Paleontology

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.

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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.

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