Actinoceras
Encyclopedia
Actinoceras is the principal and root genus of the Actinoceratidae
, a major family in the Actinocerida
, that lived during the Middle and Late Ordovician
.
or Lambeoceras
but not as much as in Armenoceras
. (Flower 1957) The diameter typically becomes smaller with respect to that of the shell in the forward or anterior part of the phragmocone. Septal necks are long with wide cyrtochoanitic to recumbent brims. Connecting rings are thin.(Teichert 1964)
, in some Armenoceras
, Nybyoceras
, and in Gonioceras
. (Flower 1957). The parispatium is a narrow opening or seam that forms between the inside of the connecting rings and the endosiphunclar deposits that grow forward and back from the region of the septal foremina.
and the Canadian Arctic with Actinoceras margaretae, A. aequale, and A. gradatum the earliest known, coming from the lower Blackriveran Loweville fm of Ottawa. Actinoceras concavum from the Ssuyan of southern Manchuria
is most similar to Actinoceras centrale from the Chaumont of New York.
. The derivation of Actinoceras may be in an early Armenoceras or perhaps from an early Nybyoceras, also part of the Armenoceratidae
.
Kochoceras, named by Troedsson, 1926, is a middle to upper Ordovician actinocerid similar overall to Actinoceras but with the shell flattened on the venter and with the siphuncle in broad contact with the ventral margin. Kochoceras was first discovered in northern Greenland, which during the Ordovician was near equatorial.
Leurothoceras, named by Foeste, 1921, is an actincerid with a flattened shell and narrower siphincle than in Actinoceras. Siphuncel segments in this genus tend to become strongly elongated in the mature part of the phragmocone.
Paractinoceras, named by Hyatt, in Zittel 1900, is a long, straight, slender-shelled actinocerid with the siphuncle segments in the early stage expanded as in Actinoceras, but becoming narrow in the later stages as in Ormoceras
. Paractinoceras has been found in North America in Manitoba, and in Russia in Siberia.
Saffordoceras, named by Foeste and Teichert in 1930, is an actinicerid with a ventrally flattened shell, subventral siphuncle with short segments, and sutures with broad shallow ventral lobes and narrower lateral saddles. Saffordoceras comes from the Middle Ordovician of eastern North America.
Troostoceras, also named by Foeste & Teichert in 1930, is similar to Actinoceras but with a slightly cyroconic and endogastric shell. It is known from the Middle Ordovician of eastern North America and Siberia.
Actinoceratidae
The Actinoceriatidae is a family of actinocerids named by Saemann in 1853 for those that grew to have large shells with blunt apecies and large siphuncles with widely expanded segments and a generally arcuate endosiphucular canal system. Their range is from the upper Middle Ordovician to the Lower...
, a major family in the 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...
, that lived during the Middle and Late 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...
.
Morphology
Actinoceras are generally large, with typically straight shells reaching a meter or so in length (about 3 ft), with a blunt apex, and usually with a circular to subcircular cross section. .Shell characteristics
The shells of Actinoceras are generally straight and long, although some are breviconic. Some are fusiform with the diameter decreasing from the anterior end of the phragmocone toward the aperture. Chambers are short and contain cameral deposits which are more concentrated apically and ventrally. Septa are close spaced, sutures are mostly transverse. The siphuncle, which varies in proportion to the size of the shell among species, is ventral, but not on the ventral margin. (Flower 1957)Siphucle
The siphuncle, which is ventral of the center but away from the ventral margin, is generally large and composed of segments that are expanded into the chambers, more so than in OrmocerasOrmoceras
Ormoceras is an actinocerid genus of the family Ormoceratidae which is found in North America from the late Chazyan though the early Cincinnatian of the Middle and Upper Ordovician, but continued through the Devonian worldwide....
or Lambeoceras
Lambeoceras
Lambeoceras is a rather large actinocerid with a convexly lenticular cross section from the Upper Ordovician of North America, and the sole representative of the family Lambeoceratidae.-Morphological Description:...
but not as much as 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....
