Discosorida

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(linked nautiloid and cephalopod)
(Diagnosis)
 
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==Diagnosis==
==Diagnosis==
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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 renforced 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.
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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.  
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.  
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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.  
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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.
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==Derivation==
==Derivation==

Latest revision as of 16:11, 29 November 2013

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