Rayonnoceras

Rayonnoceras is an extinct cephalopod genus that lived around 325 million years ago during the Carboniferous which although resembling earlier actinocerids is now though to belong to the Pseudorthocerida ^[2]

Rayonnoceras is characterized by large to very large straight shells with a large subcentral siphuncle composed of subglobular segments, as for the family.^[3] The endosiphuncular canal system is well developed and most chambers are filled with organic deposits. Some shells have transverse striae, others are smooth. As with earlier actinocerids. Rayonnoceras too has a canal system within the siphuncle that includes a narrow opening between the internal deposits and inner wall of the connecting ring known as a parispatium. but differs from the earlier actinocerids in having a bullet-like apex with a cicatrix and a long first chamber, characteristic of pseuorthocerids, rather than a blunt apex and short first chamber characteristic of true actinocerids.^[2]

Recently, an eight-foot-long specimen of Rayonnoceras solidiforme was found in Arkansas, believed to be the largest of its kind ever found, is now housed at the University of Arkansas Museum.

Orientation during life was horizontal. Cameral deposits, more concentrated in the apical part of the shell, are thought to have acted as ballast compensating for the weight of the animal in the forward part. In life Rayonnoceras was probably a bottom dweller that spent most of its time stationary, waiting in ambush for prey.

Rayonnoceras was among the last of the actinocerid-mimics, which finally died out in the Carboniferous.

References

External links

• Shell-Shock: Three University Of Arkansas Students Find World's Longest Nautiloid Fossil - an article about the recently found Arkansas Rayonnoceras.