Cephalopoda

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(Biology)
(Evolutionary History: paraphyly of early forms, Nautiloidea valid.)
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==Evolutionary History==
==Evolutionary History==
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The cephalopods first appeared in the late [[Cambrian]].  The first forms had gently curved shells.  During the [[Ordovician]] the group underwent an astonishing evolutionary radiation, possibly due to the new ecological niches made possible by the extinction of [[Anomalocarida|anomalocarids]] at the end of the Cambrian.  Some eight new orders appeared.  There was tremendous diversity among them.  Some had long straight shells, short straight ones, curved, lightly coiled, and tightly coiled ones evolved.  The internal structure of the shell differed greatly as well, mostly in the structure of the [[siphuncle]].  Most were probably relatively slow movers, at least compared to today's forms.  The largest ones had huge straight shells that reached 3 to 5 or even 10 metres in length. All these early forms are classed under the [[paraphyletic]] and probably [[artificial taxon]] Nautiloidea.
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The cephalopods first appeared in the late [[Cambrian]].  The first forms had gently curved shells.  During the [[Ordovician]] the group underwent an astonishing evolutionary radiation, possibly due to the new ecological niches made possible by the extinction of [[Anomalocarida|anomalocarids]] at the end of the Cambrian.  Some eight new orders appeared.  There was tremendous diversity among them.  Some had long straight shells, short straight ones, curved, lightly coiled, and tightly coiled ones evolved.  The internal structure of the shell differed greatly as well, mostly in the structure of the [[siphuncle]].  Most were probably relatively slow movers, at least compared to today's forms.  The largest ones had huge straight shells that reached 3 to 5 or even 10 metres in length. Many of these early forms were [[paraphyletic]], having given rise to newer equal or higher taxa; all are classified within the [[monophyletic]] and valid Subclass Nautiloidea.
The nautiloid cephalopods remained unchallenged through the Ordovician and [[Silurian]], finally giving way to the large predatory fish of the late Devonian.  About this time the ammonoids began to take over from the nautiloids.  The ammonoids are rare in the early Devonian, but by the end of the period and the beginning of the [[Carboniferous]] they increase greatly in diversity.  During this time, all but two the remaining nautiloid orders die out.  The Coleoidea meanwhile make their first appearance in the [[Late Mississippian]] (middle Carboniferous) but remain rare.
The nautiloid cephalopods remained unchallenged through the Ordovician and [[Silurian]], finally giving way to the large predatory fish of the late Devonian.  About this time the ammonoids began to take over from the nautiloids.  The ammonoids are rare in the early Devonian, but by the end of the period and the beginning of the [[Carboniferous]] they increase greatly in diversity.  During this time, all but two the remaining nautiloid orders die out.  The Coleoidea meanwhile make their first appearance in the [[Late Mississippian]] (middle Carboniferous) but remain rare.

Revision as of 12:25, 1 August 2009

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