Armenoceratidae
Encyclopedia
Armenoceratidae is a family of early Paleozoic nautiloid cephalopod
s belonging to the Order Actinocerida
.
The Armenoceratidae, established by Troedsson (1926) is characterized by large, straight or slightly curved shells and large siphuncle
s with strongly expanded segments between the septa. Septal necks are short and abruptly recurved along brims. Radial canals in the endosiphuncular canal system are typically arched, curving forward and backward from near the septal foramina (openings) to connect with the parispatium on either side of the middle of each segments. The parispatium is the narrow opening between the inner side of the connecting rings in actinocerids and the internal siphuncular deposits that grow forward and back from the region of the septal openings.
The Armenoceratidae have their beginning in Armenoceras
which first appeared near the beginning of the Chazyan, 2nd stage of the Middle Ordovician
in older established chronologies, in northeastern China, Manchuria., derived from Wutinoceras
. Additional genera include Nybyoceras
and Selkirkoceras, respectively from the upper Middle and Upper Ordovician, and Monocyrtoceras, Elrodoceras, and Megadiscoceras from the Silurian
,
The Armenoceratidae, Armenoceras, Nybyoceras, and Selkirkoceras first appear in North America in the diverse cephalopod Redriveran faunas of the early Upper Ordovician but are no longer found during that stage in Asia. Selkirkoceras is a large armenocerid from the Upper Ordovician with a blunt, flattened, somewhat breviconic shell.
Among the Silurian genera, Elrodoceras has a large shell with the apical part slightly curved, otherwise is straight, and a siphuncle that is narrower than in Armenoceras. Monocyrtoceras has a siphuncle lke that of Elrodoceras, but the entire shell is gently and evenly curved. Megadiscosorus is similar to Armenoceras but more breviconic and slightly exogastrically cyrtoconic and with the siphuncle in contact with the ventral wall.
The Armenoceratidae most likely gave rise through Armenoceras to Gonioceras
in the Chazyan, and later in the Middle Ordovician possibly through either an early Armenoceras or Nybyoceras to Actinoceras
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 belonging to the Order Actinocerida
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...
.
The Armenoceratidae, established by Troedsson (1926) is characterized by large, straight or slightly curved shells and large 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...
s with strongly expanded segments between the septa. Septal necks are short and abruptly recurved along brims. Radial canals in the endosiphuncular canal system are typically arched, curving forward and backward from near the septal foramina (openings) to connect with the parispatium on either side of the middle of each segments. The parispatium is the narrow opening between the inner side of the connecting rings in actinocerids and the internal siphuncular deposits that grow forward and back from the region of the septal openings.
The Armenoceratidae have their beginning in Armenoceras
Armenoceras
Armenoceras is the principal genus of the Armenoceratidae, ranging from the late Whiterockian Stage in the early Middle Ordovician, through the remainder of the period and on into the Upper Silurian....
which first appeared near the beginning of the Chazyan, 2nd stage of the Middle Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
in older established chronologies, in northeastern China, Manchuria., derived from Wutinoceras
Wutinoceras
Wutinoceras, orthoconic actinocerids with ventral siphuncles composed of broadly expanded segments.and the type genus of the Wutinoceratidae. -Distinguishing Characters:...
. Additional genera include Nybyoceras
Nybyoceras
Nybyoceras is an actinocerid genus assigned to the Armenoceratidae and similar to Armenoceras except for having a siphuncle close to the ventral side of the shell.-Morphology:...
and Selkirkoceras, respectively from the upper Middle and Upper Ordovician, and Monocyrtoceras, Elrodoceras, and Megadiscoceras from the Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
,
The Armenoceratidae, Armenoceras, Nybyoceras, and Selkirkoceras first appear in North America in the diverse cephalopod Redriveran faunas of the early Upper Ordovician but are no longer found during that stage in Asia. Selkirkoceras is a large armenocerid from the Upper Ordovician with a blunt, flattened, somewhat breviconic shell.
Among the Silurian genera, Elrodoceras has a large shell with the apical part slightly curved, otherwise is straight, and a siphuncle that is narrower than in Armenoceras. Monocyrtoceras has a siphuncle lke that of Elrodoceras, but the entire shell is gently and evenly curved. Megadiscosorus is similar to Armenoceras but more breviconic and slightly exogastrically cyrtoconic and with the siphuncle in contact with the ventral wall.
The Armenoceratidae most likely gave rise through Armenoceras to Gonioceras
Gonioceras
Gonioceras is an actinocerid with a broad, low shell; flattened ventrally, convexly rounded dorsally; top and bottom meeting at an acute angle along the sides. In most the shell is rather thin, especially along the lateral portion. The aperture is contracted...
in the Chazyan, and later in the Middle Ordovician possibly through either an early Armenoceras or Nybyoceras to Actinoceras
Actinoceras
Actinoceras is the principal and root genus of the Actinoceratidae, a major family in the Actinocerida, that lived during the Middle and Late Ordovician.- Morphology:...