How do fossils preserve?
Unaltered hardparts
The concept of unaltered remains can refer to multiple modes of preservation. Freezing, encapsulation in amber (tree resin), desiccation, and chemical preservation, such as entombment in petroleum containing sediment, are various examples. Freezing, mummification (desiccation), oil seeps, and amber can preserve both soft and hard tissues. Sometimes the soft tissues decay, but the hard parts remain unaltered. Teeth, bones, and shells may be preserved as unaltered hard parts if the chemical environment allows it. The specimen below is an ammonite that preserves its original shell nacre (‘mother of pearl’)- it had been preserved in an oil seep.
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Permineralisation
Permineralisation occurs when the pore spaces of organic tissues are impregnated with dissolved minerals. Common minerals found in permineralised fossils include silica, calcite, phosphates, pyrite, iron oxides.
The example below is a brachiopod that has been replaced by silica. The original shell of organisms like brachiopods and bivalves are made of calcium carbonate. When such organisms undergo replacement in the fossil record, they commonly have a grey colour, and are generally much more resilient to erosion/destruction than the original skeleton. Bones of vertebrate animals (e.g., dinosaurs) commonly become fossilised this way too).
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Petrifications
Petrifactions are the next step in the permineralisation fossilisation process where all of the original organic materials (in addition to the pore spaces) are completely replaced by minerals (e.g., minerals that fill in plant cells AND replace the original cell walls, such as in petrified wood). Petrifications are most common in plants rather than other organic materials such as bone, teeth, and shells.
The specimen here is a lump of petrified wood.
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Recrystallisation
A recrystallisation occurs when the original organic material is recrystallised and replaced by different geological materials. The example here is an extinct ammonite whose original skeleton (composed of aragonite) has recrystallised (the mineral is now calcite; note the large crystals on the broken ends of the specimen).
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Casts and moulds (impressions)
Organisms buried in sediment may decay or dissolve away leaving a cavity or mould – a type of impression. If the space is subsequently filled with sediment or a mineral precipitated from pore waters, an external cast can be made (these are also impressions). Moulds and casts are three dimensional and preserve the surface contours (shape) of the organism. A mould preserves a negative imprint of the surface, while a cast preserves the external or positive form of the organism. Most moulds and casts do not contain the actual remains of an organism.
The specimen below represents the mould (impression) of a Permian seed fern, Glossopteris. The red parts of the apparent leaves are actually just minerals that have later impregnated the rock (iron oxides) and stained the mould. There is no original plant material left at all in this specimen.
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Here is another impression below also showing the external mould of an ammonite. Note the concave shape of the original organism (moulds are always a negative impression of the original organism).
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And finally, the below model is a cast of an ammonite (another type of impression). A cast fossil is always a positive expression of the original organism (note that the fossil is convex, just like the original outer part of the shell would have been).
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Carbonisation
Carbonisation often shows the external surface of an organism, but flattened in a two dimensional fashion. The compression retains altered tissues composed mainly of carbon. If the layer of carbon is lost on the compression through weathering or further diagenesis then an impression remains. Thus, some fossils may preserve more than one style of preservation.
The specimen below is a seed fern from the Triassic, Dicroidium (the same type of seed fern that we saw preserved in the Tingalpa Formation at Toohey Forest). This specimen represents both coalified remains (the darker, black portion), as well as impressions (the lighter-coloured parts where the original leaves have now been lost).
Click here to see a full screen 3D model of this specimen.
