Fossilization

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Schematic representation of the fate of two dinosaur carcasses ( sauropods ) with death, embedding, mineralization (fossil diagenesis) and finally the excavation of the fossils

As fossilization is called the formation of fossils . These are complex processes that take place in geological time periods. The Fossilisationslehre or Taphonomy ( Greek . Taphos "grave") also deals with the evaluation preserved fossils and draws on the findings of several other disciplines. This includes chemistry and biochemistry , geology and biophysics and physiology (for traces of movement ). The main features of this science were first elaborated in 1940 by Ivan Antonovich Yefremov .

Formation of fossils

Video on the formation of fossils using the example of Solnhofen limestone

The process of fossil formation takes place in successive phases, which are described in more detail below:

Depending on the circumstances, these phases can take place repeatedly or in a different order. It can be that an organism is embedded immediately after its death or that it dies in the first place through the embedding (in media such as bitumen , quicksand or ice ). An organism can also be exposed again long after it has been embedded and then decomposed in order to finally be embedded again. This often happens with ice corpses, which are released from a glacier after thousands of years of embedding and are attacked by recent microorganisms and macroorganisms before they are again, then finally embedded in sediment and fossilized to rock after geological time .

death

The carcass of a herring gull ( Larus argentatus ) on the beach at Düne, which is in the early stages of embedding . However, the potential of its future preservation as a fossil is very low, even if it is embedded on land (see text), since it will most likely be completely destroyed in this life-friendly environment even after a complete burial.

Fossilization begins with the death of the organism. It is advantageous if the death is not noticed by contemporaries who feed on cadavers . For good preservation it is also important that death is not caused by highly destructive forces, such as a rock fall. Death by illness or drowning is cheaper .

In principle, every body can be preserved under suitable conditions, regardless of how large its content of hard and soft parts is, but suitable conditions for the preservation of soft and filigree hard parts occur much less frequently than suitable conditions for the preservation of solid hard parts.

decomposition

Fossils are basically incomplete remains of living things. The incompleteness is more or less pronounced depending on the conditions that a fossil-handed organism has experienced after its death or what processes it has gone through. It is usually very pronounced.

decay

As an aerobic decomposition process, putrefaction is not a stage of fossilization. Nonetheless, an organism that has been handed down in fossil form is often initially exposed to putrefaction, because this begins immediately after death. During putrefaction, the organic compounds of the body tissues ( proteins , fats , sugars ) are finally broken down into simple inorganic compounds ( carbon dioxide , salts ) (see mineralization ). Microorganisms are primarily involved in the chemical part of the putrefaction , while the mechanical part is carried out by necrophageous microorganisms and vertebrates ( scavengers ) that eviscerate cadavers and abduct or simply eat body parts. If the organism remains on the surface of the sediment, the soft tissue will disappear completely. If it is initially partially embedded, the decomposition can mainly affect the non-embedded areas.

Decay does not progress equally on every part of the body. Especially the areas around natural ( eyes , mouth , anus , etc.) or 'unnatural' body orifices ( injuries ) decompose much faster. In vertebrates, the area around the mouth decomposes particularly quickly, which often leads to the loss of the lower jaw , especially in the case of free-floating cadavers or those that are repositioned during the decomposition. These parts are then missing in such fossils.

Current observations of recent seagulls have shown that bird carcasses can only be preserved completely fossilized on land or on the shore. In the water, the body disintegrates while it drifts around on the surface due to its high buoyancy (because of the hollow bones).

Decay is accelerated by high ambient temperatures and a humid environment, made more difficult at temperatures below freezing or, for example, in dry drafts in desert areas.

Putrefaction

If the body of a dead living being enters an anoxic environment , decomposition does not occur or it is stopped early or temporarily. Because of the lack of oxygen, putrefaction occurs , which breaks down body materials without oxygen. In these cases, only anaerobic microorganisms can participate, but they leave behind significantly more soft tissue in the body. (see also biostratinomy )

Mummification

The woolly mammoth calf "Dima" at the location in the northeast Siberian Kolyma basin in the former Beringia . A worker found the superbly preserved ice mummy in 1977 while extracting gold. The male, 6 to 8 months old when he died, died about 39,000 years ago

