In All You Need Is Biology we often make reference to fossils to explain the past of living beings. But what is exactly a fossil and how is it formed? Which is the utility of fossils? Have you ever wondered how science knows the age of a fossil? Read on to find out!
WHAT IS A FOSSIL?
If you think of a fossil, surely the first thing that comes to your mind is a dinosaur bone or a petrified shell that you found in the forest, but a fossil is much more. Fossils are remnants (complete or partial) of living beings that have lived in the past (thousands, millions of years) or traces of their activity that are preserved generally in sedimentary rocks. So, there are different types of fossils:
- Petrified and permineralized fossils: are those corresponding to the classical definition of fossil in which organic or hollow parts are replaced with minerals (see next section). Its formation can leave internal or external molds in which the original material may disappear.
Petrified fossil of horseshoe crab and its footsteps. CosmoCaixa. Photo: Mireia Querol Rovira - Ichnofossils (trace fossils): traces of the activity of a living being that are recorded in the rock and give information about the behavior of the species. They may be changes in the environment (nests and other structures), traces (footprints), stools (coprolites -excrements-, eggs …) and other traces such as scratches, bites…
Dinosaur eggs (nest). CosmoCaixa. Photo: Mireia Querol Rovira 
Coprolites, CosmoCaixa. Photo: Mireia Querol Rovira - Amber: fossilized resin of more than 20 million years old. The intermediate state of amber is called copal (less than 20 million years) old. The resin, before becoming amber can trap insects, arachnids, pollen… in this case is considered a double fossil.
Piece of amber with insects inside, CosmoCaixa. Photo: Mireia Querol Rovira - Chemical fossils: are fossil fuels like oil and coal, which are formed by the accumulation of organic matter at high pressures and temperatures along with the action of anaerobic bacteria (bacteria that don’t use oxygen for metabolism).
- Subfossil: when the fossilization process is not completed the remains are known as subfossils. They don’t have more than 11,000 years old. This is the case of our recent ancestors (Chalcolithic).
Ötzi a subfossil. It is Europe’s oldest natural mummy. He lived during the Chalcolithic (Copper Age) and died 5300 years ago. Photo: Wikimedia Commons - Living fossils: name given to today’s living organisms very similar to species extinct. The most famous case is the coelacanth, it was believed extinct for 65 million years until it was rediscovered in 1938, but there are other examples such as nautilus.
Comparison between the shell of a current nautilus (left) with an ammonite of millions of years old (right). CosmoCaixa. Photo: Mireia Rovira Querol - Pseudofossils: are rock formations that seem remains of living beings, but in reality they are formed by geological processes. The best known case is pyrolusite dendrites that seem plants.
Obviously fossils became more common after the appearance of hard parts (shells, teeth, bones …), 543 million years ago (Cambrian Explosion). The fossil record prior to this period is very scarce. The oldes tknown fossils are stromatolites, rocks that still they exist today formed by the precipitation of calcium carbonate because of the activity of photosynthetic bacteria.
The science of fossils is Paleontology.
HOW A FOSSIL IS FORMED?
The fossilization can occur in five ways:
- Petrifaction: is the replacement of organic material by minerals from the remains of a living being buried. An exact copy of the body is obtained in stone. The first step of petrificationis permineralization: the pores of the body are filled with mineral but organic tissue is unchanged. It is the most common method of fossilized bones).
- Gelling: the body becomes embedded in the ice and don’t suffer transformations .
- Compression : the dead body is on a soft layer of soil, such as clay, and is covered by layers of sediment .
- Inclusion : organisms trapped in amber, or petroleum .
- Impression: organisms leave impressions in the mud and the trace is preserved until the clay hardens.
Fossilization processes and resulting fossils. Unknown author UTILITY OF FOSSILS
- Fossils give us information on how living things were in the past, resulting in evidence of the biological evolution and help to establish the lineages of living things today.
- Allow analyzing of cyclical phenomena such as climate change, atmosphere-ocean dynamics and even orbital perturbations of the planets.
- Those who are of a certain age can be use to date the rocks in where they are found (guide fossils).
- They give information of geological processes such as the movement of the continents, the presence of ancient oceans, formation of mountains…
- The chemical fossils are our main source of energy .
- They give climate information from the past, for example, studying the growth of rings in fossilized trunks or deposition of organic matter in the glacial varves.
