Ammonites are present in the geological record from the Devonian period until the end of the Mesozoic period, when the asteroid that hit Central America resulted in the demise not only of the dinosaurs but also of the ammonites. Little is known about the biology and physiology of the ammonites other than that they were found in all marine environments in the Paleozoic and Mesozoic periods. They were shelled cephalopods, with shell configurations varying from conical and elongated to tightly coiled. Scientists assume they had multiple arms such as those of the modern nautilus. Because of their unique shell configurations and sizes—the latter ranging from very small to more than a couple meters in diameter—ammonites have become the fixation of many fossil collectors and museums. Valuable books on ammonites include Ulrich Lehman’s The Ammonites: Their Life and Their World; Kate Marricott, Andrew Bartholmew, and Donald Prothero’s Evolution of the Ammonoids; Neale Monks and Philip Palmer’s Ammonites; and E. T. Tozer’s The Trias and Its Ammonoids: The Evolution of a Time Scale.
On the other hand, belemnites have been found only in the Jurassic and Cretaceous periods. They would have looked like modern squid, except that a shell enclosed their bodies. Because of the shell, rocks have preserved their fossils very well, and therefore belemnites are very useful in biostratigraphic studies.
In the Mesozoic period, the Pierre Seaway divided North America into two parts: the eastern side was home to the ancestral Appalachians, and the western side the ancestral Rockies. Both ammonites and belemnites have been found in the shale in the seaway. Neal L. Larson et al. provide information on fossilized cephalopods in their informative guide Ammonites and the Other Cephalopods of the Pierre Seaway.
Ammonites and Other Cephalopods of the Pierre Seaway by 