Discosorida

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The Discosorida is an order of nautiloid cephalopods from the early Paleozoic characterized by their rather unique siphuncle in which the connecting rings are zoned along their length and hook into the previous septal foramina as well as being expanded into the chambers. Discosorids produced shells that were generally breviconic or cyrtoconic. Cyrtoconic shells were typically compressed and were either endogastric with the venter longitudinally concave or exogastric with the venter longtiuinally convex.

Contents

Diagnosis

The discosorid siphunlce is composed of generally recumbent septal necks connected by thick, laterally expanded connecting rings. Septal necks are cyrtochoanitic and often crimped back or recumbent. The space formed between the back side of the septum and the brim formed by the folded back neck is reinforced by dense, amorphous calcite in a structure called the vinculum. The forward end of the connecting ring attaches to the vinculum. This part of the connecting ring, where free standing, is composed of granular calcite, and takes up about half its length. Behind this, in the posterior part of the free standing connecting ring is a zone of yellowish organic material, known either as the chitiniferous zone or the conchiliniferous zone, and bordered at either end by dark amorphous calcite as between parentheses.

One of the diagnostic features of the Discosorida is a structure in the siphuncle called the bullette. The Bullette is formed where the connecting ring wraps around the previous septal neck, filling part of the septal opening. In cross section bullettes appear as bulbous expansions. True bullettes, found only in the Discosorida, are layered. There is a layer of dense amorphous calcite that attaches to the septal neck overlain by a layer of laminar calcite. The amorphous calcite is external with respect to the middle of the siphuncle, the laminar calcite, internal.

Some orthocerids and pseudorthocerids have annular deposits in the septal openings, but these are homogenous and are not part of the basic siphuncle structure. Some oncocerids have expanded connecting rings where they connect to the previous septum, but these too lack the diagnostic layered structure of the discosorid bullette.

Derivation

The earliest known discosorid is Reudemannoceras which comes from the lower Middle Ordovician (Whitrockian age) strata of the Champlain Valley of New York State in North America. The closest species with respect to similar features is the Upper Cambrian Plectronoceras. Both have connecting rings that are expanded where not restricted as siphuncular bulbs. The problem is that there are no Plectronocerids known to have crossed into the Lower Ordovician, or any Lower Ordovician forms known to have such features.

Taxonomic Comment

Interestingly enough, while the Actinocerida and Endocerida were placed in separate subclasses of their own by Curt Teichert, respectively the Actinoceratoidea and Endoceratoidea, something Flower rejected, no such treatment for afforded the no less unique Discosorida. Mary Wade, 1988, did divide the Nautiloidea into (a number of) superorders, and so established the Discosoritoidea to contain the Discosorida, as do the superorders Aciinceratoidea and Endoceratoidea respectively contain the Actinocerida and Endoceida

Phylogeny

The phylogeny of the Discosorida is along three evolutionary lines, beginning with the Reudemannoceratidae in the lower Middle Ordovician. From the primitive reudemannocerids came the compressed, endogastric Cyrtogomphoceratidae of the Middle and Upper Ordovician which gave rise to the Silurian Phragmoceratidae with their constricted apertures. The Cyrtogomphoceratidae also gave rise in the Middle Ordovician to the exogastric Westonoceratidae which gave rise to the short lived Lowoceratidae from the lowermost Silurian, which in turn gave rise to the Silurian Discosoridae. Finally the Reudemannoceratidae provide the most likely ancestry for the essentially orthoconic Silurian Mandeloceratidae and Mesoceratidae.


<==Discosorida  F&T57, T64
   `--Ruedemannoceratidae 
     |
     |--Mandaloceratidae    
     |    `--Mesoceratidae
     |
     `---Cyrtogomphoceratidae     	 
       |  `--Phragmoceratidae
       |
      	`--Westonoceratidae
         `-Lowoceratidae
           `-Discosoridae

References

Flower, Rousseau H. and Curt Teichert, 1957; The Cephalopod Order Discosorida, in University of Kansas Paleontological Contributions, Mollusca Article 6, pp 1-144 (plates, figs) July1, 1957.

[P68] Purnell, L. R. 1968. Catalog of the Type Specimens of Invertebrate Fossils. Part I: Paleozoic Cephalopoda. United States National Museum Bulletin 262: 1-198.

[T64] Teichert, C. 1964. Nautiloidea – Discosorida. In Treatise on Invertebrate Paleontology pt. K. Mollusca 3. Cephalopoda – General Features – Endoceratoidea – Actinoceratoidea – Nautiloidea – Bactritoidea (R. C. Moore, ed.) pp. K320-K342. The Geological Society of America and the University of Kansas Press.

Wade. M, 1988; Nautiloids and their descendants: cephalopod classification in 1986; New Mexico Bureau of Mines and Mineral Resources Memooir 44, pp15-25, October 1988.

Credits

Descriptive text JM 8/01/09; dendrogram Christopher 23:57, 22 January 2011 (UTC)

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