Ammonites were marine creatures which lived from the Devonian Period and became extinct after the Cretaceous Period along with the dinosaurs. They resembled the living Nautilus with its coiled hard chambered shell however ammonites are more closely related to the squid, cuttlefish and octopus.
Ammonite fossils uncovered thus far range in size from about 6 inches to over 7 feet in diameter. Ammonites were abundant and widespread geographic marine creatures. These ancient sea creatures existed for approximately 290 million years from the Mid-Devonian through the Cretaceous period. Typical ammonite fossilization would occur in the open sea where they would die, sink to the depths and be buried by sediment. The fine-grained sediment would cover and preserve these creatures exhibiting extremely fine details. Ammonites make excellent index fossils due to their abundance in nature, extent of their fossil record, unique morphology and ornamentation.
Like octopus today it is believed some ammonite species squirted ink as a defense mechanism. Ink sack outlines preserved in some fossil specimens supports this belief. Like the nautilus the body of the creature would compress itself inside its shell controlling its floating and sinking ability controlling or maintaining its buoyancy.
Ammonites evolution is apparent when examining their various types of ornamentation development through the various geologic periods. Various patterns of ribs, spiral ridges and even spines on their shells developed up to and throughout the Cretaceous Period. Ammonites evolutionary development is most noticeable when comparing the various fossils from the earliest Geologic Periods such as the Devonian Period to the Cretaceous Period. It is believed by scientists that ammonites commonly possessed beaks, were carnivorous and possessed ten arms or tentacles. Also, that there is no evidence that any ammonite species dwelt in fresh or brackish water. They seemed to have been strictly sea going creatures.
The mass extinction during the late Cretaceous Period caused the extinction of the ammonites as well as many other marine creatures including the dinosaurs. This mass extinction was caused by a huge meteor impact at the end of the Cretaceous Period. According to the fossil record many of the ammonite species were already in serious decline by the end of the Cretaceous Period. This event sealed the faith of all ammonite species to extinction along with many other ancient creatures.