Ammonites are an extinct group of marine animals of the subclass Ammonoidea in the class Cephalopoda, phylum Mollusca. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods.
Ammonites' closest living relatives are probably octopus, squid, and cuttlefish.
Their fossil shells usually take the form of planispirals, although there were some helically-spiraled and non-spiraled forms (known as "heteromorphs"). Their name came from their spiral shape as their fossilised shells somewhat resemble tightly-coiled rams' horns. Pliny the Elder (d. 79 A.D. near Pompeii) called fossils of these animals ammonis cornua ("horns of Ammon") because the Egyptian god Ammon (Amun) was typically depicted wearing ram's horns. Often the name of an ammonite genus ends in ceras, which is Greek for "horn" (for instance, Pleuroceras).
Originating from within the bactritoid nautiloids, the ammonoid cephalopods first appeared in the Late Silurian to Early Devonian (circa 400 million years ago) and became extinct at the close of the Cretaceous (65 m.y.a.) along with the dinosaurs.
The above fossil of an ammonite image is from the Ralph Thoresby display at Leeds City Museum. Although no claim is made that the collection was ever owned by Thoresby, it is meant as a visual representation of artifacts likely to have been found in his own museum.
Ralph Thoresby (16 August 1658 – 16 October 1724), born in Leeds and is widely credited with being the first historian of the city. He was besides a merchant, non-conformist, fellow of the Royal Society, diarist, author, common-councilman in the Corporation of Leeds, and museum keeper.
Ralph Thoresby was the son of John Thoresby, a Leeds merchant who for a time was an officer under Fairfax during the English Civil War. He was by inclination an antiquarian and of his wife Ruth, from the village of Bulmer, north of York.
According to the preface of The Diary of Ralph Thoresby F.R.S., father and son were alike, deeply religious and both with strong attachments to antiquarian pursuits. John Thoresby established for himself a museum of coins and medals, purchasing at great expense two pre-existing collections owned by the Fairfax family and another family called Stonehouse.
Ralph was educated at Leeds Grammar School, and on the death of his older brother became the eldest son of the family. He was sent at age eighteen to the house of a relation in London, as part of his grooming as a merchant. He maintained a diary from this point, fairly consistently, throughout the rest of his life; an edited version survives him in print. And for this reason there is easily available a detailed chronology of his life.