Mummification occurs under certain conditions , for example when the ambient temperature is low and the air is dry , drafty or when toxic influences prevail. Then mummies are created which, when embedded, develop into indestructible fossils while preserving the soft tissues. Mummies alone do not become fossils without embedding, and the like. a. because there are no areas on earth where ice above the earth's surface or dry, undisturbed climatic areas can last longer. Nowhere is there a place that has been freezing or very dry on the surface of the earth for hundreds of millions of years. If the environmental conditions change, such bodies usually disintegrate completely. Dry mummies disintegrate very quickly with the assistance of microorganisms when moisture enters, ice mummies may even be sought out and scattered again by scavengers. Exposed mummies will weather over time. Mummies from geological periods are therefore not known. However, complete woolly mammoths , woolly rhinos and other ice corpses have been found from the permafrost soil in Siberia and northern North America, which have not or only slightly thawed since the end of the Ice Age . However, such fossils show a high temperature sensitivity and are particularly damaged by uncontrolled thawing processes.

Coalification

Another fossilization process is coalification. In the absence of air, a conversion of the organic material takes place in which mainly the elements oxygen, hydrogen and nitrogen are removed, whereby the carbon is relatively enriched until almost only carbon remains. With increasing coalification, lignite or hard coal can develop. The process occurs with plant material.

Embedding

Primary and secondary embedding

The primary embedding is the first embedding without the organism being reburied again. Secondary embedding occasionally occurs in desert animals that have mummified after death of thirst, lie in protected areas for a long time and at some point (possibly several times) are blown away and buried in sand. Ice mummies can be exposed, thawed and transported by water to another location, where they are re-buried (in the river sand).

There are many possibilities for the fate of a body before it is finally embedded. When the organism finally finds its final resting place, it comes into the substrate that decisively determines its further development. Depending on the type of rock that forms, typical fossil shapes are created.

Influence of the substrate on conservation

Different substrates are differently well suited for maintaining a body. The ideal case is that an organism is embedded in a substrate immediately after its death , which protects it from air supply and is suitable for forming a fossil.

Organisms can, for example:

  • being blown in by desert sand
  • be washed around by alluvial sand in river areas and thus completely covered
  • to be buried in mud
  • sink into quagmire and thus die in the first place
  • be embedded in ice

Embedding in alluvial substrate such as clay or mud is particularly beneficial. However, pure sand deposits (sandstone) rarely contain fossils, as these are destroyed in later pebble processes (diagenesis). Salt marshes are well suited to initially completely preserving and drying out the organism, but they do not allow fossils to develop, since the salt also dissolves the organism in the further geological course. This is why salt seams do not contain fossils. Even the initially relatively well-preserved bog corpses are only passed on in fossil form when the substrate falls dry and fossils itself or when it is reburied.

Embedding in sand

Embedding it in sand is very effective and allows the substance to be well preserved. However, it can easily lead to the destruction of the fossil in diagenesis .

Embedding in mud

In mud , due to its fine grain, bodies can be preserved in great detail. Sludge occurs in all aquatic environments, but only in still water zones . In particular, stratified water bodies at the bottom can create a chemical environment that is excellently suited for fossil conservation due to the lack of oxygen (cf. →  chemocline , →  sapropel , →  black slate ). Some of the most famous fossil sites had their origins in a muddy sediment, including the Maotianshan slate , the Burgess slate , the Hunsrück slate , the Posidonia slate from Holzmaden , and the oil shale from the Messel pit . Sludge is often subject to a relatively strong compaction during diagenesis, so that fossils are usually handed down in the corresponding rocks flattened ("two-dimensional").

Embedding in salt

Embedding in brine leads to very good preservation of the soft tissues, but rarely forms old fossils. A high salt concentration inhibits microbial decomposition.

Embedding in asphalt

Embedding in asphalt (“weathered” oil ) also occurs. If a large terrestrial vertebrate , such as a larger dinosaur or mammal, accidentally gets into an asphalt lake or pond, sinks into the tough, sticky mass and can no longer break free, its panic sounds attract large carnivores, which in turn return to the asphalt can be caught. This is how autochthonous thanatocoenoses often develop in asphalt pools . One of the most famous examples of such a fossil deposit is the La Brea Tar Pits in California.

