Rayonnoceras

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Rayonnoceras

Scientific classification

Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	Nautiloidea
Order:	Actinoceratida
Family:	Carbactrinoceratidae
Genus:	Rayonnoceras Croneis, 1926

Species

all extinct

Rayonnoceras is a genus of extinct cephalopod of the order Actinoceratida and that lived around 325 million years ago, during Ordovician to Carboniferous times. They were among the last of the Actinoceratida, which were highly specialized nautiloids characterized by possessing a long straight, sometimes large and nearly cylindrical shell, which, like the endoceroids, contains an unusually large and heavy siphuncle. However, in Rayonnoceras the siphuncle segments are globular or more slender, smaller than in other Actinoceratida.

The straight, conical shell of Rayonnoceras had a smooth surface. The centrally-placed siphuncle has a complex structure. Both the siphuncle and the convex shell chambers are always filled with calcareous deposits formed during the life of the nautiloid. These deposits are thought to have acted as ballast, to make the animal heavier. It lived in the lower, larger part of the straight shell and used the narrower part of the shell to maintain buoyancy.

Rayonnoceras would probably have lived like modern squid, mating, laying eggs and dying within a 3 or 4-year lifetime. They were swimmers, living in shallow marine environments and preying on small animals such as trilobites.

Originating in the Ordovician, by the Devonian period Actinocerids became rare; perhaps they were unable to compete with the more compact and maneuverable coiled nautiloids and ammonoids and cope with the arrival of jawed fish. Rayonnoceras was among the last of the actinocerids, which finally died out in the Carboniferous.

Recently, an eight-foot-long specimen of Rayonnoceras solidiform was found in Arkansas. It is believed to be the largest nautiloid fossil ever found. It is now housed at the University of Arkansas Museum. Since Rayonnoceras probably died soon after mating (when it was still quite small) it is thought that specimens such as this may have been rendered infertile by parasites, allowing them to live longer and hence grow much bigger.