Shifting Continents and Greenhouse Climates
By the beginning of the Cretaceous, the supercontinent of Pangea had broken up into two smaller continents. The northern one, Laurasia, included what are today the continents of Europe, Asia, and North America. The other, Gondwana, was formed from the southern landmasses of present-day Africa, South America, Australia, India, Antarctica, and Madagascar. By the earliest Late Cretaceous (100 million years ago), rising sea level had caused the Western Interior Seaway to flood the central United States. This seaway extended from Texas into British Columbia. By 94 million years ago, the Tethys Ocean covered most of southern Europe, separating Laurasia from Gondwana. Southern Europe was represented only by a chain of islands. By this time, Laurasia and Gondwana had begun to split into the continents we know today. Africa and South America had nearly completely separated from each other, and India was an island in the Indian Ocean moving toward southern Asia. The climate was also much warmer than it is today, with no ice caps in polar regions.

The Pangean breakup led to an increase in seaways and shorelines, which may have affected the diversification of terrestrial plants and animals by creating several isolated continental regions. In addition, rapid seafloor spreading during the final breakup of Pangea caused a tremendous release of carbon dioxide gas (CO₂). This began in the Early Cretaceous and led to dramatic global climate warming that culminated about 92 million years ago, with nearly tropical temperatures extending to very high latitudes (polar regions). Such extreme warmth resulted in ice-free polar regions that were populated by dinosaurs, diverse forests and abundant insects. Fossils of crocodile-like champsosaurs, which could not tolerate extended periods of sub-freezing temperatures, have been found in Late Cretaceous sediments well north of the Arctic Circle. Conditions much warmer than those of today lasted into the Eocene.