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Extinction of Species

As long as members of a species survive and reproduce themselves, they perpetuate themselves. However, if all the members of a species die, then the species becomes extinct. Extinction is local if a species disappears from a part of its range, but still exists elsewhere. A global extinction is the total disappearance of all the members of a species so that none are left alive on the earth.

Paleontologists have discovered millions of species of plants and animals that experienced extinction in the approximately 550–600 million years that life has existed on earth. The fossil remains of living creatures show clearly that life was teeming on the earth during the Cambrian Era. Many scientists believe that life probably emerged in the chapter of the biography of the earth called the Precambrian Era. However, these were probably soft bodied-fauna and flora. In the absence of shells, skeletons, or hard body parts, they have been lost to the fossil record. What is certain is that in the Cambrian Era, life forms exploded in number. New species seemed to have developed very rapidly.

Despite the presence of life and of numerous species, it is now known from the fossil record that massive deaths of whole species also occurred. Paleontologist, biologists, and other life scientists have estimated that extinction is a fact of biological life. It is estimated that at least 99.7 percent of all the species that have ever lived on earth are now extinct. The law of life is extinction.

Mass Extinctions

There have been an estimated five mass extinctions in the history of life on earth. Fossil evidence strongly suggests that massive dying of species occurred in the Ordovician, Devonian, and Permian geological eras. The most obvious example is the disappearance of the dinosaurs. These mass extinctions have been uncovered by paleontologists as they have examined the fossil record. The pattern has been the development of a few species, then an explosion in the numbers of new species, followed by a period of little change, followed by a deep dip in the number of species. The extinction-causing event is followed by a new period of at first slow development and then an explosion in numerous new species and then another massive loss.

Mass extinctions have been best seen in the fossil record of marine animals. The sedimentary record is clearer because better fossil specimens have been deposited in marine sediment than in other kinds of sediment, and show that the Paleozoic Era of the Cambrian Period was a time of rapid expansion, which was followed by massive extinctions in the Ordovician Era. Approximately 50 percent of the animal families disappeared. This massive die-off included many trilobites.

Species continued to diversify between 500 and 350 million years ago when the Ordovician extinction was followed by a Devonian extinction. At that time, about 30 percent of the species disappeared in animal families. Again many trilobites disappeared, along with many agnathan and placoderm species of fish.

In the Permian Era, about half of all animal families vanished. These included many of the new species that had not been found in the fossil record prior to the Devonian extinction. Swept away were 95 percent of marine species, great numbers of trees, amphibians, most bryozoans and brachiopods and all trilobites. The Triassic extinction occurred about 180 million years ago. It destroyed many reptiles, animal families and many marine mollusks. It is estimated that 35 percent of the animal families disappeared.

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