More Than Index Fossils

Ammonites do indeed make wonderful index fossils thanks to their abundance and long presence in the global fossil record. The name ammonite is one of the oldest in paleontology, with Pliny the Elder referring to these fossils as "Ammon's horns" (ammonis cornua). Ammon was an Egyptian god often portrayed with a ram's horn helmet. Pliny the Elder was a scholar who died when Vesuvius erupted near Pompeii in AD 79. The spiral shape of ammonites was not actually universal (the ammonite on my desk rescued from the shores of Lake Wilson, is an interesting spiral but a spiral nonetheless). Helical spirals, non-spiraled, and planispiraled individuals are all known from various fossil locations, genera, and species throughout the family. There is a rather nicely put together discussion on cephalopod shell shape page hosted by The Octopus News Magazine Online (as weird as that site sounds). This page answers questions about shell shape much better than I know how to alone without much more research. The fact that ammonites, typically depicted with simple and predictable shell patterns, were highly variable in shell morphology makes a lot of sense and seems to go somewhat without saying, but it is something not often thought about.