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fragmentation bioerosion corrosion and dissolution disarticulation fragmentation abrasion disarticulation

bioerosion

corrosion and dissolution loss of ridges loss of ridges loss of thin edge

burial flattening diagenesis bivalve shell (aragonite)

burial flattening diagenesis bivalve shell (aragonite)

(calcite)

Figure 3.6 Processes of breakage and diagenesis of fossils. Dead organisms may be disarticulated (a) or fragmented (b) by scavenging or transport, abraded (c) by physical movement, bioeroded (d) by borers, or corroded and dissolved (e) by solution in the sediment. After burial, specimens may be flattened (f) by the weight of sediment above, or various forms of chemical diagenesis, such as the replacement of aragonite by calcite (g) may take place.

ones), the energy of currents and grain size of surrounding sedimentary particles (large grains abrade skeletal elements more rapidly than small grains), and the length of exposure to the processes of abrasion.

In certain circumstances shells, bones and wood may undergo bioerosion, the removal of skeletal materials by boring organisms such as sponges, algae and bivalves (Fig. 3.6d). Minute boring sponges and algae operate even while their hosts are alive, creating net works of fine borings by chemical dissolution of the calcareous shell material. This process continues after death, and some fossil shells are riddled with borings that may remove more than half of the mineral material of any single specimen. Other boring organisms eat their way into logs, and heavily modify the internal structure.

Before and after burial, skeletal materials are commonly corroded and dissolved by chemical action (Fig. 3.6e). The minerals within many skeletons are chemically unstable, and they break down after death while the specimen lies on the sediment surface, and also for some time after burial. Carbonates are liable to corrosion and dissolution by weakly acidic waters. The most stable skeletal minerals are silica and phosphate.

Burial and modification_

Animal and plant remains are typically buried after a great deal of scavenging, decay, breakage and transport. Sediment is washed or blown over the remains, and the specimen becomes more and more deeply buried. During and after burial, the specimen may undergo physical and chemical change.

The commonest physical change is flattening by the weight of sediment deposited above the buried specimen, and this may occur soon after burial. These forces flatten the specimen in the plane of the sedimentary bedding. The nature of flattening depends on the strength of the specimen: the first parts to collapse are those with the thinnest skeleton and largest cavity inside. Greater forces are required to compress more rigid parts of skeletons. Ammonites, for example, have a wide body chamber cavity that would fill up with sand or water after the soft body decayed. This part collapses first (Fig. 3.6f) and, because the shell is hard, it fractures. The other chambers are smaller, fully enclosed and hence mechanically stronger: they collapse later. Plant fossils such as logs are usually roughly circular in cross-section, and they flatten to a more ovoid cross-section after burial. The woody tissues are flexible and they generally do not fracture, but simply distort.

These are examples of diagenesis, and they may occur early, very soon after burial (for example, flattening and some chemical changes), or thousands or millions of years later, as a result of the passage of chemicals in solution through rocks containing fossils. Other examples of late diagenesis include various kinds of deformation by metamorphic and tectonic processes, often millions of years after burial (Box 3.2).

The calcium carbonate in shells occurs in four forms: aragonite, calcite (in two varieties: high magnesium (Mg) calcite, and low Mg calcite), and combinations of aragonite +

calcite. The commonest diagenetic process is the conversion of aragonite to calcite. After burial, pore fluids within the sediment may be undersaturated in CaCO3, and the aragonite dissolves completely, leaving a void representing the original shell shape. Later, pore fluids that are supersaturated in CaCO3 allow calcite to crystallize within the void, thus producing a perfect replica of the original shell. This process of replacement of ara-gonite by calcite occurs commonly, and may be detected by the change of the crystalline structure of the shell (see Fig. 3.6g). The regular layers of aragonite needles have given way to large irregular calcite crystals (sparry calcite) or tiny irregular calcite crystals (micrite).

A common diagenetic phenomenon is the formation of carbonate concretions, bodies that form within sediment and concentrate CaCO3 (calcite) or FeCO3 (siderite). Carbonate concretions generally form early during the burial process, and this is demonstrated by the fact that enclosed fossils are uncrushed, having been protected from compaction by the formation of the concretion. Carbonate concretions form typically in black shales, sediments deposited in the sea in anaerobic conditions. Black shales contain abundant organic carbon, and, when this is buried, bacterial processes of anaerobic decay begin. These decay processes reduce oxides in the sediment, and produce bicarbonate ions that may combine with any calcium or iron ions to generate carbonate and siderite concentrations. Such concentrations may grow rapidly to form concretions around the source of calcium and iron ions, usually the remains of an organism.

Another early diagenetic mineral that occurs in anaerobic marine sediments is pyrite (FeS2). It is also produced as a by-product of anaerobic processes of microbial reduction within shallow buried sediments. Pyrite may replace soft tissues such as muscle in cases of rapid burial, and replaces hard tissues under appropriate chemical conditions. Wood, for example, may be pyritized, and dissolved ara-gonite or calcite shells may be entirely replaced by pyrite. In both cases, the original skeletal structures are lost.

Phosphate is a primary constituent of vertebrate bone and other skeletal elements. In some cases, masses of organic phosphates are

Box 3.2 Retrodeformation of deformed fossils

Some fossils may be heavily deformed or distorted, so that they do not retain their original shapes. These distortions may be the result of collapse or diagenesis, but they may indicate metamorphism - that is, processes connected with tectonic activity, faulting, folding and mountain building. If a mudstone is folded and, under high pressure, is changed into a slate, any contained fossils are likely to be stretched and distorted. The deformation is very clear in symmetric fossils (e.g. Fig. 3.7), where the form is stretched in such a way that the original symmetry has been lost. In a slab where numerous fossils lie at different orientations, they will clearly be deformed in different ways, all subject to the same forces in the rocks.

