Introduction
Depending on the original character of organisms, fossils are found in many forms. For example, the type of material in which it was buried, the chemical action to which it was subjected after its burial.
On the basis of the method of preservation, fossils are arranged into four groups.
A. Original Soft Parts of Organisms
Generally, the hard parts of organisms are fossilized but, under exceptionally favorable conditions, even the soft parts are also fossilized. This is maybe due to the fact that organisms are preserved in an intact medium that prevents them from decay. These media can be ice, oil, amber, or saturated soil.
Examples
- Ice or Frozen soil
The best examples of fossils preserved in ice or frozen soil are found in Siberia and Alaska and are of Wooly mammoths. These elephant-like mammals apparently died due to the heavy glaciation and were buried in permafrost around thousands of years ago.
The ice preservation is so perfect that hair, blood, flesh, skin, and other parts of the body are intact.
- Saturated soil
The fossils of extinct rhinoceros have been obtained in Eastern Poland from oil-saturated soil. This method is uncommon but has resulted in well-preserved flesh and skin.
- Amber
Amber is another captivating means of preservation of fossils. The insects of prehistoric times became entrapped in sticky gum-like resin that was secreted out from the trunk of trees. This yellowish color substance is known as Amber.
After it is being dried and solidified, the insects remained embedded in the amber as a well-preserved fossil which even shows histological details like the fresh specimen.
- Deserted forms
These may be dried out by extremely hot and dry desert winds after that buried in shifting sands resulting in well-preserved skin and flesh.
The soft part fossils are however less in number than that of hard parts fossils.
B. Original Hard Parts of Organisms
Hard parts of animals and plants are most likely capable of fossilization. The fossils of hard parts may be in the following forms:
I. Calcitic remains
These are shells of corals, foraminifera, the test of echinoderms, and brachiopods. Calcite mineral is the common constituent of all of them.
II. Aragonitic remains
These are the shells of gastropods, cephalopods, and pelecypods. These are unstable form of mineral calcite.
III. Phosphatic remains
These include bones of vertebrates.
IV. Siliceous remains
These are the skeleton of some sponges, radiolarian shells, etc.
V. Chitinous remains
These consist of exoskeletons of arthropods.
C. Altered Hard Parts of Organisms
With the passage of time, the original hard structure of many organisms may undergo substantial changes. These changes may occur in various different ways, depending on the body material of organisms, habitat, and environmental conditions where organisms lived, or the conditions under which the remains of organisms deposited.
These can be:
- Carbonization or Distillation
The process in which soft organic material may be preserved as a result of loss of oxygen, hydrogen, and nitrogen, only a thin film of carbonaceous material remains. This is called carbonization or distillation.
This carbon residue may have characteristics of the original organism. If carbon film of preserved fossil is lost in sediment, a replica called impression may still show considerable details. Trees, parts of trees, jellyfish, fish, etc. have been fossilized in this manner.
- Permineralization or Petrification
Fossils, often become petrified i.e., turned into stone, which means that the small internal cavities or pores of the original structure are filled with precipitated mineral matter. Hard parts of many fossils are preserved in this way.
Such types of fossils are heavy and stone-like. The most common example of this method of preservation is petrified wood.
- Mineralization
When circulating water dissolves chemicals from bones or shells and leaves them light and spongy, such preservation is called mineralization. More often minerals are dissolved, the gaps behind them are filled by the deposition of other minerals.
There are more than fifty types of replacing minerals. Most common of them are lime, quartz, silica, pyrites, etc.
D. Traces of Organisms
There are numerous types of fossils in addition to the above-mentioned ones. Most of them are impressions or traces of prehistoric life. The examples of such traces, or evidence are following:
I. Mold
The impression of the organism in the surrounding material is called mold. For instance, when a bone or shell or any other such substance is buried in sediment and after that dissolved by groundwater, the cavity is left and a mold is formed.
The mold only reflects the shape and surface markings of organisms. But this does not mention or reveal any information concerning the internal structure, genetic details, etc. of that organism.
- The molds of thin forms such as leaves of plants are called Imprints.
II. Cast
The natural cast is formed when the mold or hollow cavity is subsequently filled with some minerals or matter like that. Petrification differs from the cast in the way that the latter only retains the shape or form of the organism, not its structure.
An artificial cast is created when the mold or cavity is filled with the dental wax, liquid rubber, or plaster of Paris.
III. Traces and Trails
Tracks or traces are basically footprints made by animals as they walk over the surface or ground. These are most likely to be preserved in arid and semi-arid areas. Occurring in series, footprints, may indicate the shape and size of foot, posture, length of limbs and type of gait, etc.
Footprints of dinosaurs have been found in excellent shape in the Redstone of the Connecticut Valley in U. S. A.
- The impressions made by the bodies of organisms when they crawl over the ground are called trails. The borings and burrows of mollusks and worms have also been preserved as fossils in this way.
IV. Gastroliths
These are smooth rounded stones present in the rib cages of dinosaurs. These probably help the dinosaurs in digestion just like modern-day birds and pigeons have gravel in their gizzards to crush the grains. These gastroliths are only found in dinosaurs.
V. Coprolites
Coprolites are the castings of animal droppings or fossilized fecal pallets. Coprolites are usually found in association with the animals that made them. The study of fossil excreta may provide valuable information pertaining to the food habitats, predator-prey relationships, diet patterns of these organisms.