Embedding in tree sap

Tree resins are ideal as an embedding medium and can preserve the structure of animals and plants in great detail. Small animals can be enclosed in a drop of tree sap, which over time turns into amber . Such inclusions are called inclusions . Most of the animals that have been preserved in amber are insects and arachnids , but worms or snails and even small reptiles also occur occasionally. In addition to animals, parts of plants such as pollen , seeds , leaves, bark and sprouts are also preserved as amber inclusions. However, when it was included in tree resins, ancient fossils never formed, since amber disintegrates during diagenesis. For example, no amber has been preserved from the carbon . Most of the amber inclusions come from the Tertiary and Cretaceous periods.

degasing

When degassing is called an anaerobic process in which all, of microorganisms energetically usable components of the body are depleted. This happens with the formation of carbon dioxide , hydrogen , ammonia , hydrogen sulfide and other gases. The soft tissue goes under, but provides other substances that fill the gap. Over time, the carcass loses a lot of substance and leaves secondary structures in the surrounding sediment. The gases escape upwards through the embedding substrate. If traces of these gases are obtained, you can later determine the spatial position of the body in this phase. This also creates dragonflies , gas-filled cavities that later fill with new substances that can be identified on the fossil (geological spirit levels). The extent of the small canals, which later fill with fine sand or other substances, shows the extent to which the carcass still contained soft tissue before it was embedded. Ideally, it was undamaged, but it was often pitted.

When the carcass is embedded in sand or soft mud, which can lead to the formation of extremely durable fossils, very rarely soft tissue is preserved.

Mussels that die in the sand often create typical degassing funnels . It also happens that gas-filled cavities cannot connect to the outside world and remain intact over geological times. In the course of time, such inclusions fill with stable crystalline deposits or are disfigured by fractures or rearrangements.

Possible conclusions:

Diagenesis and Metamorphosis

Diagenetic transformation of calcite into marcasite (dark) with a trochite opened lengthways

The embedded and degassed carcass is subject to the same fate as the substrate surrounding it. It is increasingly covered (otherwise no fossils are formed) and comes under the influence of increased pressure and often also increased temperature.

A first stage of transformation is called diagenesis ; it is decisive for the further fate of the hard substance of living beings. It begins when sedimentary rocks are formed from the softer sediments through solidification and the originally deposited sediment is transformed. This transformation also affects the stored remains of living beings, which develop into actual fossils.

Diagenesis begins with the transformation of loose sediment into solid substrate as the pressure continues to rise. Diagenesis also means that fossils often no longer contain the original material from which the dead organisms were made. It is often replaced by silicon compounds (silicification). One then speaks of rock metamorphosis .

Rocks that are transformed under high pressure and temperature lose their structure and no longer contain fossils. These rocks are called metamorphic rocks or metamorphic rocks .

Stages of diagenesis

Different levels can be distinguished:

  • drainage
    • drainage occurs with increasing pressure
    • Fossil bodies are flattened and then conform to the image of the fossil photographer
  • Compaction
    • further compaction of the resulting fossil through rock pressure
    • it sometimes shrinks considerably, primarily vertically
  • Leaching
    • in several stages
    • Salt solutions gradually equalize their concentrations
    • Fossil adopts the same crystalline structure as the surrounding material
    • much of the original material is lost
    • Concentration gradients of the different salt ion classes play a role here. In most cases, levels silicon compounds one.
  • Breakage and mechanical deformation
    • Deformations and fractures that are again subject to chemical transformation. No cavity, no matter how small, can last longer without salts being deposited and filling it.
  • Recrystallization
    • the chemical structure of the fossil continues to change.
    • Gradual material regroupings in the surrounding rock continue
    • in extreme cases, the rock becomes metamorphic and loses its fossil information
    • Fossils enclosed in effluent rock often behave somewhat differently, since unweathered effluent rock is itself very compact. Tree trunks are known that were quickly surrounded and enclosed by lava: their surfaces are usually recognizable in every detail.
  • Separation of binders
    • Binders are various inorganic substances or chemical decomposition products of organic origin that are chemically stable.
    • They are deposited or converted from the substrate over time
  • Creation of concretion
    • The material that has migrated from the fossil into the surrounding rock often does not get very far from the spot. It remains - depending on the substance - in the immediate vicinity and enriches the rock there with elements and compounds. Effects that arise in the environment are, for example, concretions.
    • Mineral deposits represent a kind of aura
    • in younger fossils, these changes can be seen with the naked eye, and excavations can be used to prepare for the find.

Due to diagenesis, the age of a fossil can often not be determined from its substrate.

conservation

Organic leftovers

Organic residues can still be found in an inorganic matrix, especially with younger fossils or incomplete fossilization. It is important that the material is quickly sealed off with oxygen , so that fossils are usually found in places with a high sedimentation rate, such as swamps , moors , lakes or shallow seas . However, only a very small amount of the total converted biomass is affected by fossilization, although this is very much based on regional conditions.

