Table of Contents
In this process of formation of fossils post we have briefly explained about process of formation of fossils, determination of age of fossils, geological time scale.
Process of Formation of Fossils
During the formation of sedimentary rocks, the dead animals of the sea or large lakes and the land carried to the sea or large lakes by river, sink down and get buried in the rocks. Supply of oxygen is limited in this condition and prevents the decay or reduces the decay rate to minimal. As a result, animal remains preserved in the rock and have formed the fossil.
The hard remains of dead animals got preserved layer by layer in the sedimentary rocks. The fossils present in deeper layer are earlier and upper layer had more recent fossils. There are seven different kinds of fossils. These fossils are distributed in amber, asphalt, ice, volcanic ash, peat bogs, storm dust and sand dunes.
Process of Formation of Fossils; Different kinds of fossils, their formation and examples
Determination of Age of Fossils
1. Relative Dating Methods
In early days, mechanism of absolute dating was not present and as a result relative dating technique was used to determine the age of rock and fossils. In this technique, the position and erosin rate of rock in particular environment. Older rocks are situated in deeper had ancient fossils and superficial rocks had fossils of recent fossils.
2. Absolute Dating Methods
These methods are using spontaneous decay of unstable radioactive nuclei into stable radioactive nuclei at a constant and known rate. Absolute dating technique uses radioactive nuclei in three different techniques:
Uranium-Lead Technique: This technique was introduced by Baltwood in 1907. A rock contains uranium (U238) in the form of mineral zircon. Uranium decay spontaneously to Lead as per the given scheme (Scheme 6.1). It has a half-life of 4.5 billion years which means it will take 4,500,000,000 years to decay 50% uranium. So determination of content of uranium and lead in a rock or fossil can be used to determine the age of rock or fossil.
Carbon (C14) Dating Technique: this method was introduced by W.F.Libby in 1950. Radioactive carbon (C14) in found naturally in rock. C14 has a half-life of 5600 years and C14 decay gives nitrogen (N14). Carbon dating technique can be used to measure fossil age up to 25,000 years old.
Potassium-Argon Technique: Radioactive Potassium (K40) is usually found in the rock of all kinds. Its life is 1.3×109 years and disintegrate to form Argon.
Geological Time Scale
Use of the radioactive dating has allowed the determination of fossils found in different sedimentary rock samples. It has allowed calculating the presence of different organisms preserved in the rock samples in the form of fossils. In addition, it helped scientist to predict that earth is almost 4-5 billion year old and life appeared on earth almost 4000 million year ago. Since then the earth’s history has been divided into 5 different time frames known as eras. Few of these eras are divided into the periods and which in turn split up into the epochs. The different Eras are as follows:
Process of Formation of Fossils: Geological Time Scale
Archeozoic (4600-3500 million years): it is the first era and begins with the formation of earth and presence of solar system. There is no fossil form available from this era.
Proterozoic (3000-1000 million years): it is the second era and begins with the origin of prokaryotes, primitive metazoans and eukaryotes. Reports are available about scanty fossils in this era.
Palaeozoic (570-280 million year): it is the third era and known as era of ancient life. It saw the appearance of invertebrate, fishes, amphibian etc. Reports of spore bearing plants, tree ferns and origin of conifers is available. Initial Reports are available about scanty fossils in this era.
Mesozoic (225-135 million years): The appearance of tooth birds, therian mammals, reptiles and dominance of dinosaurs. In addition, placental mammals are also found. Reports of cycads and flowering plants is available.
Cenozoic (Modern era): This is the modern era and it witnessed the dominance of present age man, modern mammals, birds, fishes and insects.