How do you know if it s a fossil?

Many of you have found some fossilized pieces of bones, small remains of ancient organisms, shells. Some have even been lucky enough to accidentally discover a stone with the imprint of an unprecedented fish or a fossilized tooth of a fossil shark. Someone turned the fossil in their hands and threw it away, others kept their find as a “memory”, and some forever remembered this strange feeling when you hold in your hands a part of an organism that lived on Earth hundreds of millions of years ago, and you realize that you have touched stories.
And these third ones, who remembered this strange feeling for the rest of their lives, will never part with their passion for paleontology, excavations, searches for the remains of ancient organisms, collecting, classifying and storing fossils.

What are “fossils”?

Fossils are the remains of ancient organisms (or their imprints) that inhabited the Earth many millions of years ago and have been preserved in sedimentary or rock formations to this day. Depending on the type and degree of preservation, they are divided into several separate categories:

Subfossils

This category contains fossils in which, in addition to the skeleton, some soft tissue has been preserved. These also include some plant remains (phytoleims): seeds, nuts, cones of coniferous trees, wood, that is, modified remains of plants in which the cellular structure has been preserved. Subfossils also include well-preserved remains of mammoths, birds and rhinoceroses. In this case, the preservative is permafrost, or volcanic ash and bitumen.

Euphosilia

This term refers to entire fossilized skeletons of ancient organisms or their fragments. In addition to skeletons, this group includes mollusk shells, organic remains of leaves, fruits, plant seeds, spores and fungal shells.

Ichnofosilies

These are traces of the vital activity of organisms. Most often these are prints and traces of crawling, grazing, various passages and burrows, traces of the movement of large (mostly) vertebrates along the surface of the earth.

Coprofosilia

Unlike the previous category, here are the remains of waste products of fossil organisms. These include digestion products of vertebrate animals (fossilized feces), or incompletely digested remains of other organisms. Most often, fragments of ancient organisms are found (individual bones, teeth, fragments of shells), but sometimes entire fossilized remains are found. For example, on the coast of the Baltic Sea and in some other regions, pieces of amber (fossilized resin) are found with insects, leaves, grains of ancient plants, pieces of wood, etc. perfectly preserved inside the stone.
How does the process of fossilization occur (this is what mineralization of biological remains is called)? After the remains of an animal or plant fall into the ground, all the voids in them are quickly filled with sea water (if it is the seabed) or groundwater, and mineralization occurs. The faster the dead organism was hidden by sedimentary rocks (it fell into bottom silt or was covered with sand), the better it will be preserved.
Thanks to mineralization processes, the fossilized remains of most living organisms that inhabited the Earth many hundreds of millions of years ago have reached us, and paleontologists have been able to see the detailed structure of many ancient animals. For example, the fossilized remains of small pterosaurs have helped scientists understand their structure down to the smallest detail. In order for the remains to be well preserved, they had to be kept without oxygen for a long time; this was provided by water. Naturally, the remains found in natural depressions were better and more completely preserved. In this case, rapid accumulation (accumulation of water and sedimentary rocks) occurred, and, therefore, rapid burial.
If the dead animal ended up on any peak, ledge in the rock, or other similar place, the preservation and fossilization of the remains was hindered by the process of denudation. During this process, the soil was washed away by water flows and blown away by winds, exposing the underlying rocks, making burial almost impossible. Unfortunately, the remains of most large animals were not so lucky. They did not have time to completely sink into sedimentary rocks and become fossilized, so only parts of their skeletons or other fragments of carcasses have survived to this day. We know very well the structure of the feathers of small prehistoric birds, but we know almost nothing about the structure of many representatives of sauropods (lizard-hipped dinosaurs).
Often you can find fossilized remains not of the organism itself, but of its form, a cast. This happens if the organism completely gets into the sedimentary rock and, as it were, leaves its imprint there. Over time, the organism itself completely decomposed, and minerals filled the void that appeared after its decomposition, a fossil was formed, repeating the ancient organism in all its curves. How to determine the age of a fossil? How many thousands or millions of years ago did the found organism live? How was it determined which period the formation belonged to? Geologists and paleontologists have two main dating methods: relative and absolute. The relative method is faster but less accurate. Relative dating uses already studied organisms, rocks and formations. For absolute dating, a laboratory is needed to study the find. By knowing the rate of decay of certain isotopes, and how much they decayed, we can fairly accurately determine the age of the organism or, more often, the rock in which it was found.

