''Rayonnoceras'' is a genus of extinct

cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

s that lived around 325 million years ago during the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

. Although they resemble earlier actinocerids they are now though to belong to the

Pseudorthocerida Pseudorthocerida is an order of generally straight longiconic orthoceratoids with a subcentral to marginal cyrtochoanitic siphuncle composed of variably expanded segments which may contain internal deposits that may develop into a continuous p ...

Kröger, Björn, & Mapes, Royal 2007. Actinoceratoid Nautiloidea (Cephalopoda)— A new perspective; Journal of Paleontology Vol 81, No 4, pp 714-724, July 2007 ''Rayonnoceras'' is characterized by large straight shells with a large subcentral siphuncle composed of subglobular segments, as for the family.Teichert 1964, Treatise on Invertebrate Paleontology, K 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'' has a canal system within the

siphuncle The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and ...

that includes a narrow opening, the parispatium, between the internal deposits and inner wall of the connecting ring. It 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. Orientation during life was horizontal. Cameral deposits, more concentrated in the apical part of the shell, are thought to have acted as ballast that compensated 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. In 2003 a specimen of ''Rayonnoceras solidiforme'' eight feet long (2.4m) was found in

Arkansas Arkansas ( ) is a landlocked state in the South Central United States. It is bordered by Missouri to the north, Tennessee and Mississippi to the east, Louisiana to the south, and Texas and Oklahoma to the west. Its name is from the Osage ...

. It is believed to be the largest of its kind ever found and is housed at the

University of Arkansas The University of Arkansas (U of A, UArk, or UA) is a public land-grant research university in Fayetteville, Arkansas. It is the flagship campus of the University of Arkansas System and the largest university in the state. Founded as Arkansas ...

Museum.

References

External links

Shell-Shock: Three University Of Arkansas Students Find World's Longest Nautiloid Fossil
- an article about the Arkansas ''Rayonnoceras'' found in 2003. {{Taxonbar, from=Q3866731 Orthoceratoidea Carboniferous cephalopods of North America