With the disappearance of the glaciers, sea level rose (with some fluctuations) from the Early Triassic to the Late Jurassic, and continued rising to a maximum during the Cretaceous.
Vail sea-level curve showing major cycles of sea level changes. Note the rise in sea level during the Jurassic and Cretaceous.
Sea level rise was also related to the rifting and fragmentation of Pangea. The mid-Atlantic ridge system developed as the Atlantic Ocean widened. The basaltic rocks that were extruded along the mid-ocean ridge system were hot and thermally inflated. As a result, they displaced a considerable volume of sea water onto the continents.
In the Late Jurassic and Cretaceous, epicontinental (or epeiric) seas flooded large areas of North America and Europe. These epicontinental seas also contributed to the warmer climate because the water carried heat.
These epicontinental seas provided an extensive habitat for shallow marine organisms and probably led to the increase in diversity.
Note the similarity between the sea level curve (above) and the graph of marine diversity (below) during the Jurassic and Cretaceous.
Diversity of marine animals through geologic time, as indicated by number of known fossil genera.
Both curves show an increase through the Jurassic and Cretaceous, a dip at the end of the Jurassic and a peak in the Late Cretaceous.
The paleogeographic maps below show the spread of epicontinental seas over North America through the Mesozoic.
Triassic paleogeography of North America. Note absence of epicontinental seas in the central part of the continent.
Jurassic paleogeography of North America. Note the open seas in the Great Plains area, characterized by sandy and shaley deposits, limestones and evaporites.
Cretaceous paleogeography of North America. Note the epicontinental seas covering the western interior of North America as well as the Coastal Plain areas in the south and east.