It is possible to restore the original shape of the fossil, a process called retrodeformation, meaning "back deformation". The outlines of one, or preferably several, deformed fossils are drawn, usually in two dimensions, and these can be most easily restored to original symmetry in a standard computer drawing software program by manipulating the shape dimensions. This method also allows the analyst to calculate the amount by which the fossil was retrodeformed, and in which direction. This can tell us much about the nature of the tectonic forces that were in operation.

Deformed fossils become commoner the farther back in time one goes, simply because of the greater likelihood than any particular fossiliferous sediment has undergone metamorphism and tectonism.

Find web references about retrodeformation of fossils at http://www.blackwellpublishing. com/paleobiology/.

Figure 3.7 (a) Numerous examples of deformation of the brachiopod Eoplectodonta: in a tectonized mudstone from the Silurian of Ireland. (b) A single deformed example (c. 20 mm wide) of a Cambrian Billingsella fossil from the Himalayas (Bhutan) and (c) the same example retrodeformed to its original shape.

Figure 3.7 (a) Numerous examples of deformation of the brachiopod Eoplectodonta: in a tectonized mudstone from the Silurian of Ireland. (b) A single deformed example (c. 20 mm wide) of a Cambrian Billingsella fossil from the Himalayas (Bhutan) and (c) the same example retrodeformed to its original shape.

Figure 3.8 Different modes of plant preservation. (a) Permineralization, a silicified plant stem from the Rhyme Chert (Early Devonian, Scotland) (x 50). (b) Coalified compression, leaves of Annularia from the Late Carboniferous, Wales (x 0.7). (c) Authigenic preservation, a mold of Lepidostrobus from the Late Carboniferous, Wales (x 0.5). (d) Direct preservation of a microscopic fossilized diatom in the original silica (scale bar, 20 |im). (a, courtesy of Dianne Edwards; b, c, courtesy of Chris Cleal; d, courtesy of David Ryves.)

Figure 3.8 Different modes of plant preservation. (a) Permineralization, a silicified plant stem from the Rhyme Chert (Early Devonian, Scotland) (x 50). (b) Coalified compression, leaves of Annularia from the Late Carboniferous, Wales (x 0.7). (c) Authigenic preservation, a mold of Lepidostrobus from the Late Carboniferous, Wales (x 0.5). (d) Direct preservation of a microscopic fossilized diatom in the original silica (scale bar, 20 |im). (a, courtesy of Dianne Edwards; b, c, courtesy of Chris Cleal; d, courtesy of David Ryves.)

modified by microbial decay, which releases phosphate ions into the sediment. These may combine with calcium ions to form apatite, and this can entirely replace dissolved calcareous shells. In other cases, the microbial processes enable soft tissues, and entirely soft-bodied organisms, to be replaced by phosphate. Coprolites, fossil dung, may also be phosphatized. In these cases, apatite has been liberated from the organisms themselves, and from surrounding concentrations of organic matter, and the replacement destroys most, or all, of the original skeletal structures.

Plant preservation_

We deal with plant preservation separately because some modes are different from those seen for fossil animals. Plant parts are usually preserved as compression fossils in finegrained clastic sediments, such as mudstone, siltstone or fine sandstone, although three-dimensional preservation may occur in exceptional situations. There are four main modes of plant preservation (Schopf 1975): cellular permineralization, coalified compression, authigenic preservation and hard-part preservation (Fig. 3.8).

Plant fossils preserved by cellular permin-eralization, or petrifaction, may show superb microscopic detail of the tissues (Fig. 3.8a), but the organic material has gone. The plant material was invaded throughout by minerals in solution such as silicates, carbonates and iron compounds that precipitated to fill all spaces and replaced some tissues. Examples of cellular permineralization are seen in the Devonian Rhynie Chert and the Triassic wood of the Petrified Forest, Arizona. The most studied examples of permineralized plant tissues are from coal balls. Coal balls are irregular masses, often ball-shaped, of concentrated organic plant debris in a carbonate mass, that are commonly found in Carboniferous rocks in association with seams of bituminous coal. Huge collections of coal balls have been made in North America and Europe, and cross-sections of the tissues can reveal astounding detail.

The second common kind of plant preservation is coalified compression, produced when masses of plant material lose their soluble components and are compressed by accumulated sediments. The non-volatile residues form a black coaly material, made from broken leaves, stems and roots, and with rarer flowers, fruits, seeds, cones, spores and pollen grains. Coalified compressions may be found within commercially workable coal beds, or as isolated coalified films impressed on siltstones and fine sandstones (Fig. 3.8b).

The third mode of plant preservation, authigenic preservation or cementation, involves casting and molding. Iron or carbonate minerals become cemented around the plant part and the internal structure commonly degrades. The cemented minerals produce a faithful cast of the external and internal faces of the plant specimen, and the intervening space may be filled with further minerals, producing a perfect replica, or mold, of the original stem or fruit. Some of the best examples of authigenic preservation of plants are ironstone concretions, such as those from Mazon Creek in Illinois and from the South Wales coalfields (Fig. 3.8c).

The fourth typical mode of plant preservation is the direct preservation of hard parts. Some microscopic plants in particular have mineralized tissues in life that survived unchanged as fossils. Examples are coralline algae, with calcareous skeletons, and diatoms, with their silicified cell walls.

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