Although we now have an increasing number of opportunities to observe the progress of extinction directly with our own eyes, or at least “broadcast live”, the greatest amount of information on their progress and mechanisms can be obtained from studying fossils. Fossils are the remains of living organisms that, because of the interplay of favorable circumstances, found themselves posthumously in conditions under which they were not decomposed biologically, chemically or mechanically, but rather fossilization occurred, i.e. a process during which the components of tissues that normally undergo decomposition are replaced by resistant, usually inorganic material that enables preservation of the basic structure of the organism. Most frequently, the hard parts of organisms are preserved by fossilization; during the lifetime of the organisms, they were mostly formed of inorganic material, i.e. particularly hard shells and bones. However, in some cases, the conditions for fossilization were so favorable that soft tissues were also fossilized, so that we now have available unique finds that show us the appearance of the embryos of some marine invertebrates. Basically, ideal conditions for preservation of an organism occurred under circumstances where the individual was encased in plant resin. Thus petrified resin – amber – occasionally contains the perfectly preserved fossils of various organisms, understandably primarily tiny arthropods.
Microfossils constitute a special category; these are the fossilized remains of microscopic organisms, mostly algae and blue-green algae, which have even been found in the oldest rocks which, through a combination of happy coincidences, were not exposed to high pressures and temperatures during their geological history and were thus not metamorphosed. Sensitive microanalytical methods even allow detection of the presence of substances of biological origin in these microfossils and thus confirm that these not always morphologically differentiated structures actually correspond to the fossilized remains of unicellular organisms.
The absolute age of fossils and the surrounding rocks can be determined by physical methods, most frequently by determining the ratio of radioactive isotopes and their decomposition products. Understandably, the results of this dating are accompanied by a statistical error, frequently of orders of a percentage point for modern methods. In practice, relative dating by the stratigraphic correlation method is employed, based on the two Lyellian principles: on the superposition principle (under normal circumstances, older layers are located lower down than younger layers) and the leading fossils principles (layers containing the same leading fossils are of the same age). This approach utilized the ability to differentiate layers of the same age on the basis of common physical, chemical, lithological or paleontological features. They are used most frequently for differentiating the same age of layers of paleontological features, specifically the occurrence of the same leading (index) fossils, i.e. typical fossils that are found in the particular layer and that, where possible, occur widely. The absolute age of individual layers defined on the basis of leading fossils is generally at least approximately known, as it has been possible to quite exactly date a number of boundaries between layers using a number of direct physical methods. Because of the error accompanying absolute dating, relative dating using leading fossils is usually more sensitive, i.e. enables more exact assignment of the layer in time.