Rayonnoceras

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'''''Rayonnoceras''''' is an orthoceroid cephalopod from the [[Carboniferous]] with an internal structure resembling that of earlier Ordovician and Silurian [[Actinocerida|actinocerids]]],now thought to be a late homeomorphic development within the [[Pseudorthocerida]].  
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'''''Rayonnoceras''''' is an orthoceroid [[cephalopod]] from the [[Carboniferous]] with an internal structure resembling that of earlier [[Ordovician]] and [[Silurian]] [[Actinocerida|actinocerids]], now thought to be a late homeomorphic development within the [[Pseudorthocerida]].  
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Rayonnoceras was originally placed in the Actinoceratidae by C Croneis, 1926, on the basis of its actinocerid-like nummulitic siphuncle and presence of strong cameral and siphuncular deposits. Teichert, 1964, placed it in Schindewolf's (1948) Carbactinoceratidae. Kroger and Mapes, 2007, noting the differences in the apecies of true Ordovician and Silurian actinocerids and those of Devonian and Carboniferous actinocerid look-a-likes (actinoceroids) removed the Carbactinoceratidae from the Actinocerida and added them to the Pseudorthocerida, carrying ''Rayonnoceras'' along.
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==Originally classification==
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''Rayonnoceras'' was originally placed in the Actinoceratidae by C Croneis, 1926, on the basis of its actinocerid-like [[nummulitic]] [[siphuncle]] and presence of strong cameral and siphuncular deposits. Teichert, 1964, placed it in Schindewolf's (1948) Carbactinoceratidae. Kroger and Mapes, 2007, noting the differences in the apecies of true Ordovician and Silurian actinocerids and those of Devonian and Carboniferous actinocerid look-a-likes (actinoceroids) removed the Carbactinoceratidae from the Actinocerida and added them to the Pseudorthocerida, carrying ''Rayonnoceras'' along.
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The apices of unquestionable actinocerids are straight and blunt; the first septum strongly concave; the siphuncular perforation of the first septum measuring more than half the shell diameter. Also the length of the first chamber is generally much shorter than in Carboniferous actinoceroids (homeomorphic pseudorthocerids). The apices of pseudorthocerids, into which Rayonnoceras has been placed are described as bullet shaped, with a cicatrix. The first chamber in the Carbactinoceratidae, which includes Rayonnoceras, is long and conical, and the first segment of the siphuncle is elongated.
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==Rayonnoceras and actinocerids compared==
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The apices of unquestionable actinocerids are straight and blunt; the first septum strongly concave; the siphuncular perforation of the first septum measuring more than half the shell diameter. Also the length of the first chamber is generally much shorter than in Carboniferous actinoceroids (homeomorphic pseudorthocerids). The apices of pseudorthocerids, into which Rayonnoceras has been placed are described as bullet shaped, with a cicatrix. The first chamber in the Carbactinoceratidae, which includes ''Rayonnoceras'', is long and conical, and the first segment of the siphuncle is elongated.
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The holotype, ''R solidiforme'' is discribed as having camerae completely filled with organic deposits (presumed to be calcite), although perhaps not necessarily all camerae, and a large siphuncle, solid throughout. If this truely represented the condition during the life of the animal it would have rendered it virtually immobile.  Any claim of an actively mobile nektonic preditor simply ignores the facts.  Not only is any rigid longiconic orthocone or cyrtocone difficult to meneuver in the first place, even when not overburdened with deposts, but the added weight provided by completely filled cameral and siphuncular spaces would have added great difficulty to any attempt of mobility.
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===Rayonnoceras appear heavy and immobile===
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The holotype, ''R. solidiforme'' is discribed as having camerae completely filled with organic deposits (presumed to be calcite), although perhaps not necessarily all [[camera]]e, and a large siphuncle, solid throughout. If this truly represented the condition during the life of the animal it would have rendered it virtually immobile.  Any claim of an actively mobile nektonic predator simply ignores the facts.  Not only is any rigid longiconic orthocone or cyrtocone difficult to meneuver in the first place, even when not overburdened with deposts, but the added weight provided by completely filled cameral and siphuncular spaces would have added great difficulty to any attempt of mobility.
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The heavy phagmocone and siphuncle, weighted by deposits secreted during the life of the animal, surely preclude any swimming mobility, especially in larger adults. While smaller juveniles may have had some "swimming" mobility above the substrate and the ability to move from place to place, actively hunt and escape predation, larger individuals probably spent most or all of their time in one place on the bottom, waiting in ambush for passing prey, much like modern wobegong or angelfish.  
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The heavy phagmocone and siphuncle, weighted by deposits secreted during the life of the animal, surely preclude any swimming mobility, especially in larger adults. While smaller juveniles may have had some "swimming" mobility above the substrate and the ability to move from place to place, to actively hunt and escape predation, larger individuals probably spent most or all of their time in one place on the bottom, waiting in ambush for passing prey, much like modern wobegong or angelfish.  
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No comparison can be made between the heavy carbactinoceratids including ''Rayonnoceras''. tentitive pseudorthocerids, or the morphologically similar actinocerids, and the Middle and Upper Ordovician Endoceratidae, other than for having, sometimes large, straight shells. Endocerids are simply a very different critter with different and unique internal structures and undoubtedly different abilities and habits.  
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=== Actinocerids behaved differently===
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No comparison can be made between the heavy carbactinoceratids including ''Rayonnoceras'', tentitive pseudorthocerids, or the morphologically similar actinocerids, and the [[Middle Ordovician|Middle]] and [[Late Ordovician|Upper Ordovician]] [[Endocerida|Endoceratidae]], other than for having, sometimes large, straight shells. Endocerids are simply a very different critter with different and unique internal structures and undoubtedly different abilities and habits.  
    
    
==References==
==References==
* Croneis, C. 1926, New Cephalopoda from the Fayetteville Shale. Bulletin of the Museum of Comparative Zoology (Harvard College), Vol. LXVII. No. 10.[http://www.archive.org/stream/bulletinofmuseum67harv/bulletinofmuseum67harv_djvu.txt]
* Croneis, C. 1926, New Cephalopoda from the Fayetteville Shale. Bulletin of the Museum of Comparative Zoology (Harvard College), Vol. LXVII. No. 10.[http://www.archive.org/stream/bulletinofmuseum67harv/bulletinofmuseum67harv_djvu.txt]
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*Kroger, B and Mapes, R 2007. Carboniferous Actinoceratoid Nautiloidea (Cephalopoda)- a New Perspective. Journal of Paleontolgy, July 2007  
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*Kroger, B and Mapes, R 2007. Carboniferous Actinoceratoid Nautiloidea (Cephalopoda)- a New Perspective. Journal of Paleontolgy, July 2007 [[http://findarticles.com/p/articles/mi_qa3790/is_200707/ai_n19434227/?tag=content;col1]]
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[[http://findarticles.com/p/articles/mi_qa3790/is_200707/ai_n19434227/?tag=content;col1]]
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* Teichert, C. 1964. Actinoceratoidea. Treatise on Invertebrate Paleontology Part K, Mollusca 3. Geological Society of America and University of Kansas press.  
* Teichert, C. 1964. Actinoceratoidea. Treatise on Invertebrate Paleontology Part K, Mollusca 3. Geological Society of America and University of Kansas press.  
John M 12/12/09
John M 12/12/09

Latest revision as of 15:30, 13 December 2009

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