However, radiation into many closely related species is difficult to explain by genetic drift alone. Ammonites are a group of shelled molluscs, which were very successful during beginning in the Cambrian, with the explosion in the number and diversity of invertebrates. The present-day nautilus is a relative of this formerly successful, and now extinct group. Since the ammonites were shelled, a good fossil record was left behind. This record shows that many taxa which consisted of very many species became extinct early on while taxa which lasted for a long time often arose from minor branches in the evolutionary tree.
Ammonites are very popular with fossil collectors.
You may have even seen some ammonites at a collectibles shop at some mall. Steven J. Gould has discussed the Burgess Shale fauna. The Burgess Shale is a site in Canada which contains soft-bodied fossils dating back to the Cambrian. Soft-bodied organisms sometimes form fossils which look very much like a squashed blackened replica of the original organism. Some people originally thought that the black color was the result of the carbon in the biological structure but it is now believed that the carbon has been replaced by salts in a process not yet fully understood.