Michelinoceratida

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The Michelinoceratida  is a Lower Ordovician (Cassinian) to Upper Triassic assemblage of orthoconic and cyrtoconic nautiloid cephalopods 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.  
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The Michelinoceratida  is a Lower Ordovician (Cassinian) to Upper Triassic order of nautiloid cephalopods. 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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'''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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Families within the Michelinoceratida include the [[Michelinoceratidae]], [[Geisonoceratidae]], [[Clinoceratidae]],
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[[Orthoceratidae]],[[Proteoceratidae]], [[Sactorthoceratidae]], and possibly the [[Brachycycloceras|Brachycyloceratidae]].  
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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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The Michelinoceratidae is the ancestral group, which along with the Geisonoceratidae is the most persistant. Derivation is from the [[Baltoceratidae]].
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The [[Pseudorthoceratidae]], in Sweet,1964, are removed as the [[Pseudorthocerida]] with subfamilies elevated to family status. The Orthocerataceae and Pseudorthocerataceae (Sweet, 1964) are abandoned as meaningless.  
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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. 
 

Latest revision as of 12:04, 24 May 2012

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