Mollusca, Class Cephalopoda, Subclass Nautiloidea; Order Actinoceratida,
mm long specimen
Site: Fayetteville Shale, White County, Arkansas
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..