Pre-Cambrian

Doushantuo Formation Ediacara Hills Cambrian

Maotianshan Shales, Chengjiang Emu Bay Shale Sirius Passet House Range Burgess Shale "Orsten" Ordovician

Soom Shale Silurian

Ludlow Bonebed Devonian

Rhynie Chert Hunsruck Slates Gilboa

Gogo Formation, Canowindra Carboniferous Mazon Creek Hamilton Quarry Triassic

Karatau Jurassic

Posidonienschiefer, Holzmaden La Voulte-sur-Rhone Solnhofen Limestone Cretaceous

Yixian Formation Las Hoyas Crato Formation Xiagou Formation Santana Formation Auca Mahuevo Eocene

Green River Formation Monte Bolca Messel Oil Shale London Clay Florissant Formation Oligocene-Miocene Dominican amber Riversleigh Miocene

Clarkia Fossil Beds Ashfall Fossil Beds Pleistocene

Rancho La Brea Tar Pits

600 Ma 565 Ma

525 Ma 525 Ma 518 Ma 510 Ma 505 Ma 500 Ma

435 Ma

420 Ma

400 Ma 390 Ma 380 Ma 360 Ma

300 Ma 295 Ma

213-144 Ma

160 Ma 160 Ma 149 Ma

30-10 Ma 25-15 Ma

20-17 Ma 10 Ma

20,000 ya

Guizhou Province, China South Australia

Yunnan Province, China South Australia Greenland Western Utah, USA British Columbia, Canada Sweden

South Africa

Shropshire, England

Scotland

Rheinland-Pfalz, Germany

New York, USA

New South Wales, Australia

Illinois, USA Kansas, USA

Kazakhstan

Württemberg, Germany France

Bavaria, Germany

Liaoning, China Cuenca, Spain Northeast Brazil Gansu, China Northeast Brazil Patagonia, Argentina

Colorado/Utah/Wyoming, USA Italy

Hessen, Germany UK

Colorado, USA

Dominican Republic Queensland, Australia

Idaho, USA Nebraska, USA

California, USA

Mineralization

Figure 3.2 The relative rates of decay and mineralization determine the kinds of tissues that may be preserved. At minimum decay rate and with very early mineralization, highly labile muscle tissues may be preserved. When decay has gone to a maximum, and when mineralization occurs late, all that is left are the non-organic tissues such as shells. (Based on Allison 1988.)

Mineralization

Figure 3.2 The relative rates of decay and mineralization determine the kinds of tissues that may be preserved. At minimum decay rate and with very early mineralization, highly labile muscle tissues may be preserved. When decay has gone to a maximum, and when mineralization occurs late, all that is left are the non-organic tissues such as shells. (Based on Allison 1988.)

halted by mineralization (Fig. 3.2). In the process of fossilization, then, it is possible to think of a race between rates of decay and rates of pre-burial mineralization: the point of intersection of those rates determines the quality of preservation of any particular fossil.

Early mineralization of soft tissues may be achieved in pyrite, phosphate or carbonate, depending on three factors: (i) rate of burial; (ii) organic content; and (iii) salinity (Fig. 3.3a). Physical and chemical effects, such as these, that occur after burial, are termed diagenesis. Early diagenetic pyritization (Fig. 3.3b) of soft parts is favored by rapid burial, low organic content and the presence of sulfates in the sediment. Early diagenetic phos-phatization (Fig. 3.3c) requires a low rate of burial and a high organic content. Soft-part preservation in carbonates (Fig. 3.3d) is favored by rapid burial in organic-rich sediments; at low salinity levels, siderite is deposited, and at high salinity levels, carbonate is laid down in the form of calcite. In rare cases, decay and mineralization do not occur, when the organism is instantly encased and preserved in a medium such as amber (Fig. 3.3e) or asphalt.

