What is a fossil?

When an organism dies, the parts that normally are not affected by decomposition are the hard, mineralized parts : shells and exoskeletons, bones and teeth, scales and plates. When the soft tissues have dissolved, the mineralized parts may be transported by water or by gravity and accumulated in fossil deposits where they become part of the sediments that include them, and may be preserved for millions of years. In these cases, complete skeletons are rarely found, and fossil remains are generally mixed together, often with organisms of different species. Furthermore, the mode of transportation, as in the case of river currents, may lead to a further selection of the remains, for example, only the larger sized fragments may be accumulated or, on the contrary, only the smaller fragments are transported. In exceptional cases, as, for example, in the case of a very rapid burial under a blanket of very fine sediments, with a poor availability of oxygen, the soft organic parts may be preserved, leaving an impression in the sediments which, at times, is incredibly clear cut and full of very fine details such as feathers, scales, bark, traces of skin.
The Jurassic fossils, wonderfully preserved in their finest details, of the German areas of Holzmaden and Solnhoven, the extremely salty ancient lagoons which were real death “traps” for the organisms that were carried by the waves, are very famous. In other cases, the tissues may be dissolved in the water circulating in the sediments and may be replaced, molecule after molecule, with other minerals, as for example, calcites, surely the most diffused mineral, and also by silica, as in the case of silicified tree trunks and wood, that often form real “petrified forests” (as the fossil palms of the Sahara and the coniferous forest in Arizona), quartz or pyrites, and in this case real natural “jewels” are formed, as for example, the well known piriticized ammonite fossils with their characteristic “metallic” appearance. At times this process also preserves the most minute details of the organism.
The more interesting fossils are the ones that besides remaining whole, are found in their “living position”, i.e. in the position they were living in, perhaps surprised by death as they carried out their activities : hunting, sleeping, in battle, giving birth. These fossils not only provide indications of the physiognomy, but also important proof of the environment they lived in, their life-style and their interaction with other living creatures.
This usually occurs if the organism dies due to catastrophic events that provoke an immediate burial : as for example a volcanic eruption that covers an area with ashes, a landslide that immediately buries all that it finds in its way, a flood or an accidental fall into natural “traps”, such as lakes, natural tar deposits (as the skeleton of the Sabre Tooth Tiger at Rancho La Brea near Los Angeles), wells or crevasses, or in the case of more recent organisms, ice or permafrost, as in the case of the Siberian mammoths. As far as smaller organisms are concerned, as for example insects, even the spilling of resin on the trunk of a coniferous tree may be the cause of a dramatic event, leading to a rapid death and instant fossilization : this is the case of the small arthropods that have been magnificently preserved in amber, a natural fossil resin.
The state of conservation of these small fossils provided the initial idea of Crichton’s famous book, in which the blood taken from the stomach of a mosquito fossilized in amber provides the possibility of reconstructing the DNA of the future guests of the Jurassic Park. Other fossils are not real organisms, but only the traces of their activities : tracks of animal paws or of the bodies of animals that crept on the ground, dens, excretion. In many cases it has been possible to find the origins or the “culprit”, but in the case of some of the more ancient fossils, they are the only traces of organisms that are still unknown at the present date.

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