I.

Introduction

Paleontology, study of prehistoric animal and plant life through the analysis of fossil remains. The study of these remains enables scientists to trace the evolutionary history of extinct as well as living organisms (see Evolution). Paleontologists also play a major role in unraveling the mysteries of Earth’s rock strata (layers). Using detailed information on how fossils are distributed in these layers of rock, paleontologists help prepare accurate geologic maps, which are essential in the search for oil, water, and minerals. See Dating Methods.

Most people did not understand the true nature of fossils until the beginning of the 19th century, when the basic principles of modern geology were established. Since about 1500, scholars had engaged in a bitter controversy over the origin of fossils. One group held the modern view that fossils are the remains of prehistoric plants and animals. This group was opposed by another, which declared that fossils were either freaks of nature or creations of the devil. During the 18th century, many people believed that all fossils were relics of the great flood recorded in the Bible.

II.

Fossils and Stratigraphy

Paleontologists gain most of their information by studying deposits of sedimentary rocks that formed in strata over millions of years. Most fossils are found in sedimentary rock. Paleontologists use fossils and other qualities of the rock to compare strata around the world. By comparing, they can determine whether strata developed during the same time or in the same type of environment. This helps them assemble a general picture of how Earth evolved. The study and comparison of different strata is called stratigraphy.

Fossils provide most of the data on which strata are compared. Some fossils, called index fossils, are especially useful because they have a broad geographic range but a narrow temporal one—that is, they represent a species that was widespread but existed for a brief period of time. The best index fossils tend to be marine creatures. These animals evolved rapidly and spread over large areas of the world. Paleontologists divide the last 542 million years of Earth’s history into eras, periods, and epochs. The part of Earth’s history before about 542 million years ago is called Precambrian time, which began with Earth’s birth, about 4.5 billion years ago.

The earliest evidence of life consists of microscopic fossils of bacteria that lived as early as 3.8 billion years ago. Most Precambrian fossils are very tiny. Most species of larger animals that lived in later Precambrian time had soft bodies, without shells or other hard body parts that would create lasting fossils. The first abundant fossils of larger animals date from about 600 million years ago.

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.

IV.

The Mesozoic Era

The Mesozoic Era is often called the Age of Reptiles, because the reptile class was dominant on land throughout the entire age. The Mesozoic Era lasted about 186 million years, and includes the Triassic, Jurassic, and Cretaceous periods. Index fossils from this era include a group of extinct cephalopods called ammonites, and extinct forms of sea urchins.

A.

Triassic Period

The most notable of the Mesozoic reptiles, the dinosaur, first evolved in the Triassic Period (251 million to 200 million years ago). The Triassic dinosaurs were not as large as their descendants in later Mesozoic times. They were comparatively slender animals that ran on their hind feet, balancing their bodies with long tails, and seldom exceeded 4.5 m (15 ft) in length. Other reptiles of the Triassic Period included such aquatic creatures as the ichthyosaurs, and a group of flying reptiles, the pterosaurs.

The first mammals also appeared during this period. The fossil remains of these animals are fragmentary, but the animals were apparently small in size and reptilian in appearance. In the sea, Teleostei, the first ancestors of the modern bony fishes, made their appearance. The plant life of the Triassic seas included a large variety of marine algae. On land, the dominant vegetation included various evergreens, such as ginkgos, conifers, and palms. Small scouring rushes and ferns still existed, but the larger members of these groups had become extinct.

B.

Jurassic Period

During the Jurassic Period (200 million to 145 million years ago), dinosaurs continued to evolve in a wide range of size and diversity. Types included heavy four-footed sauropods, such as Apatosaurus (formerly Brontosaurus); two-footed carnivorous dinosaurs, such as Allosaurus; two-footed vegetarian dinosaurs, such as Camptosaurus; and four-footed armored dinosaurs, such as Stegosaurus. Winged reptiles included the pterodactyl, which, during this period, ranged in size from extremely small species to those with wingspreads of 1.2 m (4 ft). Marine reptiles included plesiosaurs, a group that had broad, flat bodies, with long necks and large flippers for swimming; Ichthyosauria, which resembled dolphins; and primitive crocodiles.

The mammals of the Jurassic Period were smaller than small modern dogs. Many insects of the modern orders, including moths, flies, beetles, grasshoppers, and termites, appeared during the Jurassic Period. Shellfish included lobsters, shrimp, and ammonites, as well as the extinct group of belemnites, which resembled squid and had cigar-shaped internal shells. Plant life of the Jurassic Period was dominated by the cycads, which resembled thick-stemmed palms. Fossils of most species of Jurassic plants are widely distributed in temperate zones and polar regions, indicating that the climate was uniformly mild.

C.

