The name ammonite, from which the scientific term was derived, was inspired by the spiral shape of their petrified shells, which somewhat resembles the twisted horns of ram. Pliny, the Elder, in the year 79 AD, according to the Julian calendar, called fossils of these animals Ammonis cornua (horns of Amun), because the Egyptian god Amon was depicted with the horns of ram. The name of ammonite genera often ends with -ceras, which in Greek means ‘horn’.
Originating from the early Nautiloids, Ammonoid Cephalopods first appeared in Devonian, about 400 million years ago and became extinct at the end of Cretaceous, 65 million years ago, along with the dinosaurs.
The classification of Ammonoids is partly based on the ornament and the structure of the partitions of shell chambers. While all the nautiloids exhibit soft curved seams, the ammonoid seam line (the intersection of the partition with the outer shell) has waved like structure that forms a grooved surface.
Few Ammonites from the lower and the middle parts of the Jurassic period reached a diameter greater than 9 inches. In later rocks top of the Jurassic and lower chalk found larger forms such as Titanite of Portland stone Jurassic southern England (2 feet), and Parapuzosia seppenradensis Cretaceous Germany, which is one of the largest known ammonites, reaching upto 6.5 feet in diameter.