Ammonoidea

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  [[Cephalopoda|<==]]Ammonoidea
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The Subclass Ammonoidea comprises externally shelled (ectocochliate) cephalopods with a common ancestor in the Bactritida. The Ammonoidea are characterized by moderately to hightly complex sutures,  a narrow tubular siphuncle that is generally, though not exclusively , positioned along the ventral margin, and at least in more advanced forms forward projecting prosiphonate septal necks and convexly curved forward septa. 
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Ammonoids produced highly varied shells. Most are planospiral; a few became uncoiled, others are trochoidal (assymetrically coiled along a spire), and still others, the heteromorphs, changed their form radically during ontogeny. Common, mostly planospiral, ammonoid shells may be evolute with all whorls exposed and hardly any dorsal impression or in varying degrees Involute with the outer whorl covering most if not all of the inner whorls. Some are discoidal or lenticular, others subglobose to subspherical.  Some are smoooth, others ornamented with ribs, nodes, tubercles, lirae, and/or ventral keels or grooves.
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==Taxonomy==
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The Ammonoidea are divided (Teichert 1988) into the following 10 orders.
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:Bactritida L Dev - U Perm
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:Anarcestida L Dev - U Dev
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:Goniatitida M Dev - (?) L Trias
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:Clymeniida U Dev
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:Prolecanitida  L Carb - L Trias
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:Ceratitida L Perm - U Trias
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:Phyllocerida U Trias - U Cret
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:Lytocerida L Jura -  U Cret
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:Ammonitida    L Jura - U Cret
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:Ancylocerida  U Jura - U Cret
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This follows the taxonomy in the Treatise, Part L, except that there the ancylocerids are a superfamily, the Ancylocerataceae within the Lytocerina and that the Bactritida are placed in the Bactritoidea, discussed in Part K.  Also in the Treatise the principal subdivision are shown as suborders rather than orders.
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[[user:John M|JM]]
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[[Cephalopoda|<==]]Ammonoidea
     |  ''i. s.'': Adrianitinae [Adrianitidae, Adrianitoidea]
     |  ''i. s.'': Adrianitinae [Adrianitidae, Adrianitoidea]
     |          |--''Neocrimites'' Ruzhencev 1940 [incl. ''Metacrimites'' Ruzhencev 1950]
     |          |--''Neocrimites'' Ruzhencev 1940 [incl. ''Metacrimites'' Ruzhencev 1950]

Revision as of 19:06, 2 August 2009

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