Dehydration (hypohydration) is the removal of water from an object. In physiological terms, it entails a relative deficiency of water molecules in relation to other dissolved solutes. Dehydration, thus, is slightly different from hypovolemia, which defines water deficiency only in terms of overall volume rather than in terms of solute concentrations.
Replacement is the molecule-by-molecule substitution of another mineral of different composition for the original material. The fine details of shell structures are generally preserved. Minerals which commonly replace hard parts are silica and pyrite. Look for fossils which should be calcareous (crinoids, molluscs, brachiopods, corals), but which don't fizz in acid.
Recrystallization. Many modern shells are made of aragonite. Aragonite is a metastable form of calcium carbonate (CaCO3). With time, the aragonite will alter or recrystallize to calcite, a stable form of CaCO3. Paleozoic shells which fizz in acid are probably recrystallized from the original aragonite to calcite (except for echinoderms which are originally calcite).
External molds are imprints of the outside of a shell in the rock. If the original shell was convex, the external mold will be concave.
Internal molds are imprints of the inside of the shell in the rock. Look for such features as muscle scars which are present on the inside of bivalve shells. Internal molds are produced when a shell is filled with sediment which becomes cemented, and then the shell is dissolved away. Internal molds are sometimes called steinkerns.
In hot spring environments, silica that is precipitated to form sinter is primarily precipitated in the form or phase of silica called Opal-A. As a primary precipitate this form of opaline silica is characteristically amorphous and hydrated with variable amounts of water. For precipitation the hydrothermal solutions must be supersaturated with respect to Opal-A. In the reservoir at depth, the concentration of dissolved silica is controlled by the solubility of quartz
cast may be produced if a mold is filled with sediment or mineral matter. A cast is a replica of the original. Casts are relatively uncommon. (A rubber mold of a fossil can be filled with modelling clay to produce a replica or artificial cast of the original object.)
Carbonization (or distillation) preserves plants or animals as a thin carbon film, usually in fine-grained sediments (shales). Fine details of the organisms may be preserved. Plant fossils, such as ferns, in shale generally are preserved by carbonization. Soft-bodies animals may also be preserved as carbonaceous films in black shales. (Example: Cambrian Burgess Shale fauna.)
The shells of invertebrates and single-celled organisms, or vertebrate bones and teeth may be preserved unaltered. The different compositions of original material are detailed below.
- Hard parts made of calcite, such as echinoderms and foraminifera may be preserved unaltered.
- Aragonite
shells of clams, snails, or scleractinian corals may be preserved unaltered in Cenozoic deposits, but they are generally dissolved or recrystallized in older deposits. This is because aragonite is more soluble than calcite, and because aragonite is metastable, and in time recrystallizes to calcite. - Hard parts made of phosphate, such as the bones and teeth of vertebrates, conodonts, and the outer covering of trilobites. The shiny scales of fossil fish are phosphatic.
- Hard parts made of silica, such as the skeletons of diatoms and radiolarians, and some types of sponges, may be preserved unaltered in some deposits.
- Organic hard parts, made of resistant materials such as chitin, cellulose, keratin, sporopollenin, or collagen are present in some groups of organisms. Many arthropods, including the insects, have chitinous skeletons (an organic material similar in composition to our fingernails). Plant hard parts (wood) are composed of cellulose.
Burrows are the excavations of an animal made into soft sediment. Burrows are probably used as feeding and/or dwelling structures. Continued burrowing or bioturbation of the sediment will destroy primary sedimentary structures, and result in a massive, homogeneous, structureless rock.
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