Shaping The World Of The Dinosaurs

Whereas the Jurassic Period was relatively quiet geologically, the geography of the Cretaceous Period witnessed a dramatic resurgence of tectonic plate movements. Continental drift during the Cretaceous Period caused the continued breakup of the northern and southern supercontinents of Laurasia and Gondwana, separating them into landmasses approximating their present-day configuration. The northern Laurasia included areas that became North America, Europe, and Asia. The southern landmass of Gondwana included the regions now known as South America, Africa, India, Madagascar, Australia, and Antarctica. During the Cretaceous, the Atlantic Ocean spread open, submerging the land bridges that once



Span (millions of years ago)

Duration (millions of years)

Organismal Milestones

Early Triassic



Diversification and dispersal of amniotes, particularly synapsids and diapsids

Middle Triassic



Rise of pterosaurs and sauropterygian marine reptiles

Late Triassic



Earliest frogs, turtles, crocodylians, dinosaurs, and mammals

Diversification of conifers

Mass extinction

Casualties: most nonmammalian synapsids, phytosaurs, placodonts, nothosaurs

Early Jurassic



Radiation of carnivorous and herbivorous dinosaurs, first crocodilians

Middle Jurassic



Rise of armored, plated, and ornithopod dinosaurs, diversification of sauropods

Late Jurassic



Dominance of sauropods, diversification of theropods, first birds and horned dinosaurs

Early Cretaceous



Continued diversification of dinosaurs, birds, marine reptiles (other than ichthyosaurs, which were already becoming extinct), and pterosaurs

First marsupial and placental mammals

Rise of angiosperms—the flowering plants

Late Cretaceous



Largest known theropods and sauropods, diversification of horned dinosaurs, appearance and radiation of hadrosaurs, snakes, mosasaurs, and modern-style birds,

Mass extinction

Casualties: Non-avian dinosaurs, opposite birds, pterosaurs, ammonites, all marine reptiles except turtles

connected North America with Europe and South America with Africa. India broke from Gondwana and began its northerly drift toward Asia. North America was divided by a vast inland sea that, at its greatest extent, dissected the continent from the northwest Arctic to the Gulf of Mexico.

World Map of the Cretaceous Period

Cretaceous Period World Map
The geologic positioning of the major continents during the Cretaceous Period

The northwest coast and southern tip of South America were largely under water. North Africa was largely under water, as was the northern edge of India. Much of Great Britain and the northern half of Africa were under water as well, as was much of southern Europe and northeastern Australia. High water levels reached their peak during the middle of the Cretaceous, about 100 million years ago, when about one-t hird of the present land area of Earth was submerged. This dramatic rise in sea level was caused by an acceleration of seafloor spreading that enlarged ocean ridges, formed undersea mountains, and displaced water to areas that once were dry land. Such seafloor spreading also spurred the continued breakup of the continents. The rise of the oceans was also affected by the melting of polar ice caps because of the greenhouse climate of the middle Cretaceous. By the end of the Cretaceous, a general slowdown in tectonic activity took place, reducing the size of ocean ridges and underwater mountains and forming deeper ocean basins that allowed continents to shed some of the water that had overflowed onto them.

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    What did the continents look like in the cretaceous period?
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