Cretaceous Period

The reptiles were still the dominant form of animal life in the Cretaceous Period (145 million to 65 million years ago). Many of the main groups of dinosaurs found in the Jurassic also lived during this period, and new types, notably the horned dinosaurs, also appeared. By the end of the Cretaceous, about 65 million years ago, all of these creatures had become extinct. The largest of the pterodactyls lived during this period. Pterodactyl fossils discovered in Texas have wingspreads of up to 10.5 m (35 ft). Other reptiles of the period include the first snakes and early lizards. Several types of Cretaceous birds have been discovered, including Hesperornis, a diving bird about 1.8 m (about 6 ft) in length, which had only vestigial wings and was unable to fly. Mammals of the period included the first marsupials, which strongly resembled the modern opossum, and the first placental mammals, which belonged to the group of insectivores. The first crabs developed during this period, and several modern varieties of fish also evolved.

The most important evolutionary advance in the plant kingdom during the Cretaceous Period was the development of deciduous plants, the earliest fossils of which appear in early Cretaceous rock formations (see Angiosperm). Fig, magnolia, sassafras, and poplar were among the first to evolve. By the end of the period, many of the modern varieties of trees and shrubs had made their appearance. They represented more than 90 percent of the known plants of the period. Mid-Cretaceous fossils include remains of beech, holly, laurel, maple, oak, plane tree, and walnut.

V.

The Cenozoic Era

The Cenozoic Era (65 million years ago to the present time) is divided into the Paleogene Period (65 million to 23 million years ago) and the Neogene Period (23 million years ago to the present). However, because scientists have so much more information about this era, they tend to focus on the epochs that make up each period. During the first part of the Cenozoic Era, an abrupt transition from the Age of Reptiles to the Age of Mammals occurred, when the large dinosaurs and other reptiles that had dominated the life of the Mesozoic Era disappeared.

Index fossils of the Cenozoic tend to be microscopic, such as the tiny shells of foraminifera. They are commonly used, along with varieties of pollen fossils, to date the different rock strata of the Cenozoic Era.

A.

Paleocene Epoch

The Paleocene Epoch (65 million to 56 million years ago) marks the beginning of the Cenozoic Era. A number of groups of Paleocene mammals are known. All of them appear to have developed in northern Asia and to have migrated to other parts of the world. These primitive mammals had many features in common. They were small, with no species exceeding the size of a small modern bear. They were four-footed, with five toes on each foot, and they walked on the soles of their feet. Almost all of them had slim heads with narrow muzzles and small brain cavities. The predominant mammals of the period were members of three groups that are now extinct. They were the creodonts, meat-eating mammals that were not ancestral to modern carnivores; the amblypods, which were small, heavy-bodied animals; and the condylarths, which were light-bodied herbivorous animals with small brains. The Paleocene groups that have survived are the marsupials, the insectivores, the primates, and the rodents.

B.

Eocene Epoch

During the Eocene Epoch (56 million to 34 million years ago), a number of direct evolutionary ancestors of modern animals appeared. Among these animals—all of which were small in stature—were the horse, rhinoceros, camel, rodent, and monkey. The creodonts and amblypods continued to develop during the epoch, but the condylarths became extinct before it ended. The first aquatic mammals, ancestors of modern whales, also appeared in Eocene times, as did such modern birds as eagles, pelicans, quail, and vultures. Changes in vegetation during the Eocene Epoch were limited chiefly to the migration of types of plants in response to climate changes.

C.

Oligocene Epoch

During the Oligocene Epoch (34 million to 23 million years ago), most of the archaic mammals from earlier epochs of the Cenozoic Era disappeared. In their place appeared representatives of a large number of modern mammalian groups. The creodonts became extinct, and the first true carnivores, resembling dogs and cats, evolved. The first anthropoid apes also lived during this time, but they became extinct in North America by the end of the epoch. Two groups of animals that are now extinct flourished during the Oligocene Epoch: the titanotheres, which are related to the rhinoceros and the horse; and the oreodonts, which were small grazing animals.

D.

Miocene Epoch

The development of mammals during the Miocene Epoch (23 million to 5.3 million years ago) was influenced by an important evolutionary development in the plant kingdom: the spread of grasses. These plants, which were ideally suited for forage, encouraged the growth and development of grazing animals such as horses, camels, and rhinoceroses, which were abundant during the epoch. During the Miocene Epoch, the mastodon evolved, and in Europe and Asia a gorilla-like ape, Dryopithecus, was common. Various types of carnivores, including cats and wolflike dogs, ranged over many parts of the world.

E.

Pliocene and Pleistocene Epochs

The paleontology of the Pliocene Epoch (5.3 million to 1.8 million years ago) does not differ much from that of the Miocene, although the period is regarded by many zoologists as the climax of the Age of Mammals. The Pleistocene Epoch (1.8 million to 11,500 years ago) in both Europe and North America was marked by an abundance of large mammals, most of which were basically modern in type. Among them were buffalo, elephants, mammoths, and mastodons. Mammoths and mastodons became extinct before the end of the epoch. In Europe, antelope, lions, and hippopotamuses also appeared. Carnivores included badgers, foxes, lynx, otters, pumas, and skunks, as well as now-extinct species such as the giant saber-toothed tiger. In North America, the first bears made their appearance as migrants from Asia. The armadillo and ground sloth migrated from South America to North America, and the musk-ox ranged southward from the Arctic regions. Modern human beings also emerged during this epoch.

See also Geology and individual articles on the eras, periods, and epochs mentioned.