Relative method

The relative dating method is used when we know what we found and where. For example, we found a Tyrannosaurus rex tooth from the Hell Creek Formation. We do not need a laboratory to determine the exact age. We know when Tyrannosaurus rex lived and what period the Hell Creek Formation covers. But what to do if we have unexplored layers of rocks in front of us? In this case, so-called index fossils come to our aid. As a rule, these are specific organisms characteristic only of a certain period. Their age has already been determined by the absolute method. For example, brachiopods are valuable index fossils. Thanks to these invertebrates, we can determine not only the age of the rocks, but also the physical and geographical situation of the studied area. Thus, brachiopods found in 2014 on the banks of the Shiderty River told researchers that the age of the rocks was 345-400 million years, and the area was a warm sea with an average annual temperature of +5 to +25 degrees. Accordingly, the remains of other organisms found in the same layer as these brachiopods would also date back to the Devonian period. You can also use multiple indexes. Let’s imagine that we found a formation in which the brachiopods already known to us were preserved, with an age of 345-400 million years. In the same layers, next to brachiopods, we also find trilobites dating back to 410-390 million years ago. Simple arithmetic and we get the age of the formation from 400 to 390 million years. Also in the relative dating method it is worth remembering that the layers are applied sequentially. If we find fossils whose age we know, the layer above them will be younger, and the layer below it will be older.

Absolute method

Accurate determination of the age of fossils using the absolute dating method occurs using radiometry (radioisotope dating). Radiometry uses various radioactive isotopes, which work like clockwork. The uniform radioactive decay of isotopes can help us establish very precise ages for rocks from different geological eras. From the tools of our ancestors to the exact age of the planet itself. Most often, volcanic rocks, which are layered, help us determine the age of a fossil. By dating the volcanic layers below and above the fossil, we can tell the age of the remains found. The difficulty of the absolute method is that we cannot always find the isotope we need for a particular era. For example, when it comes to radioisotope dating, the first thing that comes to mind is radiocarbon dating. But it is very rarely used for dating fossils, its degree of accuracy is good for remains younger than 60 years. During this time, the C-000 isotope goes through 14 half-life cycles and decreases by 10 times. Half-life is the time during which half of a certain amount of isotopes decays. For the C-14 isotope, this is 5730 ± 40 years. That is, in 5730 years the isotope decays by half, in another 5730 years the remaining part of it decays by half, and so on. But what if we need to set the age to millions and hundreds of millions of years? There are other methods for this that use other isotopes. Uranium-lead the analysis involves the use of uranium isotopes: uranium-235 or uranium-238. The uranium-lead method is one of the oldest and best-studied methods for dating rocks hundreds of millions and billions of years old. The accuracy of this method is very high; for rocks 2 billion years old the error will be ± 2 million years (0,1%). One of the advantages of this method is its large age coverage. The half-life of uranium-235 with its transformation into lead-207 has a period of 700 million years, and that of uranium-238 into lead-206 has a period of 4,5 billion years. Sometimes the uranium-thorium-lead method with the thorium-232 isotope is used. The transformation of thorium-232 into lead-208 has a period of 14 billion years. Lead-lead The analysis examines the presence of three isotopes in rocks: lead-206, lead-207 and lead-204. This method is used to determine the age of meteorites and rocks that have lost the isotopes uranium-235 and uranium-238. Thanks to the lead-lead method, the age of the Earth was determined. The ratio of lead-207 to lead-206, as a result of the decay of uranium-235 and uranium-238, respectively, in Earth rocks and meteorites suggested the date of the planet’s formation. To date, the most accurate figure is 4.567.200.000 ± 600.000 years. Potassium-argon The good thing about the analysis is that potassium is found in many materials: mica, clay minerals, volcanic sediments, evaporites. Because of its long half-life, the potassium-argon method is used to date fossils over 100 years old. These are not all the ways to determine the age of a find, both relative and absolute. We have no difficulty when it comes to dating a fossil whose species and location are known. It is more difficult when the sample has lost or does not have accurate data on the location of discovery. This applies to old museum exhibits, finds from amateur paleontologists, or fossils recovered from black diggers.