III.

Periods of the Phanerozoic Eon

The Phanerozoic Eon is the most recent eon of the earth and is divided into the Paleozoic Era (570 million to 240 million years before present), the Mesozoic Era (240 million to 65 million years before present), and the Cenozoic Era (65 million years before present to the present).

The periods of the Paleozoic Era are the Cambrian Period (570 million to 500 million years before present), the Ordovician Period (500 million to 435 million years before present), the Silurian Period (435 million to 410 million years before present), the Devonian Period (410 million to 360 million years before present), the Carboniferous Period (360 million to 290 million years before present), and the Permian Period (290 million to 240 million years before present). The rocks of the Paleozoic Era contain abundant and diverse fossils, so each period is marked by both geologic and biological events.

The rocks of the Cambrian Period contain many fossils of shelled animals such as trilobites, gastropods, and brachiopods that are not present in earlier rocks. The Ordovician Period is characterized by an abundance of extinct floating marine organisms called graptolites. One of the greatest mass extinctions of the Phanerozoic Eon occurred at the end of the Ordovician Period.

Rocks from the Silurian Period reveal the first evidence of plants and insects on land and the first fossils of fishes with jaws. In the Devonian Period, the first animals with backbones appeared on land. The Devonian was the first period to produce the substantial organic deposits that are used today as energy sources (see Fossil Fuel).

The rocks of the Carboniferous Period contain about one-half of the world’s coal supplies, created by the remains of the vast population of animals and plants of that period. Besides the abundance of terrestrial vegetation, the first winged insects appeared during the Carboniferous.

During the Permian Period all the continents on the earth came together to form one landmass, called Pangaea. The shallow inland seas of the Permian created an environment in which invertebrate marine life flourished. At the end of the period, one of the greatest extinctions in the earth’s history occurred, wiping out most species on the planet.

The Mesozoic Era is composed of the Triassic Period (240 million to 205 million years before present), the Jurassic Period (205 million to 138 million years before present), and the Cretaceous Period (138 million to 65 million years before present). During the Triassic Period, the supercontinent of Pangea began to break apart. Dinosaurs first appeared during the Triassic, as did the earliest mammals.

The continents continued to break apart during the Jurassic Period. Reptiles, including the dinosaurs, flourished, taking over ecological niches on the land, in the sea, and in the air, while mammals remained small and rodentlike. The continents continued to drift toward their present locations during the Cretaceous Period. Another mass extinction, which killed off the large reptiles such as the dinosaurs, occurred near the end of the Cretaceous.

The Cenozoic Era is divided into the Tertiary Period (65 million to 1.6 million years before present) and the Quaternary Period (1.6 million years before present to the present). During the Tertiary Period the continents assumed their current positions. Mammals became the dominant life forms on the planet during this period, and the direct ancestors of humans appeared at the end of the Tertiary. The most recent ice age occurred during the Quaternary Period. The first humans appeared during the Quaternary. The changing climate and melting of the glaciers, possibly combined with hunting by humans, drove many of the large mammals of the early Quaternary to extinction, making way for the animal life on the earth today.