The protoconch of Bacritida was globular and small, while the nautiloid protoconch was large. The eggs of bacritids and ammonids contained a small, embryonic shell called a protoconch. Both bacritids and ammonids produce vast quantities of eggs, while nautiloids do not. Bactrida were straight-shelled cephalopods of the Devonian (415 mya), though they appeared as early as 390 mya. Ammonites are more closely related to coleoids, which includes the living, soft bodied cephalopods like squid and the octopus.Īmmonites descend from an animal called a bacrite. Ammonoidea, Coleoidea and Nautiloidea are sub-classes of Cephalopoda. Their class Cephalopoda includes the gastropods (snails and slugs), however the cephalopods are have greater neurologic development. Some ate plankton.Īmmonites belong to the large phylum Mollusca, which includes invertebrates. Ammonites likely had a radula and beak, a crop, and ten arms to grasp prey of small fish, crustaceans, crinoids, and even small orthoceras. The last and largest of the chambers, the body chamber, was once believed to be protected by a hard aptychus- though now some scientists think this fossil remnant is part of the jaw. The septae of a nautiloid resemble a dish curving away from the body chamber. The septa of the ammonoid are convex, with the curve pushing outward to the opening, while nautiloid septa are concave with the curve pushing inward to the older parts of the shell. The beautiful patterns would have been hidden behind the protective, outer part of the shell on the living animal.
The sutures are on the inside of the shell, where the septa meet the inner wall. The septae form suture patterns which have an elaborate, folded design. The phragmacone pertains to the chambers which control buoyancy and are separated by septae. Most are planispiral, though some have helical forms or no spiral at all.
By Nobu TamuraĪmmonites shells are found with a variety of spiral types.
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