Mineralization of soft tissues occurs in three ways. Rarely, soft tissues may be replaced in detail, or replicated, by phosphates. Permin-

eralization occurs very early, probably within hours of death, and may preserve highly labile structures such as muscle fibers (Fig. 3.3b), as well as more refractory tissues such as cellulose and chitin. The commonest mode of mineralization of soft tissues is by the formation of mineral coats of phosphate, carbonate or pyrite, often by the action of bacteria (Box 3.1). The mineral coat preserves an exact replica of the soft tissues that decay away completely. The third mode of soft tissue mineralization is the formation of tissue casts during early stages of sediment compaction. Examples of tissue casts include siliceous and calcareous nodules that preserve the form of the organism and prevent it from being flattened or dissolved.

The mode of accumulation of fossils also determines the nature of fossil Lagerstätten. Fossil assemblages may be produced by concentration, the gathering together of remains by normal processes of sedimentary transport and sorting to form fossil-packed horizons (see p. 65), or by conservation, the fossiliza-tion of plant and animal remains in ways that avoid scavenging, decay and diagenetic destruction (Fig. 3.5). Exceptionally preserved fossil assemblages are produced mainly by processes of conservation. Certain sedimentary regimes, in the sea or in lakes, are stagnant, where sediments are usually anoxic, and are devoid of animals that might scavenge carcasses. In other situations, termed obru-tion deposits, sedimentation rates are so rapid that carcasses are buried virtually instantly, and this may occur in rapidly migrating river channels or at delta fronts and other situations where mass flows of sediment are deposited. Some unusual conditions of instant preservation are termed conservation traps. These include amber, fossilized resin that oozes through tree bark, and may trap insects, and tar pits and peat beds where plants and animals sink in and their carcasses may be preserved nearly completely.

Breakage and transport_

The hard parts left after scavenging and decay have taken their toll may simply be buried without further modification, or they may be broken and transported. There are several processes of breakage (Fig. 3.6), some physical (disarticulation, fragmentation, abrasion)

Figure 3.3 The conditions for exceptional preservation. (a) The rate of burial and organic content are key controls on the nature of mineralization of organic matter in fossils. Pyritization (high rate of burial, low organic content) may preserve entirely soft-bodied worms, as in an example from the Early Devonian Hunsruckschiefer of Germany (b). Phosphatization (low rate of burial, high organic content) may preserve trilobite limbs such as this example of Agnostides from the Cambrian of Sweden (c). Soft parts may be preserved in carbonate (high rate of burial, high organic content), such as polyps in a colonial coral, Favosites, from the Early Silurian of Canada (d). If decay never starts, small animals may be preserved organically and without loss of material, such as a fly in amber from the Early Tertiary of the Baltic region (e). (a, based on Allison 1988; b, courtesy of Phil Wilby; c-e, courtesy of Derek Briggs.)

Figure 3.3 The conditions for exceptional preservation. (a) The rate of burial and organic content are key controls on the nature of mineralization of organic matter in fossils. Pyritization (high rate of burial, low organic content) may preserve entirely soft-bodied worms, as in an example from the Early Devonian Hunsruckschiefer of Germany (b). Phosphatization (low rate of burial, high organic content) may preserve trilobite limbs such as this example of Agnostides from the Cambrian of Sweden (c). Soft parts may be preserved in carbonate (high rate of burial, high organic content), such as polyps in a colonial coral, Favosites, from the Early Silurian of Canada (d). If decay never starts, small animals may be preserved organically and without loss of material, such as a fly in amber from the Early Tertiary of the Baltic region (e). (a, based on Allison 1988; b, courtesy of Phil Wilby; c-e, courtesy of Derek Briggs.)

and some chemical (bioerosion, corrosion and dissolution).

Skeletons that are made from several parts may become disarticulated, separated into their component parts. For example, the multielement skeletons of armored worms and vertebrates may be broken up by scavengers and by wave and current activity on the seabed (Fig. 3.6a). Disarticulation happens only after the scavenging or decay of connective tissues that hold the skeleton together. This may occur within a few hours in the case of cri-

noids, where the ligaments holding the separate skeletal elements together decay rapidly. In trilobites and vertebrates, normal aerobic or anaerobic bacterial decay may take weeks or months to remove all connective tissues.

