From South Dakota, about 70 million years old These are shells of ammonites, extinct marine animals. Found in the center of the continent, far from the present coasts, they show that a warm inland sea once extended across North America. Fossil of a palm frond. From Utah, about 75 million years old. Some scientists think that this sea, like those that spread across other continents, helped to keep the Earth's climate mild.
Ammonites build their shells using two types, or isotopes, of oxygen atoms.
Shells that grow in warm water contain more of one isotope than the other. These shells contain more of the warm-water isotope. Today, winters in Utah are generally too cold for palms to grow. This fossil from Utah and others like it from Canada show that these warm-climate plants once grew much farther north than they do now. Design and construction by Knauer Communications .