Preservation of hard parts

Hard parts are also subject to abiotic and biotic decomposition (weathering) and are not infrequently broken or weathered in various ways. However, they do not rot as quickly as soft tissues and are therefore preserved more often. Mussels and snails often have a smooth surface on their hard parts, which protects them from various attacks from their environment during their lifetime. Calcium compounds such as calcium carbonate , mother-of-pearl , apatite and others are ideal protection against various environmental influences. Built-in protein components are thus initially protected and only disintegrate during the further transformation of the embedded material in the rock.

The bone components of vertebrates, which mainly consist of inorganic substances such as calcium compounds ( calcium phosphates ), are thus preserved much more completely than the soft components before and after embedding. However, in every respect they are subject to the laws governing sedimentation and behave in the same way as the rock during all processes and transformations.

Fossils

The most common fossils are fossils. The deformation of the earth's crust is one of the reasons that ensure that fewer and fewer fossils are found in older layers of the earth. In animals, mostly only hard components such as bones , teeth or shells remain. If wood is penetrated by silicic acid , this is called silicification , the annual rings can even be preserved, which is particularly evident in the case of petrified forests . In rare cases, however, soft tissues can also be preserved, for example in the Ediacaran fauna in Australia , the Burgess Shale fossils in Canada or the Chengjiang fossils in China .

Stone cores

Stone cores arise from marine invertebrates with an exoskeleton or similar, housing-like hard parts. These hard parts are usually made of calcium carbonate and have a significantly higher maintenance potential than the soft body inside the housing. When this is completely decomposed after embedding, a hollow housing remains in the sediment. This can then be filled with sediment. If, after filling, the actual housing is destroyed (dissolved) as a result of diagenetic processes, its sediment filling , which has now solidified into rock, continues to depict the housing as a drain. The activity of the bacteria, which once decomposed the soft body, can lead to the surface of the stone core being colored differently than the surrounding sediment. The embossed stone core is a special form of stone core . Here, the outer shape of the housing is imprinted on the sediment filling that has not yet completely solidified. Stone cores are a typical form of conservation in ammonites .

Ichnofossils

Not only bodies and body parts of living things can be transformed into fossils, traces can also be preserved. Ichnofossils are among the most common fossils. These include grave marks, creep marks, running marks, eating marks or manure marks. Occasionally, traces are preserved that indicate the circumstances of death.

The preservation of tracks presupposes that the sediment that supports them and the sediment that is deposited on them consist of (at least slightly) different substrates , as otherwise the two layers will inextricably fuse and become a structureless layer. Often there are damp sandy banks with traces of animals that came to water. The sand was slowly washed over by water shortly afterwards, with river sediment settling. Occasionally, tracks in seasonal water-bearing rivers are very long because the animals use the river valleys as a game pass.

The evaluation of running tracks is very effective because, based on modern knowledge of movement physiology , but also of sports science , we know very precisely how which types of prints are created under which circumstances. In this way, conclusions can be drawn about the weight of the animal, its running speed , its age , the structure of the pool and any injuries.

literature

  • Steffen Berg, Renate Rolle, Henning Seemann: The archaeologist and death. Archeology and Forensic Medicine Munich / Lucerne 1981.
  • RG Bromley: Trace Fossils - Biology, Taphonomy, Applications . Springer, Berlin 1999. ISBN 3-540-62944-0 .
  • CC Emig: Death: a key information in marine palaeoecology. In: Current topics on taphonomy and fossilization . Collecio Encontres, 5: 21-26, Valencia 2002.
  • RL Lyman: Vertebrate Taphonomy . Cambridge University Press, Cambridge 1994, ISBN 0-521-45215-5 .
  • RE Martin: Taphonomy. A process approach . Cambridge Paleobiology Series. Cambridge University Press, Cambridge 1999, ISBN 0-521-59833-8 .
  • Arno Hermann Müller: Textbook of paleozoology. Gustav Fischer, Jena 1957, 1994. ISBN 3334002233 .

Web links

Individual evidence

  1. ↑ Efremov, JA (1940): Taphonomy: New branch of paleontology . Pan American Geologist 74/2, 81-93. text
  2. ↑ Bernhard Ziegler: Introduction to Paleobiology - Part 1. General Paleontology . 5th edition. E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart 1992, ISBN 3-510-65316-5 , p. 35.