What Are Fossils?
Fossils are the remains, molds, traces or impressions of prehistoric animals and plants found in the ground. The word comes from the Latin fodere, meaning “to dig”, and originally meant any old, curious object dug out of the earth. But since middle of 16th Century, fossils have been regarded specifically as hard objects showing evidence of earlier forms of life, often many millions of years old.
There are many kinds of fossils. Some are footprints which strange prehistoric animals left behind in mud long since turned to rock. Others are stones which were once soft substances, but still preserve the outlines of extinct plants, or of seashells, or of the bodies of animals. Sometimes even the bones of creatures have survived.
The study of fossils, which is called paleontology, has enabled scientists to fill many vital gaps in the history of the world and its inhabitants. For example, fossils have shown that rocks in great mountain ranges like the Alps or the Rocky Mountains were once below the surface of the sea. They have indicated that the United States and Europe were once covered by tropical forests.
Also, they provide evidence of the common ancestry of animals which today differ widely in appearance. The subject can be a rewarding hobby for amateurs. Many important contributions to the world’s great collections have been made by people who looked for fossils in their spare time or even came across fossils by sheer accident.
The study of fossils across geological time, how they were formed, and the evolutionary relationships between taxa (phylogeny) are some of the most important functions of the science of paleontology. Such a preserved specimen is called a “fossil” if it is older than some minimum age, most often the arbitrary date of 10,000 years.
Hence, fossils range in age from the youngest at the start of the Holocene Epoch to the oldest, chemical fossils from the Archaean Eon, up to 3.48 billion years old, or even older, 4.1 billion years old, according to a 2015 study. The observation that certain fossils were associated with certain rock strata led early geologists to recognize a geological timescale in the 19th century. The development of radiometric dating techniques in the early 20th century allowed geologists to determine the numerical or “absolute” age of the various strata and thereby the included fossils.
Like extant organisms, fossils vary in size from microscopic, even single bacterial cells one micrometer in diameter, to gigantic, such as dinosaurs and trees many meters long and weighing many tons. A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates, or the chitinous or calcareous exoskeletons of invertebrates.
Fossils may also consist of the marks left behind by the organism while it was alive, such as animal tracks or feces (coprolites). These types of fossil are called trace fossils (or ichnofossils), as opposed to body fossils. Finally, past life leaves some markers that cannot be seen but can be detected in the form of biochemical signals; these are known as chemofossils or biosignatures.