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for more detail. Courtesy of USGS and slightly modified by Norm Brown of UCSD |
Throughout most of the Cretaceous period, rapid sea-floor spreading contributed to a rise in global sea levels. Regionally, this caused massive flooding of the low-lying regions of central North America and formed the Western Interior Seaway which expanded across North America.
The seaway's coastlines, depth and length fluctuated throughout the Cretaceous period. In fact, at times it reached as far north as the Arctic Sea and as far south as the Gulf of Mexico, completely dividing the eastern and western landmasses of what we now call North America. This generally shallow waterway is estimated to have been less than 600 feet deep, with a flat, muddy bottom.
This warm seaway supported an abundance of fascinating lifeforms. Dinosaurs, reptiles and small mammals lived on the land areas surrounding the seaway. Flowering plants dominated the soil and prehistoric toothed birds (hesperornis and ichthyornis), flying reptiles (pteranodon) and insects soared through the skies. The Western Interior Seaway was home to many forms of marine vertebrates, including the Archelon. Fish, mosasaurs, pleisosaurs, aquatic birds as well as other prehistoric marine creatures swam alongside the giant Archelon. A great number of invertebrates, such as crinoids, echinoids, crabs, lobsters, ammonites, snails and clams also inhabited this prehistoric seaway.
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| Pteranodon. Courtesy of Joe Tucciarone |
The fluctuating sea levels created three types of deposits. Layers close to the shore were primarily composed of sandstone. Deposits of both shale and limestone represent areas that were once the muddy floor of the Western Interior Seaway. One such layer is known as the Pierre Shale Formation, which was created in the late Cretaceous period, 65 to 80 million years ago when the Western Interior Seaway was shrinking.
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The Cretaceous Period: Age of Reptiles
