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5.5. Fossils


The fossils are the remains of living organisms, or their activity, which have been preserved in sedimentary rocks as a result of a process of fossilization.

It is difficult for living beings to be preserved but, when they do, they fossilize the hardest and least alterable parts, such as bones, shells, shells, teeth, logs, etc. Many times there are not even these remains and only the hole that has been filled in by another mineral is found and only the mold remains, as is often the case with shells.

Sometimes, if the right conditions are given, complete animals or plants can be fossilized that can prevent putrefaction, remaining in asphalt or resin, or having died by freezing. Thus, you can find insects and arachnids preserved in amber (remember the movie Jurassic Park), rhinos on asphalt, mammoths frozen in Siberia or logs preserved under a layer of volcanic ash.

There are different types of fossils:

  • Body fossils.
    • Hard parts (shells, teeth, ...) partially or fully mineralized.
    • Soft parts. For example the frozen mammoths of Siberia or the insects preserved in amber.
  • Chemical fossils. Fossil chemicals produced by the physiological activity of living things.
  • Molds and footprints. They can be impressions marked in the rocks by soft-bodied organisms such as worms or jellyfish, footprints left when moving, galleries dug into the earth.
  • Other types of fossils. Other remains that can reach our days are fossil eggs that, even complete nests can appear, or fossilized feces (coprolites).

Interactive activity: Types of fossils.


When a living being dies, fungi and bacteria quickly decompose it, but if they are covered by a material that insulates them from contact with the atmosphere, they can fossilize.

The remains of an organism that has just died or the traces of its activity can quickly be covered by sediment. If the conditions are suitable, a mineralization of the organic remains takes place in which the components of the organism are exchanged for other minerals such as silica or carbonates. Fossilization lasts millions of years.

Interactive activity: The formation of a fossil.
Interactive activity: Crossword.

Importance of fossils

Paleontology is the part of geology that deals with the study of fossils. Fossils provide information about the anatomy and what life was like for organisms in the past, where they lived, how they fed or the environment in which the rock that contains it was formed. We can know if the environment was continental or marine, or the climate of the region.

As living things have evolved throughout history, fossils are also different. Knowing the period in which an organism lived we can know the age of the rocks that contain it. But some lived in very small areas and others for a long time, so they do not provide much information either. Those that are most valuable are the guide fossils, which lived for a short period of time and had a wide geographic dispersion. Some of the most characteristic guide fossils are:

  • Nummulites. Large foraminifera (a type of protozoa) with a flat disk-shaped shell. They lived in the warm seas of the northern hemisphere during the Miocene, in the Cenozoic.
  • Ammonites. Ancient cephalopod mollusks, such as today's octopus and squid, but with rolled outer shells. They lived in the seas in the Mesozoic.
  • Trilobites. Marine arthropods that lived during the PaleozoicThey are very diverse, with some 5,000 different known species.


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Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.