Actinoceras

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Actinoceras
Temporal range: M Ordovician - L Silurian
Actinoceras OUMNH.png
Cast of a specimen discovered at the Newbury Marble Works, Derbyshire, England that is part of the OUMNH coll.
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Subclass: Nautiloidea
Order: Actinocerida
Family: Actinoceratidae
Genus: Actinoceras
Bronn, 1885

Actinoceras is the principal and root genus of the Actinoceratidae, a major family in the Actinocerida, that lived during the Middle and Late Ordovician. It is an extinct genus of nautiloid cephalopod that thrived in the warm waters of the United States and England during the Paleozoic era.

Contents

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 Ormoceras 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)

Canal system

The canal system within the siphuncle in Arctinocers is of the single 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. Lambeoceras , 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.

Species

About 45 species have been described from North America, including Greenland 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.

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 Lambeoceratidae. The derivation of Actinoceras may be in an early Armenoceras or perhaps from an early Nybyoceras, also part of the Armenoceratidae.

Related Research Articles

<span class="mw-page-title-main">Siphuncle</span> Strand of tissue passing longitudinally through the shell of a cephalopod mollusk

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. In the case of the cuttlefish, the siphuncle is indistinct and connects all the small chambers of that animal's highly modified shell; in the other cephalopods it is thread-like and passes through small openings in the septa (walls) dividing the camerae (chambers). Some older studies have used the term siphon for the siphuncle, though this naming convention is uncommon in modern studies to prevent confusion with a mollusc organ of the same name.

Huronia is an actinocerid genus included in the Huroniidae along with Discoactinoceras and Huroniella,(Teichert 1964). Huronia is characterized by long siphuncle segments with the free part of the connecting rings only slightly inflated and by a narrow central canal and strongly curved radial canals located in the anterior part of each siphuncle segment

The Ellesmerocerida is an order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.

Lambeoceras is a genus of large actinocerids with a convexly lenticular cross section from the Upper Ordovician of North America and the sole representative of the family Lambeoceratidae.

Armenoceras is a genus of actinocerid nautiloid cephalopods whose fossils ranged from the late Whiterockian Stage in the early Middle Ordovician, through the remainder of the period and on into the Upper Silurian. It is the type genus of the family Armenoceratidae.

Gonioceras is an extinct genus of actinocerid nautiloidean cephalopods typified by 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. Sutures from broad ventral and dorsal lobes, more narrowly rounded ventro-lateral and dorso-lateral saddles, and sharp pointed lateral lobes; more complex than in later Lambeoceras. The siphuncle is typically subcentral but may be closer to the venter; armenocerid in form with short segments and very short brims and containing a straight endosiphuncular canal system.

<span class="mw-page-title-main">Actinocerida</span> Extinct order of molluscs

The Actinocerida are 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 canals and a narrow space along the inner side of the connecting ring known as a paraspatium. Septal necks are generally short and cyrtochoanitic, some being recumbent, some hook shaped. Most grew to lengths of about 60 to 90 cm but some, like the Huroniidae of the Silurian grew significantly larger.

The Reudemannoceratidae are the ancestral and most primitive of the Discosorida, an order of cephalopods from the early Paleozoic. The Reudemannoceratidae produced generally medium-sized endogastric and almost straight shells with the siphuncle slightly ventral from the center.

Winnipegoceras is an extinct nautiloid genus from the Ordovician belonging to the Order Discosorida.

Wardoceras is an extinct nautiloid genus from the late Early Ordovician of Western Utah, assigned to the orthocerid family, Michelinoceratidae

Westonoceras is an extinct nautiloid genus from the Discosorida that lived during the Middle and Late Ordovician that has been found in North America, Greenland, and Northern Europe. It is the type genus for the Westonoceratidae

Sactoceras is an extinct nautiloid cephalopod that lived during the Ordovician and Silurian in what would become North America, Europe, and Asia.

The Actinoceriatidae are a family of actinocerids named by Saemann in 1853 for those that grew to have large shells with blunt apices 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 Silurian. Actinocerids are generally straight-shelled nautiloid cephalopods with a siphuncle composed of expanded segments, typically with thin connecting rings, in which the internal deposits are penetrated by a system of canals

<span class="mw-page-title-main">Armenoceratidae</span> Extinct family of molluscs

The Armenoceratidae are a family of early Paleozoic nautiloid cephalopods belonging to the order Actinocerida.

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.

Macroloxoceras is a large pseuorthocerid from the upper Devonian of Central Colorado and Southern New Mexico with features resembling those found in actinocerids. Pseudorthocerids and actinocerids are extinct nautiloid cephalopods, generally with long straight shells and expanded siphuncle segments filled with organic deposits.

Mandaloceratidae is a family in the nautiloid cephalopod order Discosorida, from the Middle and Upper(?) Silurian characterized by short, essentially straight shells referred to as breviconic, typically with a faintly exogastric shape produced by the profile of the body chamber.

Westonoceratidae are exogastric, mostly compressed, Discosorida of moderate size from the Middle Ordovician to the Lower Silurian.

Kochoceras is an extinct nautiloid genus from the later Ordovician belonging to the family Actinoceratidae and found in North America.

Discosoridae comprise a family of endogastric discosorids, with endocones in the siphuncle, ranging from the Middle Silurian to Middle Devonian.

References