. (Flower 1957) The diameter typically becomes smaller with respect to that of the shell in the forward or anterior part of the phragmocone. Septal necks are long with wide cyrtochoanitic to recumbent brims. Connecting rings are thin.(Teichert 1964)
Canal system
The canal system within the siphuncle in Arctinocers is of the singe arc type wherein the radial canals branch off the central canal near the septal openings and sweep back and out, connecting to the parispatium in the preceding segments at their broadest expansion. This type is also found in the derivatives (descendants) of Actinoceras, e.g. LambeocerasLambeoceras
Lambeoceras is a rather large actinocerid with a convexly lenticular cross section from the Upper Ordovician of North America, and the sole representative of the family Lambeoceratidae.-Morphological Description:...
, in some 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....
, 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 in 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...
. (Flower 1957). The parispatium is a narrow opening or seam that forms between the inside of the connecting rings and the endosiphunclar deposits that grow forward and back from the region of the septal foremina.
Species
About 45 species have been described from North America, including GreenlandGreenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
and the Canadian Arctic with Actinoceras margaretae, A. aequale, and A. gradatum the earliest known, coming from the lower Blackriveran Loweville fm of Ottawa. Actinoceras concavum from the Ssuyan of southern Manchuria
Manchuria
Manchuria is a historical name given to a large geographic region in northeast Asia. Depending on the definition of its extent, Manchuria usually falls entirely within the People's Republic of China, or is sometimes divided between China and Russia. The region is commonly referred to as Northeast...
is most similar to Actinoceras centrale from the Chaumont of New York.
Phylogeny
Actinoceras is one of seven known genera in the Family Actinoceratidae.(Teichert 1964) Actinoceras is thought to have given rise to such genera as Kochoceras and Paractinoceras in the Actinoceratidae and to Lambeoceras of the LambeoceratidaeLambeoceras
Lambeoceras is a rather large actinocerid with a convexly lenticular cross section from the Upper Ordovician of North America, and the sole representative of the family Lambeoceratidae.-Morphological Description:...
. The derivation of Actinoceras may be in an early Armenoceras or perhaps from an early Nybyoceras, also part of the Armenoceratidae
Armenoceratidae
Armenoceratidae is a family of early Paleozoic nautiloid cephalopods belonging to the Order Actinocerida.The Armenoceratidae, established by Troedsson is characterized by large, straight or slightly curved shells and large siphuncles with strongly expanded segments between the septa. Septal necks...
.
Related genera
The following actinoceratid genera are closely related to Actinoceras although differing in various details.Kochoceras, named by Troedsson, 1926, is a middle to upper Ordovician actinocerid similar overall to Actinoceras but with the shell flattened on the venter and with the siphuncle in broad contact with the ventral margin. Kochoceras was first discovered in northern Greenland, which during the Ordovician was near equatorial.
Leurothoceras, named by Foeste, 1921, is an actincerid with a flattened shell and narrower siphincle than in Actinoceras. Siphuncel segments in this genus tend to become strongly elongated in the mature part of the phragmocone.
Paractinoceras, named by Hyatt, in Zittel 1900, is a long, straight, slender-shelled actinocerid with the siphuncle segments in the early stage expanded as in Actinoceras, but becoming narrow in the later stages as in Ormoceras
Ormoceras
Ormoceras is an actinocerid genus of the family Ormoceratidae which is found in North America from the late Chazyan though the early Cincinnatian of the Middle and Upper Ordovician, but continued through the Devonian worldwide....
. Paractinoceras has been found in North America in Manitoba, and in Russia in Siberia.
Saffordoceras, named by Foeste and Teichert in 1930, is an actinicerid with a ventrally flattened shell, subventral siphuncle with short segments, and sutures with broad shallow ventral lobes and narrower lateral saddles. Saffordoceras comes from the Middle Ordovician of eastern North America.
Troostoceras, also named by Foeste & Teichert in 1930, is similar to Actinoceras but with a slightly cyroconic and endogastric shell. It is known from the Middle Ordovician of eastern North America and Siberia.