Arkansas Mississippian Nautiloid Rayonnoceras

Name: Rayonnoceras solidiforme

Phylum Mollusca, Class Cephalopoda, Subclass Nautiloidea; Order Actinoceratida, Family Carbactrinoceratidae

Geologic Age: Mississippian

Size: 170 mm long specimen

Fossil Site: Fayetteville Shale, White County, Arkansas

Rayonnoceras solidiformeDescription: This large nautiloid is a Rayonnoceras from the Mississippian of Arkansas that is free of matrix. The fossil has been split and polished to show interior details. The primitive suturing pattern distinguishes these cephalopods from the ammonites. The Order Actinoceratida originated in the middle Ordovician, and radiated extensively in the Silurian to become one of the most important groups of cephalopods. They 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. By the Devonian, they had become rare, and went extinct in the Carboniferous. The straight shells show them to be distantly related to another group of Orthocones, the Endoceratids, some of which grew to 6 meters in length. Rayonnoceras have been found in Arkansas at lengths up to 2.5 meters. Like other nautiloids, the siphunule was centrally located in contrast to the dorsal positioning in ammonites. It was quite complex in shape as seen here, and filled with calcareous deposits thought to have served as ballast. This intricate structure is well displayed within this fine example. Rayonnoceras probably lived much like modern squids as swimming predators, primarily hunting in shallow marine waters and likely feeding on small animals..

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