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144 TO 65 MILLION YEARS AGO
During the Cretaceous, the supercontinent of Pangaea completed its breakup into present day continents, although their positions were substantially different. Though Gondwana was still intact in the beginning of the Cretaceous, Gondwana itself broke up as South America, Antarctica and Australia rifted away from Africa. The Indian Ocean was newly formed. Such active rifting lifted great undersea mountain chains along the welts, raising sea levels worldwide. Laurasia started to rotate clockwise and moved northward with North America to the north, and Eurasia to the south.
The environment was warm and sub-tropical and there was no ice at the poles. The sea levels were high. Broad leaf trees replaced the conifers and flowering plants appeared for the first time and became the dominant flora. The landscape looked very similar to the landscape today, but there were still no grasses. On land, mammals were a small and still relatively minor component of the fauna. In the skies, Pterosaurs were common in the early and middle Cretaceous, but as the Cretaceous proceeded faced growing competition from the adaptive radiation of birds. Dinosaurs continued to diversify and dominate the landscape. The herbivorous Hadrosaurs, Ankylosaurs and Ceratopsians emerged. New carnivore groups developed including the Tyrannosaurs, Spinosaurs and Dromaeosaurs. One of the most widely accepted explanations for the mass extinction of animal and marine life at the end of the late Cretaceous is the collision of a meteor into the earth. Many changing environmental elements contributed to the demise of the dinosaurs; however, it seems the impact of an extra-terrestrial body was the decisive factor.
208 TO 144 MILLION YEARS AGO
At the beginning of the Jurassic period the Earth's continents were still jammed together, making much of the inland area dry and desert-like, but they were beginning to drift apart and there was no polar ice. There had been a minor extinction at the end of the Triassic period, which gave rise to an abundance of dinosaurs in the Jurassic.
In the mid-Jurassic period, the supercontinent Pangaea started to drift apart. A north-south rift formed in the mid-Jurassic, and by the late Jurassic, the separation of the continents of Laurasia and Gondwana was almost complete. During the Jurassic period sea levels began to rise, probably due to an increase in sea-floor spreading. This caused flooding of large areas of the continents. As a result, the deserts began to retreat, and continental temperatures stabilized. Pangaea also began to break up into smaller units, which brought more land area in contact with the ocean. The presence of nearby oceans also increased humidity, so that climates worldwide became wetter as well as warmer. This effect created an environment that was warm and tropical. The deserts were replaced by lush forests of conifer trees and vast open fern prairies.
The herbivorous sauropod had become the largest creatures to ever walk the earth. Allosaurs emerged as the alpha predators. Some of the smaller carnivores developed feathers and took to the air, including Archaeopteryx (Ancient Wing). There was a minor mass extinction toward the end of the Jurassic period. During this extinction, most of the stegosaurid and enormous sauropod dinosaurs died out. The cause of the extinction is unknown.
245 TO 208 MILLION YEARS AGO
Roughly 248 million years ago (mya), the Permo-Triassic extinction occurred. This is the largest extinction known. About 95% of all species and about 60% of the genera died out, including many marine animals (like the trilobite). The cause of the Permian extinction might have been global cooling, volcanic eruptions, or a decrease in the continental shelf area during the formation of the giant continent, Pangaea. This catastrophic extinction and continental rearrangement opened the way for the rise of the dinosaurs and mammals.
There were no dinosaurs at the beginning of the Triassic, but there were many amphibians and some reptiles and dicynodonts (like Lystrosaurus). During the early Triassic, corals appeared and ammonites recovered. The Triassic meadows and prairies would have been covered in ferns, low cycads, club mosses and horsetails as these plants dominated the land; in the Northern hemisphere, conifers flourished. There was no grass and no flowers.
The first half of the Triassic was dry and highly seasonal, with particularly large annual temperature variations in the vast continental interior of Pangaea. Low sea levels probably exaggerated these temperature extremes. The environment was harsh, with deserts and dry wilderness. 230 million years ago a unique breed of reptile appears, the dinosaur. Most Triassic dinosaurs were small bipeds, but at the end of the Triassic the first large sauropods appear.