Carbactinoceratidae
Encyclopedia
Carbactinoceratidae is a family
of extinct cephalopod
s with external shells that lived around 325 million years ago, during the Carboniferous
period. They were the last group of orthocones to attain sizes exceeding one meter (three feet) in length. One specimen of the carbactinoceratid species Rayonnoceras
solidiforme, recently found in Arkansas
, was measured at 2.5 meters (8 feet) long.
of the shell was high and conical. The siphuncle
was central or subcentral, and its first segment elongated. The first septum was slightly convex, had a cyrtochoanitic septal neck, and with septal perforation about one third of the shell diameter. Siphuncular segments that formed later in the animal's growth were subglobular (rounded but not spherical). The wide siphuncle was filled with deposits that formed around central, radial canals and a perispatum.
Both groups share because both groups share collar-like deposits within the siphuncle, and radial canals.
However, there are some problems with the actinocerid hypothesis. Carbactinocerids such as Rayonnoceras appear much later than the last definite actinocerids, leaving a large stratigraphic gap between the two.
Furthermore, the carbactinoceratids share more characters from early development with the pseudorthocerids than with the actinocerids. Among them are a bullet-shaped apex that is not constricted, and a first segment of the siphuncle that is relatively narrow.
Moreover, the pseudorthocerids abounded in the Carboniferous period and so eliminate the stratigraphic gap.
Family
In human context, a family is a group of people affiliated by consanguinity, affinity, or co-residence. In most societies it is the principal institution for the socialization of children...
of extinct cephalopod
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...
s with external shells 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"...
period. They were the last group of orthocones to attain sizes exceeding one meter (three feet) in length. One specimen of the carbactinoceratid species Rayonnoceras
Rayonnoceras
Rayonnoceras is an extinct cephalopod genus that lived around 325 million years ago during the Carboniferous which although resemble earlier actinocerids are now though to belong to the Pseudorthocerida...
solidiforme, recently 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...
, was measured at 2.5 meters (8 feet) long.
Characters
The shells of carbactinoceratids was usually straight or slightly curved. The first chamber in the apexApex (mollusc)
Apex is an anatomical term for the tip of the mollusc shell of a gastropod, scaphopod, or cephalopod mollusc.-Gastropods:The word "apex" is most often used to mean the tip of the spire of the shell of a gastropod...
of the shell was high and conical. 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...
was central or subcentral, and its first segment elongated. The first septum was slightly convex, had a cyrtochoanitic septal neck, and with septal perforation about one third of the shell diameter. Siphuncular segments that formed later in the animal's growth were subglobular (rounded but not spherical). The wide siphuncle was filled with deposits that formed around central, radial canals and a perispatum.
Relationships to other cephalopods
Some specialists have classified the carbactinoceratids among the actinocerids.Both groups share because both groups share collar-like deposits within the siphuncle, and radial canals.
However, there are some problems with the actinocerid hypothesis. Carbactinocerids such as Rayonnoceras appear much later than the last definite actinocerids, leaving a large stratigraphic gap between the two.
Furthermore, the carbactinoceratids share more characters from early development with the pseudorthocerids than with the actinocerids. Among them are a bullet-shaped apex that is not constricted, and a first segment of the siphuncle that is relatively narrow.
Moreover, the pseudorthocerids abounded in the Carboniferous period and so eliminate the stratigraphic gap.
External links
- Shell-Shock: Three University Of Arkansas Students Find World's Longest Nautiloid Fossil - an article about the recently found Arkansas Rayonnoceras.