Bellerophontacea

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The '''Bellerophontacea''' (=nov nom Bellerophonoidea) is a superfamily of protogastropods, more advanced than the monoplacophoran order Triblidoidea but not as evolved as the protobranch Pleurotomariacea or Murchisoniacea. Shells are by far mostly planospiral, coiled or cyrtoconic. A very few are slightly asymmetric Most all have some sort of emargination, sinus or slit, at the middle of the upper lip of the aperture. Their stratigraphic range is from the Upper Cambrian to the Lower Triassic.  
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The '''Bellerophontacea''' (=nov nom Bellerophonoidea) is a superfamily of protogastropods, more advanced than the monoplacophoran order Triblidoidea but not as evolved as the prosobranch Pleurotomariacea or Murchisoniacea. Shells are by far mostly planospiral, coiled or cyrtoconic. A very few are slightly asymmetric Most all have some sort of emargination, sinus or slit, at the middle of the upper lip of the aperture. Their stratigraphic range is from the Upper Cambrian to the Lower Triassic.  
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In the Treatise on Invertebrate Paleontology (here on simply the Treatise) Part I, Mollusca, Gastropoda, it is argued that the Bellerophontacea should be included in the protobranch gastropod order Archaeogastropoda, partly on the basis of certain similarities with the conospiral Pleurotomariacea which also have an exhalent (anal) emargination or slit close to the middle of the apertural lip, and partly on the timing of larval torsion which Crofts (1955) showed preceded muscle development and therefor was not the cause of asymmetry.  Bellerophons, which appeared earlier, simply retained the ancestral symmetry of the shell.  
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In the Treatise on Invertebrate Paleontology (here on simply the Treatise) Part I, Mollusca, Gastropoda, it is argued that the Bellerophontacea should be included in the prosobranch gastropod order Archaeogastropoda, partly on the basis of certain similarities with the conospiral Pleurotomariacea which also have an exhalent (anal) emargination or slit close to the middle of the apertural lip, and partly on the timing of larval torsion which Crofts (1955) showed preceded muscle development and therefor was not the cause of asymmetry.  Bellerophons, which appeared earlier, simply retained the ancestral symmetry of the shell.  
Previously R.C. Moore (1952) had argued on the basis of the symmetry of the shell that bellerophons had not undergone torsion, assuming the two went hand in hand and included them in the Amphygastropoda. In this view the apical portion of the shell is held forward, over the head, in the primitive position as for the tryblidiids, with the anal slit to the rear.  It is further argued in the Treatise that the heavy apical portion of the shell over the head would have impeded  maneuvering and that the aperture at the rear would have made it difficult to withdraw the head and foot.  In the view of the Treatise, the aperture with emargination or slit, is to the front and the more closely coiled apex is to the rear, opposite from the primitive orientation.  
Previously R.C. Moore (1952) had argued on the basis of the symmetry of the shell that bellerophons had not undergone torsion, assuming the two went hand in hand and included them in the Amphygastropoda. In this view the apical portion of the shell is held forward, over the head, in the primitive position as for the tryblidiids, with the anal slit to the rear.  It is further argued in the Treatise that the heavy apical portion of the shell over the head would have impeded  maneuvering and that the aperture at the rear would have made it difficult to withdraw the head and foot.  In the view of the Treatise, the aperture with emargination or slit, is to the front and the more closely coiled apex is to the rear, opposite from the primitive orientation.  
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If the bellerophon animal was untorted and the apex held to the front, the Bellerophontacea then belong to the Amphygastroda, along with the tryblidiids, as suggested by R. C. Moore (1952). If however it had undergone larval torsion, as inferred in the Treatise, following Crofts (1955), with the opening to the front and apex to the rear, it then belongs to the Protobranchia as a member of the Archaeogastropoda.  Either way the Bellerophontacea realistically lie in evolutionary sequence between the Tryblidiacea and Pleurotomariacea and other true archaeogastropods.  
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If the bellerophon animal was untorted and the apex held to the front, the Bellerophontacea then belong to the Amphygastropoda, along with the tryblidiids, as suggested by R. C. Moore (1952). If however it had undergone larval torsion, as inferred in the Treatise, following Crofts (1955), with the opening to the front and apex to the rear, it then belongs to the Prosobranchia as a member of the Archaeogastropoda.  Either way the Bellerophontacea realistically lie in evolutionary sequence between the Tryblidiacea and Pleurotomariacea and other true archaeogastropods.  
   
   

Revision as of 15:32, 5 March 2011

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