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Ammonoids were a very successful order of sea creatures (Ammonoidea) among the cephalopods, related to the octopuses, squids and nautilus. (more below)
Ammonoids ruled the seas for 300 million years
Photo (c) 2006 Andrew Alden, licensed to About.com (fair use policy)
Paleontologists are careful to distinguish the ammonoids from the ammonites. Ammonoids lived from Early Devonian times until the end of the Cretaceous Period, or from about 400 million to 66 million years ago. Ammonites were a suborder of ammonoid with heavy, ornamented shells that thrived starting in the Jurassic Period, between 200 and 150 million year ago.

Ammonoids have a coiled, chambered shell that lies flat, unlike gastropod shells. The animal lived at the end of the shell in the largest chamber. The ammonites grew as large as a meter across. In the wide, warm seas of the Jurassic and Cretaceous, ammonites diversified into many different species, largely distinguished by the intricate shapes of the suture between their shell chambers. It is suggested that this ornamentation served as an aid to mating with the right species. That would not help the organism survive, but by ensuring reproduction it would keep the species alive.

All the ammonoids died at the end of the Cretaceous in the same mass extinction that killed off the dinosaurs.

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