Carbactinoceratidae is a
family
Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
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 with external shells 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 ...
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
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 ...
, was measured at long.
Characters
Shells of carbactinoceratids are usually straight or slightly curved, with a large central or subcentral siphuncle. The first chamber at the apex is long and conical, the first septum slightly convex with the a septal perforation about one third of the shell diameter and a cyrtochoanitic septal neck (tubular or funnel-like projection from the back of the septum. The first siphuncle segment is elongated, subsequent ones formed during growth of the animal are subglobular (rounded but not spherical). Organic deposits are formed within the siphuncle through which run central, and radial canals along with a seam along the connecting rings known as a perispatium.
[
]
Genera
* ''
Aploceras''
* ''
Carbactinoceras''
* ''
Rayonnoceras''
Relationships to other cephalopods
Schindewolf (1941) and Teichert (1964) classified the Carbactinoceratidae within the
actinocerids based on siphuncle similarity.
[
]
In both groups the siphuncle contains collar-like deposits and radial canals.
However, there are some problems with the actinocerid hypothesis. Carbactinocerids such as ''Carbactinoceras'' appear much later than the last definite actinocerids, leaving a large stratigraphic gap between the two.
[
]
Furthermore, carbactinoceratids share key characters from the early development of
pseudorthocerids missing from actinocerids. Among these is a bullet-shaped apex that is not constricted, and a first segment of the siphuncle that is relatively narrow.
Moreover, pseudorthocerids abounded in the Carboniferous period and so eliminate the stratigraphic gap.
References
External links
Shell-Shock: Three University Of Arkansas Students Find World's Longest Nautiloid Fossil- an article about the recently found Arkansas ''Rayonnoceras''.
{{Taxonbar, from=Q5037825
Orthoceratoidea
Prehistoric cephalopod families
Carboniferous cephalopods of North America
Mississippian animals of North America
Mississippian United States
Mississippian first appearances
Mississippian extinctions