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Fossils
Fossils are remains or impressions of the hard parts of the extinct organism preserved in the sedimentary rock or other media.
• Archaeopteryx is the connecting link between reptiles and Aves.
• Fossils are useful in search of oil and minerals.
Note: Paleontology is the study of fossils and the person who is involved in the study of fossils is a paleontologist.
Let us start from 100 million years ago
Some invertebrates on the sea bed died, and they were buried in the sand. More sand accumulated and sandstone formed under pressure.
Millions of years later, dinosaurs living in the area died, and their bodies, too, were buried in mud. This mud was also compressed into rock, above the rock containing the earlier invertebrate fossils.
Again, millions of years later, the bodies of horse-like creatures dying in the area were fossilized in rocks above these earlier rocks.
Much later erosion by sun, water flow, wears away some of the rock and exposes the horse like fossils. As we dig deeper, we will find older and older fossils.
1.0Living Fossils
The animals which underwent little change during long geological periods.
2.0Determination of the Age of Fossil
There are two ways of determining age of fossils.
Relative Method
If we dig into the earth and start finding fossils it is reasonable to suppose that the fossils we find closer to the surface are more recent than the fossils we find in deeper layers.
Using Radioactive Elements
Rocks contain some radioactive elements that decay and convert into their more stable form. This decay takes place at a constant rate for each radioactive element. e.g., Carbon dating.
Carbon dating: Carbon dioxide of air contains a small proportion of radioactive carbon (C14). CO2 is used during photosynthesis and there is an equal proportion of C14 among carbon atoms of all organisms. The radioactivity of C14 is lost at a precise rate. Half-life of C14 is about 5730 years.
If a fossil shows radioactivity if it is one fourth of that found in the living organisms, then the organisms died about 11,460 years ago (Two half-lives).