Skeletons may also become fragmented, that is, individual shells, bones or pieces of woody tissue break up into smaller pieces (Fig. 3.6b), usually along lines of weakness. Fragmentation may be caused by predators and scavengers such as hyenas that break bones, or such as crabs that use their claws to

Box 3.1 Exceptional preservation of muscle and microbes

There are now many examples of fossil animals with muscle tissue preserved. These range in age right back to the Cambrian, and there is no diminution in the quality of the specimens with geologic age. A good example is the report of a horseshoe crab from the Upper Jurassic of Germany, presented by Derek Briggs and colleagues (2005) from Yale University and the University of Bristol. The specimen of Mesolimulus walchi (Fig. 3.4a) from the Plattenkalk at Nusplingen in Baden-Württemberg

Figure 3.4 Exceptional preservation of muscle in the Jurassic horseshoe crab Mesolimulus walchi: (a) the whole specimen showing the rounded headshield (prosoma), with preserved muscle tissues in the middle; (b) muscle fibers; (c) banding across muscle fibers revealed by early decay; and (d) small coccoid microbes associated with the muscle fibers. Scale bars: 20 mm (a), 50 |im (b), 10 |im (c, d). (Courtesy of Derek Briggs.)

Figure 3.4 Exceptional preservation of muscle in the Jurassic horseshoe crab Mesolimulus walchi: (a) the whole specimen showing the rounded headshield (prosoma), with preserved muscle tissues in the middle; (b) muscle fibers; (c) banding across muscle fibers revealed by early decay; and (d) small coccoid microbes associated with the muscle fibers. Scale bars: 20 mm (a), 50 |im (b), 10 |im (c, d). (Courtesy of Derek Briggs.)

looks very like a modern horseshoe crab. The site had been known since 1839 as a source of exquisite fossils of shallow-water marine organisms such as crocodilians, fishes, ammonites and nautiloids with beaks and gut contents, crustaceans and other arthropods, as well as well-preserved land plants washed in from the nearby shore, and pterosaurs that must have fallen in the water.

The specimen was collected during an excavation by the Museum at Stuttgart, and volunteer excavator, Rolf Hugger, who found the specimen, was amazed when he saw that the major muscles of the prosoma, the broad head shield, of this horseshoe crab had survived. Chemical analysis showed that the muscles are preserved as calcium phosphate (apatite). These muscles had a variety of functions: compressing and moving food through the crop, operating the limbs, and bending the body. Under the scanning electron microscope, all the muscle fibers are clear (Fig. 3.4b), and decay had highlighted cross-banding on some of the muscle fibers (Fig. 3.4c). At higher magnification, spherical coccoids (Fig. 3.4d) and spirals could be seen, associated with the preserved muscles. These coccoids and spirals are actually preserved microbes that were presumably feeding on the muscle tissue after the animal died, and formed a so-called biofilm over the carcass.

It is well known that muscle tissue breaks down rapidly after an animal dies. Experiments have shown that the muscle here must have been mineralized as apatite within a matter of days, or at most a couple of weeks. The seabed was saturated in calcium carbonate at the time of deposition (the rock is a limestone), and pH has to be lowered slightly to allow calcium phosphate to precipitate. Perhaps the carapace of the dead horseshoe crab acted as a protective roof, inside which microbes began feasting on the muscle tissues and thereby lowered the pH locally enough for apatite to precipitate. The decaying muscle provided some calcium phosphate, but more must have been derived from the surrounding sediment.

Find web references about the Nusplingen fossils at http://www.blackwellpublishing.com/ paleobiology/.

snip their way into shelled prey. Much fragmentation is caused by physical processes associated with transport: bones and shells may bang into each other and into rocks as they are transported by water or wind. Wave action may cause such extensive fragmentation that everything is reduced to fine-grained sand.

Shells, bones and wood may be abraded by physical grinding and polishing against each other and against other sedimentary grains. Abrasion removes surface details, and the fragments become rounded (Fig. 3.6c). The degree of abrasion is related to the density of the specimen (in general, dense elements survive physical abrasion better than porous

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