Rayonnoceras
Encyclopedia
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
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
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 trilobite
s.
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.
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
genus that lived around 325 million years ago during the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
(late Mississippian) which although resemble earlier actinocerids
Actinocerida
The Actinocerida comprise an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposits formed within contain a system of radial...
are now though to belong to the Pseudorthocerida
Pseudorthocerida
Pseudorthocerida are generally straight longiconic nautiloids with a subcentral to marginal cyrtochoanitic siphuncle composed of variably expanded segments which may contain internal deposits that may develop into a continuous parietal lining.. Cameral deposits are common and concentrated ventrally...
Rayonnoceras was a highly specialized nautiloid
Nautiloid
Nautiloids are a large and diverse group of marine cephalopods belonging to the subclass Nautiloidea that began in the Late Cambrian and are represented today by the living Nautilus. Nautiloids flourished during the early Paleozoic era, where they constituted the main predatory animals, and...
characterized by possessing a long straight, sometimes large and nearly cylindrical shell, which, as with the actinocerids
Actinocerida
The Actinocerida comprise an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposits formed within contain a system of radial...
, 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
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 Spirula...
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
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
Nautiloid
Nautiloids are a large and diverse group of marine cephalopods belonging to the subclass Nautiloidea that began in the Late Cambrian and are represented today by the living Nautilus. Nautiloids flourished during the early Paleozoic era, where they constituted the main predatory animals, and...
. 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
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
.
Rayonnoceras would probably have lived like modern squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
, 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 trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
s.
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
Arkansas
Arkansas is a state located in the southern region of the United States. Its name is an Algonquian name of the Quapaw Indians. Arkansas shares borders with six states , and its eastern border is largely defined by the Mississippi River...
. It is believed to be the largest nautiloid fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
ever found. It is now housed at the University of Arkansas
University of Arkansas
The University of Arkansas is a public, co-educational, land-grant, space-grant, research university. It is classified by the Carnegie Foundation as a research university with very high research activity. It is the flagship campus of the University of Arkansas System and is located in...
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.
External links
- Shell-Shock: Three University Of Arkansas Students Find World's Longest Nautiloid Fossil - an article about the recently found Arkansas Rayonnoceras.