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Fossils

 

 

A fossil is defined as being the remains of any animal or plant organism that lived in a period prior to our own, which is preserved in layers of the earth’s crust or has left clear traces of its past existence.

 

In this section there are no postage stamps of dinosaurs or other gigantic animals.

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1 - Introduction

 

The concept of fossil includes both the remains of species that still exist today, and those belonging to species that are now extinct.  If a species is extinct, it does not necessarily mean, however, that it is a fossil. For example Alca impennis was a bird that died out in historical times, but there are no known fossil remains. The element which defines a fossil, therefore, is its age; in other words its remains must have been deposited prior to our age.

 

Fossils are the remains of organisms that lived in all kinds of environments: in the sea, on land, in lakes, etc.; for this reason they can be found in all kinds of sediments and they obviously have characteristics related to the environment in which they lived.

Fossils vary in size, from dinosaurs to the microscopic foraminifera.

Sometimes there are so many remains of organisms that they form rocks such as radiolarites, spongolites, diatomites and lumachelles. There is an extremely large number of fossil species, but not more than the number of species living today.

It should be said, however, that not all species have found conditions that are favourable to the preservation of their fossil status and, moreover, the number of known fossil species is not definitive, since new species are constantly being reported.

 

 2 – The meaning of fossils

                                

Fossils are very important because they give geologists a great deal of information.

First and foremost, since they lived in a limited period of time, they have a stratigraphic value, so they permit scientists to establish the age of a layer and to classify it in a geological series. However, not all fossils give this kind of useful information: long-living species do not permit one to define the age of a level, whereas this information can be obtained from species that had a very short life.

 

 3 - Facies fossils

 

Some fossils can be found only in certain environments, and they allow us to learn about the environmental conditions in the period when the covering layer was deposited. These fossils are called facies fossils. 

Fossils thus have another important meaning, their paleogeographic significance, in that they give us information about the distribution of land and sea during different geological eras.

The presence of the remains of mainly marine organisms, suggests that the layer containing them was deposited on the sea floor and that it emerged later, probably during orogenesis.

Fossils also give useful information for reconstructing paleoclimates.  In this case, it is above all plant organisms that can best define the climate, since they are more influenced by it.

Diatoms in marine deposits are an indication, for example, of mild temperatures. However we have to consider that the further one goes back in geological time, the greater the differences are between today’s species and fossil species, and it is therefore more difficult to determine the climatic conditions in which those species lived.

The presence of carbonaceous rocks in the Carboniferous period, both in Central Europe and in America along the line of the 50th parallel, is a clear clue to the hot and humid climate in these areas. These carbonaceous rocks are in fact largely composed of ferns and palm trees, representative of a humid climate, now typical of the equatorial zone.

The distribution of corals that live only at temperatures above 20°C gives us extremely exact paleoclimate information and permits us to trace the different equatorial lines that occurred in time, following the change in position of the Earth’s poles.

Moreover fossils are evidence of evolution; they are the only historical proof that life evolved from simple organisms towards more complex and specialized creatures.

 

4 - The position of fossils

 

Fossils can be deposited in sediments in different ways: in the true primary position (the position of the organism during its life) or in the sub-primary position (when transportation took place between the death and burial of the organism). In general, organisms found in the primary position are organisms that lived fixed, either directly or by means of a peduncle, to the substrate. In most cases, however, this does not happen. 

After they die, planktonic and nektonic organisms migrate vertically until they reach the seafloor. Horizontal movements can be caused on land by the wind or surface water which carry organisms to the sea where they may undergo horizontal transportation by sea currents.

In the secondary position, various cases can be seen: during orogenetic movements the layers on the seafloor fold, bend and move upwards to create mountain chains. During these movements fossils preserved in the layers can be damaged, sometimes even becoming unidentifiable. Fortunately this does not always happen: it is possible to find wonderfully preserved fossils even on the highest mountain peaks.

 

 

Fossils Postage Stamps