Life of the Past

The Mesozoic Era

The Mesozoic (Middle Life) Era covers a period of about 165 million years, during which reptiles so overshadowed all other animals that it is often called the “Age of Reptiles.” Great changes took place in some invertebrates too. New forms replaced those which had become extinct at the end of the Paleozoic. Ammonites developed rapidly until countless numbers lived in the seas. Birds, mammals, flowering plants and many modern insects appeared for the first time. Elm, oak, maple and other modern broad- leaved trees became common. The development and spread of some flowering plants depended on the parallel development of insects which pollinated the flowers.

Other important geographic changes were also taking place. New patterns of lands and seas formed. New mountain ranges slowly emerged. As the result of several related geologic processes, great mineral deposits were formed. Sixty million years later, we still depend on many of these deposits for our metals and our fuels.

THE TRIASSIC PERIOD

The Triassic period (230 to 180 million years ago) was named from a threefold division of its rocks. In many places Triassic rocks resemble those of the Permian Period—thick sequences of red shales and sandstones, deposited in temporary lakes, deserts and basins. Volcanic activity was considerable, as in eastern North America from Virginia to Connecticut.

Against this background the reptiles developed and established their mastery. Their advanced body structure and shell-protected eggs enabled them to survive changing and often adverse climates, and to colonize new areas which were forbidden to the water-tied amphibia. The first dinosaurs appeared; their footprints are abundant in some rocks, as in the Connecticut Valley.

Nor was the dominance of reptiles confined to land, for in open oceans dolphin-like ichthyosaurs swept through the water. Later 15- to 20-foot long plesiosaurs paddled their way through Triassic seas.

New types of sponges and protozoans developed. The modern hexacorals appeared, and new groups of brachiopods replaced their Permian forebears. Gastropods and pelecypods increased in numbers. Ammonites flourished and underwent considerable change. Lobster-like arthropods and modern echinoids and crinoids first appeared in the Triassic.

Cycads and primitive conifers flourished on upland areas. The Petrified Forest of Arizona contains fossils of these trees. Ferns and scouring rushes prospered in
lower, moist areas.

THE JURASSIC PERIOD

Named for the Jura Mountains, this period began about 180 million years ago and lasted about 45 million years. Of all its abundant and exotic life, none was more typical than the dinosaurs of which there were three main Jurassic groups: first, the sauropods, long-necked, long-tailed, four-legged monsters, which included the largest land animal (Diplodocus, 87 ft. long); second, stegosaurs, armored reptiles that weighed up to 10 tons (with only a 3-oz. brain); third, the carnivorous theropods which walked on their hind legs, including Allosaurus, a savage, 35-ft. creature. Others were more slender, and some were only 3 feet long. A few early, duck-billed herbivorous dinosaurs lumbered across the swampy lowlands.

Flying reptiles gliding through the air included sparrow- sized species and others up to 4 feet long with slender club-like tails. Ichthyosaurs and plesiosaurs were the carnivorous masters of the oceans. Hosts of ammonites (some up to 6 feet in diameter) thronged the shallow seas, together with gastropods, pelecypods, squids (belemnites), echinoids, crinoids and foraminifera.

The Jurassic also saw the development of two groups that were later to establish their dominance. The oldest mammals are known from fossil fragments of rat-sized jaws and teeth from western United States and Europe. The Solenhofen Jurassic limestone of Bavaria contains remains of Archaeopteryx, the oldest known bird.

Jurassic plants included the now extinct cycadeoids with short, thick trunks. These were crowned with frond- like leaves and ornate reproductive structures which closely resembled modern flowers. Cycads, conifers, ferns and ginkgos were common. Ginkgos were widespread all through the Mesozoic but later became almost extinct. Today only a single species survives but this is widely planted. Over a thousand species of insects are known, including many modern forms.

CRETACEOUS PERIOD

The Cretaceous Period (named from chalk, its most characteristic deposit) began about 135 million years ago and lasted some 70 million years. It was one of the most important of all geologic periods, marked by a major advance of the sea in many parts of the world, and by the great thickness of both marine and continental sediments.

A great depression connected the Arctic Ocean with the Gulf of Mexico. Middle Europe was also submerged except for a central land mass. Towards the close of this period earth movements produced mountain ranges which are now the Andes and the Rockies, as well as mountains in Antarctica and northeastern Asia.

The Cretaceous Period marked both the culmination of Mesozoic life and the foreshadowing of animals and plants that were later to displace it. The most important new arrivals were the flowering plants (angiosperms). They first appeared in the Lower Cretaceous, but eventually they became the dominant plants on every continent.

Many familiar living trees and shrubs, including the poplar, magnolia, oak, maple; beech, holly, ivy and laurel appeared during the Cretaceous. The spread of the flowering plants also had important effects on animal life, for they provided new sources of food for mammals, birds. reptiles and insects. The subsequent expansion of mammals and birds depended very largely upon these new food supplies.

Dinosaurs extended their dominance across Cretaceous lands during this time. They are known from every continent, and included many unusual types. Horned dinosaurs (ceratopsians) were common, as were the armored ankylosaurs, and the bizarre duck-bills with their striking amphibious adaptations. The great quadruped dinosaurs declined in the Cretaceous but savage carnivores were common. Tyrannosaurus stood 20 feet high and had a skull over 3 feet long. Other carnivores were much smaller. Flying reptiles were represented by Pleranodon, a toothless, hammer- headed creature with a wingspan of 25 feet, the largest animal ever to fly.

In the seas, giant turtles (Archelon) reached a length of 12 feet, and some plesiosaurs grew over 40 feet long. Ichthyosaurs declined. Savage, serpent-like mosasaurs, some 35 feet long, were sea-going lizards.

Two well-known types of fossil birds occur in Cretaceous rocks. Ichthyornis, a slender, tern-like bird about 8 inches high, was a strong flier. Hesperornis, in complete contrast, was about 4% feet high, a diving bird, with powerful swimming legs but only vestiges of wings. It also had long, toothed jaws.

Mammals were small and relatively insignificant. Their remains are rare, represented by small primitive forms that survived from the Jurassic and also by two new groups, the opossum-like pouched marsupials and the insectivores, forerunners of the shrews. The fossils are mostly teeth and parts of lower jaws, which, because of their unique structures, are sufficient to distinguish these true mammals from mammal-like reptiles.

In the shallow seas invertebrates lived in great diversity. The dominant group was the ammonites, which showed many unusual forms. Belemnites, pelecypods and gastropods of rather modern appearance, corals, sea urchins and foraminifera also flourished. Modern bony fish (teleosts) were common. The corals, abundant locally in Cretaceous beds, show a basic sixfold symmetry. Crinoids also developed new forms, including a tree-swimming, stemless crinoid with slender arms up to 4 ft. long. Inoceramus mollusks, sometimes 3to 5 ft. across, were widely distributed.

The close of the Cretaceous saw the widespread extinction of many of the dominant animals of the Mesozoic Era. Dinosaurs, pterosaurs, ichthyosaurs, plesiosaurs, mosasaurs, ammonites, true belemnites, many pelecypods and corals all became extinct. It was as truly the end of an era in the long history of life as was the Permian, and the causes for this widespread decline are no less difficult to identify. It is probable, however, that the great geological changes, and the changes in plants exercised a profound effect on many groups of animals.



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