Oncoceratidae

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As defined by Sweet (1964), the Oncoceratidae is ancestral to a number of other oncocerid families; indeed, the oncoceratid lineage probably survives to the modern day in the Nautilidae. The Oncoceratidae excluding the descendant families were mostly cyrtoconic.
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The Oncoceratidae is a family of Middle Ordovician to Upper (Late) Silurian  [[Oncocerida|Oncocerids]] characterized by exogastric shells, typically compressed but with a few of circular or depressed section. Siphuncles are empty and ventral; tubular and suborthochoantic in early stages of early forms, becoming cyrtochoanitic with expanded segments in later stages, and throughout the entire phragmocone in later forms. A few later forms did manage to develop actinosiphonate deposits.
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The Oncoceratidae appeared almost simultaneously with the [[Graciloceratidae]], [[Tripteroceratidae]], and [[Valcouroceratidae]] early in the Middle Ordovician. Derivation is most likely from the Graciloceratidae which are the most primitive of the four. Unlike the other three, which are restricted to the Ordovician, the Oncoceratidae continued well into the Silurian.
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According to Sweet (1964), the Oncoceratidae are ancestral to a number of other oncocerid families. The Ordovician [[Diestoceratidae]] may have its origin in the oncoceratid ''Oncoceras''. The Silurian [[Hemiphragmoceratidae]] and [[Trimeroceratidae]] probably have a similar origin in the Oncoceratidae. ''Oonoceras'', another oncoceratid, likely gave rise to the [[Brevicoceratidae]] in the Middle Silurian. The [[Polyelasmoceratidae]] and [[Acleistoceratidae]] can also be traced back to the Oncoceratidae.
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'''Characters''' (from Sweet 1964): Exogastric, ventromyarian brevicones and cyrtocones; siphuncle empty, on ventral side of chamber. Siphuncles suborthochoanitic and cylindrical in early stages of early forms, cyrtochoanitic with expanded segments in later stages and later forms. Connecting rigns thin. Conch usually compressed, few species with circular or depressed section. Actinosiphonate deposits in a few advance forms.
 
==References==
==References==
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[S64] Sweet, W. C. 1964. Nautiloidea – Oncocerida. In ''Treatise on Invertebrate Paleontology'' pt. K. ''Mollusca 3. Cephalopoda – General Features – Endoceratoidea – Actinoceratoidea – Nautiloidea – Bactritoidea'' (R. C. Moore, ed.) pp. K277-K319. The Geological Society of America and the University of Kansas Press.
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Sweet, W. C. 1964. Nautiloidea – Oncocerida. In ''Treatise on Invertebrate Paleontology'' pt. K. ''Mollusca 3. Cephalopoda – General Features – Endoceratoidea – Actinoceratoidea – Nautiloidea – Bactritoidea'' (R. C. Moore, ed.) pp. K277-K319. The Geological Society of America and the University of Kansas Press.
'''Credits'''
'''Credits'''
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[[User:Christopher|Christopher]] 00:03, 16 January 2011 (UTC)
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[[User:Christopher|Christopher]] 16 Jan. 2011 <br/>
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[[User:John M|John]] Modified and expanded 7 Oct 2014

Latest revision as of 19:10, 7 October 2014

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