Fossils are the remains of organisms that lived in the remote past that have been naturally preserved in a lasting material, generally a rock. Many fossils have undergone a mineralization process and are preserved within sedimentary rocks. There are other, less common material where fossils can be found, such as frozen soils and amber.

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Several examples of fossils preserved in construction stones: [A] gastropod shells, [B] shells of ortoceras cephalopods, [C] the filling of a burrow excavated on marine sediment y [D] carbonised vegetal remains.

Palaeontology is the science that studies and interprets fossils. Palaeontologists reconstruct the biological evolution that organic species have undergone throughout the geological history of our planet. Since species last only for a given period of time, their fossil remains are used by geologists to date the rocks where they are found and infer the conditions under which these rocks were formed: temperature, energy, etc. The oil industry makes extensive use of fossils to explore and exploit crude reservoirs.
The temporal scale that is used to date fossils clearly exceeds the general human experience of time. The most common time unit used by palaeontologists to date fossils is a million years (Ma), which roughly represents 15,000 times the current life expectancy of human beings. The oldest fossils are about 3,400 Ma and have been found in Australian rocks; the have being interpreted as the remains of bacterias However, the fossil record is very poor until the Phanerozoic Eon, a geological unit that covers the last 540 million years. Phanerozoic means “Visibly Life” and is divided into three Eras: Palaeozoic, Mesozoic and Cenozoic.
The Palaeozoic Era means “Ancient Life” and spans about 320 million years, beginning with the Cambrian period and finishing with the end of the Permian period. The sedimentary rocks from this geological Era preserve fossil remains of relatively old living forms, including lineages that have completely gone extinct. During the first part of the Palaeozoic most organisms lived in the sea; land was not colonised until the middle Palaeozoic.

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Fossilised shell or an ortoceras, a species that lived in the Palaeozoic

The Mesozoic Era (”Middle Life”) lasted about 180 million years. Mesozoic organisms where quite similar to current species in terms of structure and variety, although there where obvious differences. The Mesozoic Era is also known as the Age of Reptiles because these animals flourished into several important groups, including extinct dinosaurs.

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Rudist fossils, a typical group of organisms from the Mesozoic

Cenozoic literally means “Modern Life” and includes the last 65 million years. The species that have lived during this short geological Era are very similar to current species. Although there where mammals in the Mesozoic, it was not until the Cenozoic that this group reached a great diversification, including the human lineage.

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Sea urchin fossil in a Cenozoic rock

Below you can see a geologic time scale with the main temporal divisions. It includes the three Eras of Phanerozoic (Palaeozoic, Mesozoic and Cenozoic), which in turn are divided into periods. This is the geologic time scale used in this web site to describe the temporal distribution of the main fossil groups that can be found on construction stones of North Spain towns.

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