Fossil trunks where growth rings are observed. American Museum of Natural History. Photo: Mireia Querol Rovira DATING FOSSILS
To determine the age of fossils there are indirect methods (relative dating) and direct (absolute dating). As there is no perfect method and accuracy decreases with age, the sites are often dated with more than one technique.
RELATIVE DATING
The fossils are dated according to the context in which they are found, if they are associated with other fossils (guide fossils) or objects of known age and it depends on the stratum they are found.
In geology, stratums are different levels of rocks that are ordered by their depth: according to stratigraphy, the oldest ones are found at greater depths, while the modern ones are more superficial, as the sediments have not had much time to deposit on the substrate. Obviously if there are geological disturbances dating would be wrong if there were only this method.
Stratigraphic timescale. Picture: Ray Troll ABSOLUTE DATING
This methods are more accurate and are based on the physical characteristics of matter.
RADIOMETRIC DATING
They are based on the rate of decay of radioactive isotopes in rocks and fossils. Isotopes are atoms of the same element but with different number of neutrons in their nuclei. Radioactive isotopes are unstable, so they are transformed into a more stable ones at a rate known to scientists emitting radiation. Comparing the amount of unstable isotopes to stable in a sample, scientits can estimate the time that has elapsed since the fossil or rock formed.
Carbon-14 cycle. Unknown author - Radiocarbon (Carbon-14): in living organisms, the relationship between C12 and C14 is constant, but when they die, this relationship changes: the uptake of C14 stops and decay with a descomposing rate of 5730 years. Knowing the difference between C12 and C14 of the sample, we can date when the organism died. The maximum limit of this method are 60,000 years, therefore only applies to recent fossils.
- Aluminum 26-Beryllium 10: it has the same application as the C14, but has a much greater decaying period, allowing datings up to 10 datings millions of years, and even up to 15 million years.
- Potassium-Argon (40K/40Ar): is used to date rocks and volcanic ash older than than 10,000 years old. This was the method used to date the Laetoli footprints, the first traces of bipedalism of our lineage left by Australopithecus afarensis.
- Uranium Series (Uranium-Thorium): various techniques with uranium isotopes. They are sed in mineral deposits in caves (speleothems) and in calcium carbonate materials (such as corals).
- Calcium 41: allows to date bones in a time interval from 50,000 to 1,000,000 years .
PALEOMAGNETIC DATING
The magnetic north pole has changed throughout the history of Earth and its geographical coordinates are known in different geological eras.
Some minerals have magnetic properties and are directed towards the north magnetic pole when in aqueous suspension, for example clays. But when laid on the ground, they are fixed to the position that the north magnetic pole was at the time. If we look at what coordinates are oriented such minerals at the site, we can associate it with a particular time.
This dating is used on clay remains and as the magnetic north pole has been several times in the same geographical coordinates, you get more than one date. Depending on the context of the site, you may discard some dates to reach a final dating.
THERMOLUMINESCENCE DATING AND OPTICALLY STIMULATED LUMINESCENCE (OSL)
Certain minerals (quartz, feldspar, calcite …) accumulate in its crystal structure changes due to radioactive decay of the environment. These changes are cumulative, continuous and time dependent to radiation exposure. When subjected to external stimuli, mineral emits light due to these changes. This luminescence is weak and distinct as apply heat (TL), visible light (OSL) or infrared (IRSL).
Can be dated samples that were protected from sunlight and heat to more than 500 ° C, otherwise the “clock” is reset as the energy naturally releases.
ELECTRON PARAMAGNETIC RESONANCE (ESR)
The ESR (electro spin resonance) involves irradiating the sample and measuring the energy absorbed by the sample depending on the amount of natural radiation which it has been subjected during its history. It is a complex method which you can get more information here.
REFERENCES
- Carbonell, E. (coord) (2011). Homínidos. Las primeras ocupaciones de los continentes (pg 26-37). Ariel Historia.
- Coenraads, R. (2005). Rocas y fósiles. Libros cúpula.
- Tipos de fósiles
- New Word Encyclopaedia
- Dating rocks and fossils using geological methods
- CENIEH – Centro Nacional de Investigación sobre la Evolución Humana
- Procesos químicos de fosilización
- Museo Virtual de Paleontología
- Understanding evolution
- Cover photo by Mireia Querol (Tarbosaurus)
All you need is Biology 
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