600 Million Years of Life on Earth

Time Periods of Earth’s History

Cambrian – Opening period of the Paleozoic Era. Much of North America was covered by shallow seas. Rocks of this period lack evidence of land plants, but many animal types are present. Trilobites are very common.

Ordovician – Animal groups that first appeared in the Ordovician include the corals, and very primitive armored fish that are related to lampreys. Common fossils include trilobites, brachiopods (shellfish that look like clams, although they are not closely related to them), and crinoids (sea lilies – despite the name, these are animals).

Silurian – Coral reefs are frequently found in rocks of this period. Many kinds of eurypterids (giant scorpion-like creatures that lived in near-shore areas of shallow seas) are known, especially from North America. Many giant cephalopods (relatives of the modern squids and octopus) have been found. These cephalopods were active, meat-eating animals like their modern relatives.

Devonian – This is often called the Age of Fishes. Many Devonian fish types that would appear strange to modern eyes have long been extinct. However, very modern-appearing sharks were common along with lungfish and other air-breathing fish. The Devonian saw the rise of the first forests and the appearance of numerous land animals. At the very end of the Devonian, the first land living four-footed animals appeared. These animals were amphibians, but only very distantly related to the amphibians living today (frogs and salamanders).

Mississippian – Trilobites, which were common in earlier periods, declined in abundance and importance. Crinoids (sea lilies), on the other hand, became very widespread and common. Their fragmented remains make up whole rock formations from this period. On land, numerous types of amphibians appeared and the coal deposits were formed in North America.

Pennsylvanian – This is called the Coal Age. Peat deposits accumulated in lowland swamps composed of horsetails, giant club mosses and ferns. Over time, the peat was compacted into coal. Numerous fossil amphibians, some of them quite large, lived in the humid lowlands. Insects reached their largest size and the first reptiles appeared. The earliest reptiles were small, roughly lizard-sized creatures.

Permian – Closing period of the Paleozoic Era. There was extensive glaciation across India, South Africa, South America, Australia and Antarctica. Amphibians were still common, but reptiles began to diversify and some became large, weighing hundreds of pounds. There were many primitive “mammal-like” reptiles, although at this time most of them looked much more like reptiles than like mammals. At the end of the Permian, there was a “great dying” or mass extinction. Many groups of animals disappeared completely.

Triassic – First period of the Mesozoic Era. Dinosaurs originated, along with crocodiles and pterosaurs (flying reptiles). Large meat-eating reptiles’ plesiosaurs and ichthyosaurs, lived in the sea. Toward the close of the Triassic, small, primitive mammals evolved from mammal-like reptiles. As plant life diversified the Paleozoic tree ferns gave way to cycads, gingkoes, and primitive conifers. The earliest known frog is from the Triassic.

Jurassic – Dinosaurs diversified and some became huge. Brontosaurus and Stegosaurus belong to this period. Mammals were more common than in the Triassic, but still were small. Birds appear in the Jurassic complete with feathers and teeth. The birds probably descended from small dinosaurs. Along with marine reptiles, relative of the modern squids, called ammonites and belemnites, became common. The first oysters are found in the Jurassic.

Cretaceous – The last period of the Mesozoic Era and the dawn of modern times. Flowering plants became abundant and widespread. Cretaceous plants looked much like the plants of today, and many indeed were closely related to living plants. Large reptiles, including dinosaurs, were common. Tyrannosaurus and Triceratops are some of the best known Cretaceous dinosaurs. The largest flying animals in the Earth’s history soared through Cretaceous skies – pterosaurs with a wingspan of 40 feet or more. Primitive representatives of some of the present-day mammal groups appeared, but were still fairly small. At the close of the Cretaceous, belemnites and ammonites disappeared from the seas, and dinosaurs from the land.

Tertiary – Beginning of the Cenozoic Era. Mammals became very diverse, and many were large. Most of the large Tertiary mammals are extinct. However, most of our living animals, as well as plants, are very similar to those of the Tertiary. Grasses appeared during the Tertiary.

Quaternary – This included the time known as the Ice Age, when vast glaciations occurred particularly in the northern hemisphere. Man first appears in the fossil record near the Tertiary-Quaternary boundary.