That was the end of our little trilobite’s more or less peaceful rest. Its incredibly long journey through age and time came to a preliminary end in the showcase of that fortunate collector, who, during long winter evenings, took pleasure in looking into its well preserved eye. A kingdom for the opportunity to see what that little creature had seen in the Palaeozoic!
If we take a closer and perhaps more serious look at diagenesis we can identify the following processes that play a part in its success:
With growing sedimentation and increasing pressure, water gets squeezed out of the surrounding sediment. The enclosed remains may get flattened by the pressure or remain in their original three-dimensional shape as the sometimes very thin shells do not necessarily form a cavity into which they may be crushed. After decomposition of soft tissues the remaining parts quickly become enclosed in sediment, the latter exerting consistent pressure.
By further compaction of the remains due to the continous increase in pressure the prospective fossil may shrink considerably, first and foremost in a vertical direction in relation to its embedding plain. This is a very interesting aspect as it allows for a much larger size in the living animal than could be assumed by the size of the fossil itself once excavated.
- Gradual elution
Involved salt dilutions gradually equalize their concentrations, the fossil may acquire the same crystalline structure as the surrounding matter whereby the better part of the original chemical composition of the remains gets lost. Gradients of concentration play a part in this (differences in the concentration of a chemical agent at two or more points within an enclosed area). Silicium compounds level out.
- Breaking up and mechanical distortion
In the course of ongoing diagnesis and ever-increasing pressure within the rock additional distortions and breaks may occur, giving further opportunity for chemical reaction and changes in composition. No cavity, however small, can escape the intrusion of salt dilutions over a longer period of time which fill the cavities. A good example are the calcite-filled chambers in fossilized ammonites (see illu.)
The changes in the fossil’s chemical structure continue. Chemical agents within the rock may gradually regroup. In a worst case scenario the rock turns metamorphous and loses its fossil information altogether. Fossils entombed in volcanic rocks may behave differently as unweathered volcanic rock is compact to a high degree right from the very beginning. Examples can be found in trees which were quickly engulfed by floating lava. Their surfaces are frequently well preserved in all detail.
- Precipitation of binders
Binders are various inorganic agents or chemical decay products of organic origin that are chemically stable. They may be altered in composition or become precipitated from the substrate over time. These binders can lead to the creation of geodes, i. e. strong enclosures around a fossil. These enclosures are a useful hint to look out for when collecting fossils.
- Formation of concretions
The substances precipitating from the fossil frequently do not get lost but accumulate close to it and enrich the surrounding rock with chemical elements and compounds. Results are concretions like the geodes we described above. In the field of trilobites a good example can be found in the geodes containing Devonian trilobites from South America which are collected in abundance by locals in certain areas near La Paz, Bolivia.
Depending on the initial situation and environmental conditions, that is pretty much how organic remains become fossils – the principles are always the same. Fossils are indeed travellers through space and time.
Recommend our web site!