| Rayonnoceras Temporal range: Early Devonian to Early Carboniferous[1] |
|
|---|---|
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Cephalopoda |
| Subclass: | Nautiloidea |
| Order: | Pseudorthocerida (or Actinocerida) |
| Family: | Carbactinoceratidae |
| Genus: | Rayonnoceras Croneis, 1926 |
| Species | |
|
all extinct |
|
Rayonnoceras is an extinct cephalopod genus that lived around 325 million years ago during the Carboniferous (late Mississippian) which although resemble earlier actinocerids are now though to belong to the Pseudorthocerida [2]
Rayonnoceras was a highly specialized nautiloid characterized by possessing a long straight, sometimes large and nearly cylindrical shell, which, as with the actinocerids, contains a large and heavy siphuncle composed of globular segments filled with organic deposits. As with the actinocerids, Rayonnoceras has a canal system within the siphuncle and a parispatium (the narrow opening between the intermal deposits and inner wall of the connecting ring) in each segment. Rayonnoceras differs from the earlier actinocers however 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]
The straight, conical shell of Rayonnoceras had a smooth surface. The centrally-placed siphuncle has a complex structure. Both the siphuncle and the convex shell chambers are always filled with calcareous deposits formed during the life of the nautiloid. These deposits are thought to have acted as ballast, to make the animal heavier. It lived in the lower, larger part of the straight shell and used the narrower part of the shell to maintain buoyancy.
Rayonnoceras would probably have lived like modern squid, mating, laying eggs and dying within a 3 or 4-year lifetime. They were swimmers, living in shallow marine environments and preying on small animals such as trilobites.
Rayonnoceras was among the last of the actinocerid-mimics, which finally died out in the Carboniferous.
Recently, an eight-foot-long specimen of Rayonnoceras solidiforme was found in Arkansas. It is believed to be the largest nautiloid fossil ever found. It is now housed at the University of Arkansas Museum. Since Rayonnoceras probably died soon after mating (when it was still quite small) it is thought that specimens such as this may have been rendered infertile by parasites, allowing them to live longer and hence grow much bigger.