Carbactinoceratidae

Carbactinoceratidae is a family of extinct cephalopods 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.

Characters

The shells of carbactinoceratids was usually straight or slightly curved. The first chamber in the apex 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. ^[3]

Relationships to other cephalopods

Some specialists have classified the carbactinoceratids among the actinocerids. ^[4] Both groups share because both groups share collar-like deposits within the siphuncle, and radial canals.^[3]

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.^[5] 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.^[3] Moreover, the pseudorthocerids abounded in the Carboniferous period and so eliminate the stratigraphic gap.

References

1. ^ "Actinocerida". Palaeos. http://www.palaeos.com/Invertebrates/Molluscs/Cyrtosoma/Cephalopoda/Actinocerida.html. Retrieved September 12, 2009. 
2. ^ "Carbactinoceratidae". The Paleobiology Database. http://paleodb.geology.wisc.edu/cgi-bin/bridge.pl?action=checkTaxonInfo&taxon_no=122576&is_real_user=0. Retrieved Sept 12 2009. 
3. ^ ^{a b c} Kröger, Björn, & Mapes, Royal (2007). "actinoceratoid Nautiloidea (Cephalopoda)— A new perspective". Journal of Paleontology 81 (4): 714–724. doi:10.1666/pleo0022-3360. 
4. ^ Teichert, Curt, ed. (1964), Actinoceratoidea, in the Treatise on Invertebrate Paleontology, Vol K (Nautiloidea), Geol Soc of America and University of Kansas Press, pp. K190– K216 
5. ^ Flower, Rousseau H. (1940). "Some Devonian Actinoceroidea.". Journal of Paleontology 14 (5): 442–446. 

External links

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