Discosorida

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Discosorida
Temporal range: mid Ordovician–late Devonian
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Subclass: Nautiloidea
Order:† Discosorida

Discosorida are an order of cephalopods that lived from the beginning of the Middle Ordovician, through the Silurian, and into the Devonian. Discosorids are unique in the structure and formation of the siphuncle, the tube that runs through and connects the camerae (chambers) in cephalopods, which unlike those in other orders is zoned longitudinally along the segments rather than laterally. Siphuncle structure indicated that the Discosorida evolved directly from the Plectronoceratida rather than through the more developed Ellesmerocerida, as did the other orders. Finally and most diagnostic, discosorids developed a reinforcing, grommet-like structure in the septal opening of the siphuncle known as the bullette, formed by a thickening of the connecting ring as it draped around the folded back septal neck.

Contents

Evolution

The origin of the Discosorida is unknown, thought at one time [1] to be directly from the Plectronocerida. Evolution within the order begins with the lower Middle Ordovician Reudemannoceratidae and from there diverges into three main lineages. [2] [3] Questionable discosorids have been reported as early as the Middle Tremadocian - near the start of the Ordovician, however the first bona fide examples date to the Middle Ordovician. [4]

The diversification of the Discosorida, in terms of genera, peaked at the beginning in the Middle Ordovician (modern Darriwilian stage) followed by a decline in the Upper Ordovician (modern Sandbian and Katian stages) only to peak again in the Middle Silurian. Afterwards their diversity declined drastically and remained low until their end in the late Devonian. [3] Some were endogastrically curved, with the lower, siphuncle side concave, others were exogastrically curved with the same side convex. In some, the aperture was a simple opening. In others, it became contracted into a pattern of slits. In earlier, Ordovician forms, the bullette became quite large and readily noticeable. In later forms, the bullette became reduced, in some to the point of being vestigial.

The Discosoridae, one of the last families to evolve, found in Silurian and questionably in Devonian rocks, are characterized by a rapidly expanding siphuncle with segments that extend into the adjacent chambers, and parietal deposits within the siphuncle that overlap to form endocones. [3]

Taxonomy

The Discosorida [1] [2] include these families, more or less in phylogenetic sequence beginning with the oldest:

Reudemannoceratidae
Cyrtogomphoceratidae
Westonoceratidae
Phragmoceratidae
Lowoceratidae
Discosoridae
Mandaloceratidae
Mesoceratidae

These form three basin evolutionary lineages. The first, formed by the Reudemannoceratidae, Cyrtogomphoceratidae, and Phragmoceratidae, are fundamentally endogastric with the siphuncle near the inside or longitudinally concave curvature. The second, formed by the Westonoceratidae, Lowoceratidae, and Discosoridae, are fundamentally exogastric with the siphuncle near the outside or longitudinally convex curvature, although the Discosoridae are somewhat different. The third, consisting of the Mandaloceratidae and Mesoceratidae are basically straight (orthoconic). Families differ primarily in the structural details of the siphuncle and in the nature of the aperture.

Ecology

Discosorids were probably benthic forms that crawled over the bottom in search of food or safety, or hovered close to the bottom. The general orientation during life was most likely head down, with the aperture of the shell facing the general direction of the sea floor and shell carried above. Nothing is known of what the animal itself may have looked like; how many tentacles they had and relative length or how well they may have seen.

In general form the Discosorida resembled the Oncocerida, which lived about the same time, but evolved from a completely different stock. The two convergent groups differ in their internal detail.

Related Research Articles

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

Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived.

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.

Discosorus, a genus of the Discosorida and member of the family Discosoridae. Not to be confused with Discoceras. Discosorus consists of rapidly expanding endogastric brevicones, mostly known from isolated siphuncles composed of broadly expanded segments that increase rapidly in size. Septal necks are recumbent, connecting rings thin, bullettes small. The siphuncle interior is occupied by endocones produced by layered annular deposits expanded back toward the apex, leaving a central tube running down the middle. The short phragmocone is poorly known.

