Nautilida

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(Conch Shape: orthochoaitic to siphuncle)
(Phylogenetic Relationships: revised for emphasis)
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[[Image:Ptenoceras alatum.gif|thumb|680px|center|The early nautilid ''[[Ptenoceras alatum]]'' (Barrande), Early Devonian, Czechoslovakia. A, Dorsal view; B, lateral view; C, longitudinal section of siphuncle (After Barrande, 1865, from Teichert, 1967, from Teichert 1988).]]
[[Image:Ptenoceras alatum.gif|thumb|680px|center|The early nautilid ''[[Ptenoceras alatum]]'' (Barrande), Early Devonian, Czechoslovakia. A, Dorsal view; B, lateral view; C, longitudinal section of siphuncle (After Barrande, 1865, from Teichert, 1967, from Teichert 1988).]]
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Although the ancestors of the Nautilida have previously been considered among straight-shelled (''"[[Orthoceras]]"'') [[Cephalopoda|cephalopods]] which became gradually coiled nautilids (Schindewolt, 1942), or from coiled [[Silurian]] forms of the [[Barrandeocerina|barrandeocerine]] [[Tarphycerida|tarphycerids]] ([[Barrandeoceratidae]] Flower & Kummel 1950 or [[Lechritrochoceratidae]] Dzik 1984, it is now generally accepted that the [[Oncocerida]] gave rise to the order Nautilida. Teichert (1988) suggests that the best candidates would be the [[Acleistoceratidae]] and the [[Brevicoceratidae]], both of whom have [[exogastric]]ally curved or coiled shells and [[cyrtochoanitic]] [[septa]]l necks, characteristic of early rutoceratid and ptenoceratid Nautilida.
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The  Nautilida are generally thought to be derived from the [[Oncocerida]], through the Rutoceratidae. Teichert (1988) suggested that the best candidates would be the Acleistoceratidae and the Brevicoceratidae, both of which have [[Cephalopod shell morphology|exogastrically]] curved or coiled shells and [[siphuncle|cyrtochoanitic]] septal necks, characteristic of early rutoceratid and ptenoceratid nautilids. Dzik (1984) previously proposed the alternative oncocerid Lechritrochoceratidae.
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Earlier ideas include evolution from straight-shelled ("[[Orthocerida|Orthoceras]]") cephalopods which became progressively more and more coiled (Schindewolf, 1942), or from already coiled Silurian [[Barrandeocerina|barrandeocerine]] [[Tarphycerida|tarphycerids]] (Barrandeoceratidae Flower & Kummel 1950)
==Evolutionary History==
==Evolutionary History==

Revision as of 14:23, 14 December 2009

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