III.

The Paleozoic Era

The Paleozoic Era lasted about 290 million years. It includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. Index fossils of the first half of the Paleozoic Era are those of invertebrates, such as trilobites, graptolites, and crinoids. Remains of plants and such vertebrates as fish and reptiles make up the index fossils of the second half of this era.

A.

Cambrian Period

At the beginning of the Cambrian Period (542 million to 488 million years ago) animal life was entirely confined to the seas. By the end of the period, all the modern phyla of the animal kingdom existed. The characteristic animals of the Cambrian Period were the trilobites, a primitive form of arthropod, which reached their fullest development in this period and became extinct by the end of the Paleozoic Era. The earliest snails appeared in this period, as did the cephalopod mollusks. Other groups represented in the Cambrian Period were brachiopods, bryozoans, and foraminifers (see Foraminifera). Plants of the Cambrian Period consisted of algae in the oceans.

B.

Ordovician Period

The most characteristic animals of the Ordovician Period (488 million to 444 million years ago) were the graptolites, which were small, colonial hemichordates (animals possessing an anatomical structure suggestive of a portion of a spinal cord). Primitive fish and the earliest corals emerged during the Ordovician Period. The largest animal of this period was a cephalopod mollusk that had a shell about 3 m (about 10 ft) in length. The first primitive land plants appeared by the end of the period.

C.

Silurian Period

The most important evolutionary development of the Silurian Period (444 million to 416 million years ago) was the appearance of the first air-breathing animals, including scorpions, spiders, and insects. Fossils of these creatures have been found in Scandinavia and Great Britain. The first fossil records of vascular plants—that is, land plants with tissue that carries food—appeared in the Silurian Period. They were simple plants that had not developed separate stems and leaves.

D.

Devonian Period

The dominant forms of animal life in the Devonian Period (416 million to 359 million years ago) were fish of various types, including sharks, lungfish, armored fish, and primitive forms of ganoid (hard-scaled) fish that were probably the evolutionary ancestors of amphibians. Fossil remains found in Pennsylvania and Greenland indicate that early forms of amphibia may already have existed during the Devonian Period. Early animal forms included corals, starfish, sponges, and trilobites.

In 2006 paleontologists reported the discovery of an intermediate link between fish and land animals. Several well-preserved fossil specimens from Devonian rock on Ellesmere Island in the Canadian territory of Nunavut were clearly derived from a group that included ancient lungfish. Scientists said the animals ranged from 1.2 m (4 ft) to 3 m (9 ft) in length. The scientists named the newly discovered species Tiktaalik roseae. (Tiktaalik is an Inuit word for large freshwater fish.) The fossils showed anatomical features characteristic of land animals, such as wrist and elbow bones and parts of a primitive hand embedded in the pectoral fins. This transitional creature, found in rock dating from 375 million years ago, had a flat skull with eye sockets on top resembling the skull of a crocodile, a neck, ribs, and other features similar to four-limbed animals known as tetrapods. Tiktaalik roseae preceded amphibians, reptiles and dinosaurs, and mammals. Scientists compared the significance of its discovery to that of Archaeopteryx, the feathered dinosaur that was transitional between reptiles and birds.

The Devonian is the first period from which any considerable number of fossilized plants have been preserved. During this period, the first woody plants developed, and by the end of the period, land-growing forms included seed ferns, ferns, scouring rushes, and scale trees, the modern relative of club moss. Although the present-day equivalents of these groups are mostly small plants, they developed into treelike forms in the Devonian Period. Fossil evidence indicates that forests existed in Devonian times, and petrified stumps of some of the larger plants from the period measure about 60 cm (about 24 in) in diameter.

E.

Carboniferous Period

The Carboniferous Period lasted from 339 million to 299 million years ago. During the first part of this period, sometimes called the Mississippian Period (359 million to 318 million years ago), the seas contained a variety of echinoderms and foraminifers, as well as most forms of animal life that appeared in the Devonian. A group of sharks, the Cestraciontes—or shell-crushers—were dominant among the larger marine animals. The predominant groups of land animals were primitive, lizardlike amphibians that developed from relatives of lungfish. The various forms of land plants became diversified and grew larger, particularly those that grew in low-lying swampy areas.

The second part of the Carboniferous, sometimes called the Pennsylvanian Period (318 million to 299 million years ago), saw the evolution of the reptiles, a group that developed from the amphibians and lived entirely on land. Other land animals included spiders, snails, scorpions, more than 800 species of cockroaches, and the largest insect ever evolved, a species resembling the dragonfly, with a wingspread of about 74 cm (about 29 in). The largest plants were the scale trees, which had tapered trunks that measured as much as 1.8 m (6 ft) in diameter at the base and 30 m (100 ft) in height. Primitive gymnosperms known as cordaites, which had pithy stems surrounded by a woody shell, were more slender but even taller. The first true conifers, forms of advanced gymnosperms, also developed during the Pennsylvanian Period.

F.

Permian Period

Major events of the Permian Period (299 million to 251 million years ago) included the disappearance of many forms of marine animals and the rapid spread and evolution of the reptiles on land. Permian reptiles were dominated by so-called mammal-like reptiles or synapsids. One group of synapsids led to the ancestors of mammals. Most vegetation of the Permian Period was composed of ferns and conifers. The largest mass extinction to affect life on Earth came at the end of the Permian.