Valcouroceratidae

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(Taxonomy: added discussion)
(added systematics, diagnosis section)
 
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The Valcouroceratidae were mostly breviconic (short-shelled) oncocerid cephalopods from the Middle to Upper Ordovician. In mature individuals, bladelike lamellae called actinosiphonate deposits developed within the siphuncle (Sweet 1964).
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<u>Systematics</u> <br/>
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Class: [[Cephalopoda]]<br/>
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Subclass: [[Nautiloidea]] <br/>
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Order: [[Oncocerida]] <br/>
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Family: Valcouroceratidae <br/>
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'''Characteristics''' (from Sweet 1964): Exogastric cyrtocones and brevicones with ventral, actinosiphonate siphuncle; conch changes during ontogeny from compressed to depressed or subtriangular section.
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The Valcouroceratidae are mostly breviconic (short-shelled) [[Oncocerida|oncocerid]] cephalopods from the Middle to Upper Ordovician.
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==Diagnosis==
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Shells are exogastric cyrtocones and brevicones which during ontogeny change from compressed to depreed or subtiangular in section. The siphuncle is ventral, which in mature individuals have bladelike lamellar actinosiphonate deposits.  
==Taxonomy==
==Taxonomy==
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''Valcouroceras'', the type genus, named by Rousseau Flower, 1943, for Valcour Island on Lake Champlain, where first found, has the strongest curviture (exogastric), and greatest rate of expansion of any of these.  ''Manitoulinoceras'' named by Foeste,1924, is perhaps the most opposite in that it has the slowest rate of expansion of the eight. ''Kindleoceras'' is similar but has a triangular cross section. That of ''Manitoulinoceras'' is subcircular. ''Augustoceras'', ''Fayettoceras'', ''Minganoceras'', and ''Staufferoceras'' are intermediate, with moderately curved exogastric cyrtocones.
''Valcouroceras'', the type genus, named by Rousseau Flower, 1943, for Valcour Island on Lake Champlain, where first found, has the strongest curviture (exogastric), and greatest rate of expansion of any of these.  ''Manitoulinoceras'' named by Foeste,1924, is perhaps the most opposite in that it has the slowest rate of expansion of the eight. ''Kindleoceras'' is similar but has a triangular cross section. That of ''Manitoulinoceras'' is subcircular. ''Augustoceras'', ''Fayettoceras'', ''Minganoceras'', and ''Staufferoceras'' are intermediate, with moderately curved exogastric cyrtocones.
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==Phylogeny==
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The phylogeny within the Oncocerida is difficult to acertain.  Whether the family is monophyletic, derived from some genus in the [[Graciloceratidae]], or even [[Bassleroceratidae]] or polyphyletic, derived from different genera in either or both of these is unknown.
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''Valcouroceras'', ''Augustoceras'', and ''Minganoceras'' are known only from the Middle Ordovician. Of these the strongly curved ''Valcouroceras'' and ''Augustoceras'' with its widely spaced septa may be dead end off shoots.  ''Minganoceras'' may have given rise to the ?middle and upper Ordovician ''Kindleoceras'' through the developments of a flattened dorsum and keel-like venter resulting in a triangular cross section, as well as to the middle and upper Ordovician ''Staufferoceras'' and ''Fayetoceras''.  ''Kindleoceras'' may in turn have given rise  though development of a more strongly curved, gently expanding shell, with a more rounded cross section to ''Manitoulinoceras'', known only from the Upper Ordovician.
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The inclusion of ''Actinomorpha'', which may belong to the [[Diestoceratidae]], is doubtful.
==References==
==References==
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'''Credits'''
'''Credits'''
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[[User:Christopher|Christopher]] 22:46, 14 January 2011 (UTC)
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[[User:Christopher|Christopher]] 22:46, 14 January 2011 (UTC) <br/>
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[[user: John M|JM]] 21 Nov. 2012 (Genera, phylogeny)

Latest revision as of 19:33, 6 October 2014

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