Helcionelloida

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m (Protected "Helcionelloida" [move=sysop])
(paraphyletic, clade no, taxon yess)
 
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{{mollusca}}
 
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<center><big><big><big>Helcionelloida</big></big></big></center>
<center><big><big><big>Helcionelloida</big></big></big></center>
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The '''helcionelloids''' are a little-known but important grade of primitive [[Cambrian]] [[conchifera]]n [[Mollusca|molluscs]] with both compressed and spiral shells. Some developed strange "snorkels" through which the animal would breathe, presumably while half-submerged in mud or sand. They include the earliest molluscs that possess a hard shell. Like a number of other early conchiferan stem groups, they have previously been (and are often still) included under the [[Monoplacophora]]. However, it is becoming more and more apparent that the "Monoplacophora" are a very diverse assemblage of basal molluscs, and not a natural group. The Helcionelloida were recently named as a distinct class of molluscs by Dr John Peel (1991), who distinguished them from the "[[Tergomya]]" (Tergomya + [[Tryblidiida]]) by their direction of coiling and other attributes. However, inasmuch as they represent a clearly paraphyletic assemblage (being ancestral to a number of later groups) they cannot be considered a valid [[class]] [[taxon]] or [[clade]], but constitute an early conchiferan "[[grade]]", a little more advanced than the Tryblidiida. Nevertheless, these creatures constitute a fascinating and very important development in the early evolution of shell-bearing molluscs.
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The '''helcionelloids''' comprise a little-known but important group of primitive [[Cambrian]] [[conchifera]]n [[Mollusca|molluscs]] with both compressed and spiral shells. Some developed strange "snorkels" through which the animal would breathe, presumably while half-submerged in mud or sand. They include the earliest molluscs that possess a hard shell. Like a number of other early conchiferan stem groups, they have previously been (and are often still) included under the [[Monoplacophora]]. However, it is becoming more and more apparent that the "Monoplacophora" are a very diverse assemblage of basal molluscs, and not a natural group. The Helcionelloida were recently named as a distinct class of molluscs by Dr John Peel (1991), who distinguished them from the "[[Tergomya]]" (Tergomya + [[Tryblidiida]]) by their direction of coiling and other attributes. However, inasmuch as they represent a clearly paraphyletic assemblage (being ancestral to a number of later groups) they cannot be considered a true [[clade]] but are a valid [[taxon]] a little more advanced than the Tryblidiida. Nevertheless, these creatures constitute a fascinating and very important development in the early evolution of shell-bearing molluscs.
== Exogastric or Endogastric ==
== Exogastric or Endogastric ==

Latest revision as of 17:14, 25 September 2011

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