Fossils are remains of organisms that developed in past times and that were preserved thanks to transformations of their constituent material. The word fossil derives from Latin, specifically from the verb fodere , “to dig”, and from the noun fossil , “what is excavated or unearthed”. And it is from this meaning that the key attribute of a fossil is derived. The most widespread idea of a fossil is the skeleton of an animal or a leaf preserved in its shape within a stone, but the concept is somewhat more complex.
Fossils may have formed in a number of ways: the bodies of ancient organisms preserved frozen in glaciers or in the polar permafrost ; dried or mummified, preserved in caves or on beds of salt; they can also be preserved throughout geological time in amber crystals, or remain isolated in clay matrices. These fossils are the ones that most fully maintain the characteristics of the organism, but they are also the least frequently found.
Baby mammoth found in Siberia, preserved frozen for over 9,000 years.
Mineralized organisms, such as dinosaur bones, petrified wood, or invertebrate shells, are the best-known fossils. Fossils with this shape are very varied, including microfossils such as microbes or pollen grains. But while they are relatively easy to find in many places, they are not the most common type of fossil on Earth. The footprints, nests, burrows, and droppings of ancient living things are another category called trace fossils or ichnofossils. They are exceptionally rare, but trace fossils have special value because they are records of an organism’s behavior.
Fossil of a dinosaur footprint; ichnofossil.
There are also chemical fossils, which consist of organic compounds, such as proteins, that were preserved in a geological matrix, that is, in a rock. Well-preserved sedimentary rocks contain chemical fossils, since they are materials that store a large amount of very valuable information in scientific research from past times. For example, certain compounds found in current leaves have been detected in ancient rocks, helping to study the evolution of these organisms.
Fossil Formation Processes
Fossils were preserved in soils and rocks, but soils have intense chemical and biological activity; organisms that die decompose in soils and their components are recycled. In order to preserve themselves, the organisms that generated fossils had to be isolated from decomposition processes when they died, and in particular from oxygen. Even the components of organisms that generate fossils more easily, such as bones, shells or wood, need special conditions in the environment in which they are deposited in order to be preserved. They should generally settle rapidly into a fine sediment, usually a clay. Even more specific preservation conditions are required to preserve skin and other soft tissues,
Despite this, some surprising fossils have been found, such as ammonites, which are around 100 million years old and show their mother-of-pearl shells intact in Miocene rocks. Or Cambrian jellyfish and two-celled embryos 500 million years old. There are only a few exceptional places where the Earth has provided the conditions that allowed the preservation of these fossils for such long periods, and they can be found in relative abundance; they are called lagerstätten . It is a term that in German means “deposit or deposit” and that is used to designate the places where it is possible to find a large number of fossils, in sediments or sedimentary rocks.
Once in the soil or sediment, the organic remains enter a long and complex process whereby they are transformed into a fossil. The study of this process is called taphonomy, from the Greek, taphos , “burial,” and nomos , “law.” Taphonomy interacts with other disciplines, such as ecology, geochemistry, and sedimentology.
Many fossils, especially the shells of marine organisms that were preserved in young rock, undergo recrystallization. In other cases, the organic matter of the organism decomposes, leaving an empty space with its shape, a mold, which is then filled with minerals that take the original shape of the organism. Petrification is a different process. In this case, the tissues of the organism are slowly replaced by a mineral, generating beautiful pieces of agate or opal in the shape of the fossilized organism.
After their preservation during geological times, the recovery of fossils can be complex. The natural processes produced by heat and pressure tend to disintegrate them, and if the rock that hosts them recrystallizes, their record can also disappear. The fracturing and folding that affect many sedimentary rocks can destroy a large part of the fossils impregnated in their matrices.
Fossils can be exposed during the processes of erosion of the rocks that preserve them. But this is a process that can take thousands of years to fully reveal a fossilized skeleton, and by the time it is complete the section discovered in the first place may already have disappeared. That is why it is very difficult to find a complete fossil of a large organism, such as the dinosaur shown in the figure below. Fossil recovery requires adequate techniques and instruments, applied by experienced personnel, so that valuable parts of the fossil that could provide key information to researchers are not lost.
Recovery of a large dinosaur fossil.
Eduardo Mayoral, Eladio Liñán, José Antonio Gámez Vintaned, Rodolfo Gozalo. Lower Cambrian jellyfish from Constantine (Seville) . Scientific research and conservation in the Sierra Norte de Sevilla Natural Park. Accessed October 2021.
Mauricio Anton. The secret of fossils . Aguilar, 2006.
Peter A. Allison, Dereck E.G. Briggs. Exceptional fossil record: distribution of soft-tissue preservation through the Phanerozoic. Geology, 21: 527-530, 1993.