Extinction

All life on earth, from single-celled microbes and simple fungus to dinosaurs and mammals, is compelled to adapt to changes in surroundings. If any species cannot adapt, it will simply die out, becoming extinct, in other words, totally eliminated. Those changes that may lead to extinction can be gradual background extinctions or relatively sudden mass extinctions.

Background extinctions

Gradual changes may lead to the extinction of an entire species. Extinctions caused by gradual changes are called background extinctions. These extinctions are caused by small changes in the environments: climate or habitat, depleted resources, inter- or intra-species competition; changes that require life forms to adapt and be flexible or become extinct. There is always a normal background rate of extinction, punctuated by mass extinctions. The fossil record provides strong evidence that of all the species which have ever existed, 99.9 % are now extinct.

Extinctions have been a normal part of the life cycle and are occurring throughout time. The normal background rate of extinctions is about two to five families of marine invertebrates and vertebrates per million years.

Background extinctions occurred throughout the Mesozoic Era. Most dinosaur species of the Mesazoic era probably perished because they were unable to adapt to gradual changes in the environment. Some examples of dinosaurs that probably perished in these gradual extinctions of the Mesozoic era are Allosaurus, Cetiosaurus, Deinocheirus, Dilophosaurus, Edmontosaurus, Iguanodon, Maiasaura, Ouranosaurus, Protoceratops, and Styracosaurus. A theoretical example is the change in available flora in the diet. The angiosperms (flowering plants) made their appearance during the Cretaceous period. As they became more numerous they began to displace many other plants, including conifers, until flowering plants dominated the landscape. The diet of some herbivorous dinosaurs, like Edmontosaurus, consisted exclusively of conifers. Edmontosaurus did not adapt to the change in flora and were probably unable to find enough conifers to live on. Eventually their birth rate was less than their death rate until they became extinct. The Edmontosaurus died out, unable to find enough conifers to sustain themselves.

Mass extinction

Punctuating the process of background rate extinctions are occasional mass extinctions that result in the total elimination of a large number of taxa (groups of life forms collectively categorized taxonomically as kingdom, phylum, class, order, family, genus, or species). Mass extinctions are, relatively speaking, sudden. They result in the global decrease in the number and diversity of life forms. These extinctions have taken place from time to time throughout the existence of life on Earth. Mass extinctions are defined by the four criteria:

• 1. Extinctions occur all over the world.
• 2. A large number of species are eliminated.
• 3. Many types of species are eliminated.
• 4. The extinctions are clustered in a short amount of geological time (relatively speaking, a few million years is a short time in terms of geological time).

Major mass extinction events

The Precambrian and Vendian Mass Extinctions

• Precambrian period (4.6 billion to 523 million years ago)
• Vendian period (523-543 million years ago)
• Both Precambrian and Vendian periods were host to at least one mass extinction each

Extinctions are proposed to have affected the earliest organisms on Earth. Approximately 650 million years ago, seventy percent of the dominant Precambrian flora and fauna perished in the first great extinction.

The Ordovician-Silurian Extinction (439 Ma) in which 85% of species became extinct.

• Ordovician period (510- 438 million years ago)
• Ordovician extinction (440-450 million years ago)

This extinction, in which an estimated 85% of all species became extinct, is the second only to the Precambrian extinction in devastation. It resulted in the elimination of one third of all brachiopod and bryozoan families, many groups of conodonts, trilobites, and graptolites and ended a large part of the reef-building fauna. In total, more than one hundred families of marine invertebrates ceased to exist in this extinction.

The Devonian Extinction (345 Ma) in which 85% of species became extinct.

The Permian Extinction (about 250 Ma) at the end of the Permian Period and the beginning of the Triassic in which most notably, trilobites became extinct (an estimated 15,000 species). More than ninety percent of all species became extinct including about fifty percent of all animal families.

The Triassic Extinction (213 Ma) in which 76% of species became extinct.

The Cretaceous Extinction (65 Ma) in which 20% of the families of plants and animals on land (50% in the sea) and 85% of all species became extinct. All dinosaurs became extinct.