The Lowoceratidae is a small family of discosorids, early nautiloid cephalopods, from the Middle Silurian in which the characteristic bullette is found only in early growth stages. Lowoceratids were first found in Southampton Island in the Canadian arctic.

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

Ulrichoceras is recognised as the basal cyrtogomphoceratid genus, which is the source for the rest of the Cyrtogomphoceratidae as well as for the Westonoceratidae. The Cyrtogomphoceratidae are endogastric, the Westonoceratidae, exogastric.

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

<span class="mw-page-title-main">Oncoceratidae</span> Extinct family of nautiloids

Oncoceratidae is a family of nauatiloid cephalopods in the order Oncocerida established by Hyatt, 1884, that range from the Middle Ordovician to the Upper Silurian.

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

The Phragmoceratidae is a family of extinct nautiloid cephalopods from the Order Discosorida that lived during the latter part of the Silurian.

Landeroceras is a genus of straight shelled cyrtogomphoceratid from the Middle Ordovician Big Horn dolomite of Wyoming.

The Cyrtogomphoceratidae are a family in the cephalopod order Discosorida that comprises genera commonly with compressed, endogastrically curved shells. Siphuncles lie close to the ventral side, segments are broadly inflated, connecting rings thick and apically expanded thick bullettes. Chambers are short, separated by shallow, dish shaped septa. Apertures are generally simple.

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

Teichertoceras is a discosorid genus in the family Westonoceratidae characterized by an endogastric curvature to the early portion of the phragmocone.

Hectoceras is a genus in the nautiloid cephalopod order Discosorida from the Upper Ordovician of Australia (Tasmania), known from a few isolated siphuncle specimens.

Reedsoceras is a genus of large discosorids (Nautiloidea) in the family Westonoceratidae from the middle and upper Ordovician of North America.

Simardoceras is a genus in the discosorid family Westonoceratidae from the Middle Ordovician of Quebec.

Sinclairoceras is a nautiloid cephalopod in the discosorid family Westonoceratidae from the Middle Ordovician Simard Limestone of Quebec. The shell is laterally compressed, upwardly curved, and short; an exogastric brevicone with the venter narrowly and dorsum broadly rounded. The dorsum, at the upper surface, is only slightly concave longitudinally. The venter, at the lower surface, is more strongly convex longitudinally, drawing away from the dorsum and forming a hump at the anterior end of the phragmocone where Sinclairoceras reaches its greatest width.

Apocrinoceratidae constitutes a family of Middle Ordovician nautiloid cephalopods characterized by straight or slightly curved, transversely ribbed shells having siphuncles composed of expanded segments, short recurved septal necks, and thick connecting rings. Derivation is from the Protocycloceratidae, a family of ellesmerocerids, which differ in having straight or concave siphuncle segments, but are otherwise similar in form.

Cyrtogomphoceras is a genus of nautiloid cephalopods, recognized by its large breviconic shell with a notable endogastric curvature. The shell is fusiform in profile, reaching maximum width at or near the base of body chamber, which narrows toward the aperture. The siphuncle is large and slightly removed from the ventral side, that with the concave longitudinal profile. Siphuncle segments are short, as are chambers; septal necks recurved, connecting rings thick, bullettes at the apical end of the rings swollen. Cameral deposits are lacking.

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

References

  1. 1 2 Rousseau H. Flower. 1964. The Nautiloid Order Ellesmerocerida (Cephalopods); relevant pages. Memoir 12, New Mexico Bureau of Mines and Mineral Resources, Socorro, NM.
  2. 1 2 Rousseau H. Flower and Curt Teichert. 1957. The Cephalopod Order Discosorida; University of Kansas Paleontological Contributions, Mollusca, Article 6.
  3. 1 2 3 Curt Teichert. 1964. Nautiloidea -Discosorida; Treatise on Invertebrate Paleontology, Vol. K, pp. K320.
  4. ↑ Bjorn Kroger,Yun-Bai, Zhang. 2008. Pulsed cephalopod diversification during the Ordovician. Palaeogeography, Palaeoclimatology, Palaeoecology 273: 174.
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