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The origin of fossils

A short story on a very long journey

"What cannot be preserved, disappears. Whatever is useful to man, remains in the ground." - Words of the Quran

The fact that our visitors can look at photographic images of fossils – on this website as well as on many others – we owe to the circumstance that the bodies of most animals contain hard, mineralized parts which can be preserved over a long period of time, provided the animal dies in an environment favourable to fossilization (e. g. isolation from oxygen, quick deposition of sediment to enclose the body). In most cases the chemical composition of these parts transform over time to become completely inorganic. Examples are to be found in the bones of reptiles and mammals, the teeth of sharks (phosphatization), the shells of clams and brachiopods or, last but not least, the exoskeletons of trilobites (calcification).

All fossils are a window to the past and they should be treated with utmost care and respect. Every single fossil, no matter how well or how badly preserved, deserves appropriate treatment and places an obligation on its owner to ensure its future preservation outside of its protective rocky grave. Fossils are witness to the history of the Earth and strike a chord with man’s desire to discover the origin of his very own existence and the evolution of life as well as the development of the planet as a whole.

Diagenesis

Various options :
After death, the body of a marine animal usually disintegrates rather quickly (A) or is covered by a layer of mud (B).
Despite the protective cover, the body may still fall prey to animals foraging in the sediment or get disturbed by ground swell and thereby destroyed before it can be contained (C).
Even if the animal's remains escape the perils described above they still face the risk of dissolution in the uncompacted sediment or during compaction (diagenesis), depending on the predominant environmental conditions (D).
With sediment of adequate compaction, the fossil may leave an impression (E).
Salt dilutions penetrating into the impression can form into a stable mineral framework, replacing the original body (real mould) (F).
Some exoskeletons (including those of trilobites) can be embedded without alteration to their original mineral composition (G).
With increasing depth, growing pressure, adequate temperature and millions of years passing, sedimentary rocks become deformed and altered in their mineral composition (metamorphosis). The original rock texture may disappear, losing its fossil information altogether (H).
Plate tectonics cause the upfolding of ridges, sedimentary rocks get lifted far above sea level. With weathering, they will sooner or later start to yield the fossils they carry (I).

Quelle: Cyril Walker & David Ward: Naturführer Fossilien


A trilobite fate

Walcott Quarry While our little bugs roamed across the sea floors of the world’s ancient oceans in their neverending quest to make a living by securing enough food, it frequently happened that one or the other was met with the unavoidable fate of all living things and lost its earthly life (maybe in a heart attack in view of a sudden and unexpected eruption of a huge submarine volcano.) <g>

Joking aside – the body of our unfortunate little friend immediately began to drift around, most likely it got disturbed by currents, perhaps even smashed by waves and vandalized by scavengers before it finally came to rest at some place suitable for its future preservation.

As time went by, an increasing amount of fine mud laid a sealing blanket of sediment on top of our poor friend’s remains, fully enclosing it and exposing it to growing pressure. Most likely, all organic parts had already decomposed by that time, including the antennae and legs.

More time went by – much more time – and the blanket of sediment became ever thicker, entombing our little trilobite in a massive sepulchre. Under increasing pressure, the sediment that surrounded him became more compact as sea water got squeezed out. Finally the fine-grained mud transformed into solid rock, turning into limestone or slate. In this process minerals from within the sediment intruded into the exoskeleton, giving it the colors that we see in these fossils when we unearth them today. This simplified description of fossilization and its accompanying processes are referred to as “diagenesis”.

Wheeler Shale Ages passed by until, by movement of the earth’s crust, by the unsurmountable forces of plate tectonics, the fossilized remains of our little bug got disturbed again, having spent already millions and millions of years in its rocky tomb. Old land masses gave way to new continents, oceans disappeared and new ones claimed their realms. Sea floors became hills or moutain ridges (you can find fossilized sea clams on Mt. Everest) while countrysides drowned to become shallow seas. The never-ceasing forces from within the planet pushed up what used to be down, shifted left to become right and jackknifed whole massifs.

Not to get destroyed in these geological events, to escape getting smashed by the violence of tectonics or dissolved by volcanic activities was only by a stroke of luck. As a rough estimation: less than 1 % of all trilobites stood a chance of survival as a fossil in the first place and only a negligible percentage was lucky enough to survive this trip through space and time more or less complete.

Paradoxides sp. In the course of elevating former sea floors and raising them way above sea level the never ceasing forces of weathering immediately took the upper hand. Tides, precipitation, frost and winds started to wear down the rocks with determination and continued their merciless duty over millions of years until – on one dark day in November – an attentative collector noticed some familiar shape at the breaking edge of a rock, maybe a part of the trilobite’s tail shield.

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:

- Dewatering
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.

Ammonite - Compaction
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.)

Geode - Recrystallization
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.

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Last Update : 01/30/2010 5:04 PM