Oncocerida

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==Introduction and Description==
==Introduction and Description==
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The '''Oncocerida''' constitute an order of [[Nautiloidea|nautiloid]] [[cephalopoda|cephalopods]] from the [[Ordovician]], [[Silurian]], and [[Devonian]] in which the earliest produced compressed [[cephalopod shell morphology|exogastric cyrtocones]], but which evolved to produce virtually straight breviconic, gyroconic, torticonic, and nautiliconic shells; most exogastric, some slightly or notably [[cephalopod shell morphology|endogastric]]. The [[siphuncle]] is commonly in the ventral part of the shell, in most marginal or submarginal. In the juvenile portion of earlier forms, [[siphuncle|septal necks]] are short and orthochoanitic and connecting rings are thin and tubular. In the later stages of earlier forms and throughout in most advanced forms septal necks are [[siphuncle|cyrtochoanitic]] and the connecting rings are expanded, at least on one side, into the chambers, producing an outline described as "beaded', "cylindroid", "ellipsoidal", "ovoid:. or "nummuloida". A number of genera are referred to as being actinosiphonate, in which the connecting rings are thickened internally in radially dispersed longitudinal blade-like lamellae.  
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The '''Oncocerida''' constitute an order of [[Nautiloidea|nautiloid]] [[cephalopoda|cephalopods]] from the [[Ordovician]], [[Silurian]], and [[Devonian]] in which the earliest produced compressed [[cephalopod shell morphology|exogastric cyrtocones]], but which evolved to produce virtually straight breviconic, gyroconic, torticonic, and nautiliconic shells; most exogastric, some slightly or notably [[cephalopod shell morphology|endogastric]]. The [[siphuncle]] is commonly in the ventral part of the shell, in most marginal or submarginal. In the juvenile portion of earlier forms, [[siphuncle|septal necks]] are short and orthochoanitic and connecting rings are thin and tubular. In the later stages of earlier forms and throughout in most advanced forms septal necks are [[siphuncle|cyrtochoanitic]] and the connecting rings are expanded, at least on one side, into the chambers, producing an outline described as "beaded', "cylindroid", "ellipsoidal", "ovoid:. or "nummuloidal". A number of genera are referred to as being actinosiphonate, in which the connecting rings are thickened internally in radially dispersed longitudinal blade-like lamellae.  
==Phylogeny==
==Phylogeny==
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==References==
==References==
* Sweet, Walter C, 1964. Nautiloidea -Oncocerida' Treatise on Invertebrate Paleontology, Part K. Geological Society of America and University of Kansas press.
* Sweet, Walter C, 1964. Nautiloidea -Oncocerida' Treatise on Invertebrate Paleontology, Part K. Geological Society of America and University of Kansas press.
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John M 12/14/09

Revision as of 16:06, 14 December 2009

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