Now extinct, ammonites are abundant, prehistoric sea molluscs that first appeared in the fossil record around 240 million years ago. The images of ammonites that we often see in museums and books are planispiral-shaped, but as the American Museum of Natural History curator Neil Landman explains in this video, ammonites came in a wide variety of shaped shells… an excellent example of evolution.
A bit more about their history via Wikipedia:
Many genera evolved and ran their course quickly, becoming extinct in a few million years. Due to their rapid evolution and widespread distribution, ammonoids are used by geologists and paleontologists for biostratigraphy. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geologic time periods…
The ammonoids as a group continued through several major extinction events, although often only a few species survived. Each time, however, this handful of species diversified into a multitude of forms. Ammonite fossils became less abundant during the latter part of the Mesozoic, with none surviving into the Cenozoic era. The last surviving lineages disappeared, along with the non-avian dinosaurs, 65 Mya in the Cretaceous–Paleogene extinction event.