Tarphycerida

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'''Tarphycerida''' are coiled and exogastric [[nautiloid]] [[cephalopod]]s known from the [[Lower Ordovician]] to the [[Upper Devonian]], derived from ''[[Bassleroceras]]'', or some similar form from the [[Ellesmerocerida]]. Early tarphycerids from the [[Lower Ordovician]] (middle and upper [[Canadian]]) have tubular [[siphuncle]]s with thick-walled,often layered connecting rings. The position of the siphuncle, a taxonomic indicator, may be ventral, central, or dorsal. Later, in the [[Middle Ordovician]], connecting rings became thinner and homogenous in some forms, resulting in what once was a separate order, the [[Barrandeocerida]]. These later derivates, found to have multiple tarphycerid ancestors are now encorporated within the [[Tarphycerida]]  
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'''Tarphycerida''' are exogastrically coiled [[nautiloid]] [[cephalopod]]s known from the Lower [[Ordovician]] to the Upper [[Devonian]], derived from ''[[Bassleroceras]]'', or some similar form from the [[Ellesmerocerida]]. Early tarphycerids from the Lower [[Ordovician]] (middle and upper Canadian Epoch) have tubular [[siphuncle]]s with thick-walled,often layered connecting rings. The position of the siphuncle, a taxonomic indicator, may be ventral, central, or dorsal. Later, in the Middle Ordovician, connecting rings became thinner and homogeneous in some forms, resulting in what once was a separate order, the [[Barrandeocerina|Barrandeocerida]]. These later derivatives, found to have multiple tarphycerid ancestors are now encorporated within the [[Tarphycerida]]  
Tarphycerids were the first  coiled cephalopods, a condition that evolved again in the [[Nautilida]] and [[Ammonoidea]] and in a lesser but more diverse manner in the [[Oncocerida]].  The evolutionary sequence began with an exogastric form like Basslerocers, in which at some early stage the curvature continued beyond the basic arc and eventually grew for more than one volution, producing an open spiraled gyrocone. Here we have the first undeniable tarphycerid. A similar sequence can be shown form the nautilids and for the ammonoids.  
Tarphycerids were the first  coiled cephalopods, a condition that evolved again in the [[Nautilida]] and [[Ammonoidea]] and in a lesser but more diverse manner in the [[Oncocerida]].  The evolutionary sequence began with an exogastric form like Basslerocers, in which at some early stage the curvature continued beyond the basic arc and eventually grew for more than one volution, producing an open spiraled gyrocone. Here we have the first undeniable tarphycerid. A similar sequence can be shown form the nautilids and for the ammonoids.  

Revision as of 13:49, 16 December 2009

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