Detailed Arkansas Mississippian Nautiloid Rayonnoceras

Rayonnoceras solidiforme

Nautiloidea, Actinoceratida, Carbactrinoceratidae

Size (25.4mm=1 inch): 70 mm long, 45 mm across living chamber, up to 25 mm thick

Fossil Site: Fayetteville Shale, White County, Arkansas

Fossil Code: ARC3

Price: $60.00 - sold

Description: This large nautiloid is a Rayonnoceras from the Mississippian of Arkansas which is free of matrix. This allows you the opportunity to view this fine, robust specimen from all aspects. It has been split and polished to bring out the fine interior details. The primitive suturing pattern distinguishes these cephalopods from the ammonites (I have posted it with the ammonites as they are close relatives). The Actinoceratida originated in the middle Ordovician, and radiated extensively in the Silurian to become one of the most important groups of cephalopods. By the Devonian, they had become rare, only to go 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. The reverse shows the smooth exterior.