Michelinoceratida

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The Michelinoceratida or Orthocerida were a Lower Ordovician to Upper Triassic assemblage of orthoconic or cyrtoconic cephalopods. This is perhaps the most systematically difficult group of fossil cephalopods and may be polyphyletic, with recent authors suggesting that some families (particularly the Pseudorthoceratidae) should be removed from the order.
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The Michelinoceratida is a Lower Ordovician (Cassinian) to Upper Triassic assemblage of orthoconic and cyrtoconic nautiloid cephalopods that includes both orthoconic and cyrtoconic forms with varied ontogeneses, lending to the idea this group may be polyphyletic. Systematics are particularly subject to revision, with recent authors suggesting that some families (particularly the Pseudorthoceratidae) should be removed.  
'''Characters''' (from Sweet 1964): Conchs straight to curved (never coiled), smooth or elaborately ornamented; cameral deposits typically well developed; siphuncles with orthochoanitic or secondarily cyrtochoanitic septal necks and thin connecting rings, empty or occupied by parietal or annulosiphonate deposits.
'''Characters''' (from Sweet 1964): Conchs straight to curved (never coiled), smooth or elaborately ornamented; cameral deposits typically well developed; siphuncles with orthochoanitic or secondarily cyrtochoanitic septal necks and thin connecting rings, empty or occupied by parietal or annulosiphonate deposits.
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The question regarding whether the michelinoceratids are monophyletic or  polyphyletic has to do with the derivation of cyrtochoanitic forms which make up the pseudorothoceratoids. and whether they form a monoophyletic group with a separate derivation from that of the orthochoanitic Michelinoceratida (Orthocerida sensu strictu). Sweet (1964) divides the Orthocerida (=Michelinoceratida Flower) into the Orthocerataceae, established by M'Coy, 1844 and the  Pseudorthocerataceae, established by Flower & Caster, 1936.  Siphuncle segments in the Orthocerataceae are typically straight and tubular, with orthochoanitic septal necks. Those in the Pseudorocerataceae are typically expanded , with cyrtochoanitic necks. The connecting rings in both are thin.
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Flower, 1976, showed that the Mysterioceratidae, Proteoceratidae, and Stereoplasmoceratidae (Pseudorthocerataceae sensu Sweet, 1964) were derived from the Michelinoceratidae during the Middle Ordovician, while the Pseudorthceratidae arose independently during the Silurian. 
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==Taxonomy==
==Taxonomy==
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==References==
==References==
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[D82] Dong D.-Y. 1982. Lower Ordovician stromatoporoids of northern Anhui. ''Acta Palaeontologica Sinica'' 21 (5): 577-583.
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Rousseau  H. Flower 1976. Ordovician Cephalopod Faunas And Their Role in Correlation. The Ordovician System, proceedings of a Palaeontogical Society symposium, Birmingham, Sept 1974.  
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[P68] Purnell, L. R. 1968. Catalog of the Type Specimens of Invertebrate Fossils. Part I: Paleozoic Cephalopoda. ''United States National Museum Bulletin'' 262: 1-198.
[P68] Purnell, L. R. 1968. Catalog of the Type Specimens of Invertebrate Fossils. Part I: Paleozoic Cephalopoda. ''United States National Museum Bulletin'' 262: 1-198.
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'''Credits'''
'''Credits'''
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[[User:Christopher|Christopher]] 07:41, 30 January 2011 (UTC)
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[[User:Christopher|Christopher]] 07:41, 30 January 2011 (UTC) <br>
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[[User:John M|John]] May 2012

Revision as of 01:34, 24 May 2012

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