Trassic | Jurassic | Cretaceous
[Mesozoic Era]

The Mesozoic Era is the major division of geological history, following the Paleozoic era and preceding the Cenozoic era. The Mesozoic era lasted from approximately 244 million to 66.4 million years ago. The Mesozoic era is divided into three time periods: Triassic, Jurassic, and Cretaceous.
Mesozoic means "middle animals."

The Triassic was a time of a single world continent, when the first pterosaurs took flight and dinosaurs evolved along with many other animals. In the Jurassic, both plant-eating and meat-eating dinosaurs grew enormous. The world broke into two continents. In the Cretaceous, flowering plants and many insects evolved. Most dinosaurs we know come from the end of the Cretaceous period. We know about 175 kinds of dinosaurs from just the last 25 million years of dinosaur time. That's about half the total from all 165 million years they were around.

The environment during the dinosaurian era was far different than today's environment. The days were several minutes shorter, and the sun's radiation was weaker. Carbon dioxide, a gas that traps solar heat in the earth's atmosphere, was more abundant, causing warmer temperatures that kept polar ice caps from forming.

Triassic Period

The Triassic Period, the first and shortest of the three periods of the Mesozoic. It spanned an interval of 37 million years, from 245 to 208 million years ago. [Pangaea]

250 million years ago, continents formed a single land mass surrounded by an enormous sea. This supercontinent was called Pangaea, and the oceans formed a vast world ocean called Panthalassa.

During the Triassic period, Pangaea began tearing apart. About 200 million years ago, movements of the earth's crust caused the supercontinent to begin slowly separating. Rifts developed between North America and the African portion of Gondwanaland. As the earth's crust stretched, huge seas covered portions of North America, Africa, Europe, and Asia. Thick forests bordered drier plains, and swamps and deltas lined the seacoasts.

Fossils found in Triassic strata indicate that the general climate was warm. Throughout the Mesozoic, the climate was less changable. Areas near seas and along rivers and lakes had mild, moist weather all year. Inland regions were drier and in some cases desertlike.

The terrain was dominated by evergreen trees. There were fewer than half as many species of plants and animals on land during the Mesozoic Era than there are today. Bushes and trees appear to have provided the most abundant sources of food for dinosaurs. Few flowering plants bore nuts or fruit.

Dinosaurs appeared in the Triassic period, although early ones did not attain the huge dimensions of the dinosaurs of later Mesozoic times.

Other reptilians of the Triassic were the ichthyosaurs, with bodies shaped like modern dolphins, and the plesiosaurs, who had turtlelike bodies.

The Triassic period is considered to mark the emergence of the first true mammals (evolving from the synapsid reptiles), but little is known of their physiology. Insects were represented by the first species to undergo complete metamorphosis from larva through pupa to adult.

(The smallest dinosaurs were probably from the late Triassic and early Jurassic. That's where we find the smallest ornithischian dinosaurs so far. Dinosaurs got biggest in the late Jurassic and Cretaceous.)

Jurassic Period

The Jurassic Period spanned an interval of about 208 to 144 million years ago. The period is named for strata from the Jura Mountains.

At the beginning of the Jurassic Period, Italy, Greece, Turkey, and Iran were attached to the North African portion of Gondwanaland, the southern supercontinent. Antarctica and Australia, already detached from Gondwanaland, remained joined together, while India drifted northward. North America separated from Gondwanaland and drifted westward, opening the Gulf of Mexico.

As North America ran over the Pacific Ocean floor, it triggered mountain-building events that created the North American Cordillera (the Rocky Mountains and the Sierra Nevada). South America and Africa began rifting apart, creating a long, narrow seaway between them.

Evidence suggests the Jurassic climate was warm and moist.

Reptiles were the dominant form of animal life and had adapted to life in the sea (ichthyosaurs and plesiosaurs), in the air (pterosaurs), and on the land, where some reached huge sizes (Allosaurus and Apatosaurus). Mammals remained small and rodentlike.

Cretaceous Period

The Cretaceous Period lasted from about 138 million to about 66.4 million years ago. In Europe and North America, geologists divide the period into an Early and a Late Cretaceous.

In the early Mesozoic two supercontinents - Laurasia, now the northern continents, and Gondwanaland, now the southern continents - were separated by the Tethys Sea, of which the Mediterranean is a small remnant.

In the Cretaceous period the African continental plate and India broke from Gondwanaland. Subsequent continental shifting created the roots of the European Alps, and the forerunners of the Himalayas. The South Atlantic Ocean broadened the gap between Africa and South America; and Antarctica and Australia, still joined, drifted south and westward. North America continued to move west, causing the uplift of the Rocky Mountains and California's Sierra Nevada.

During the Late Cretaceous, sea levels rose worldwide, eventually producing a warm, mild global climate. By this time the flora had taken on a modern appearance and included many of today's genera of trees. Despite mild conditions, several mass faunal extinctions occurred toward the end of the period.