Endocerida

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The Endocerida are natuiloid cephalopods from the Ordovician, extending into the Silurian, characterized by the presence of conical structures in the siphuncle known as endocones and by a phragmocone free of organic deposits. Endocerid shells are , longiconic and breviconic orthocones and endogastric cyrtocones. Septa are generally transverse and close spaced. The siphuncle is invariably ventral, though not necessarily on the venter; tubular with staight or concave segments and generally large in proportion to the phragmocone, reaching as much as about half its diameter. In size endocerid shell vary in size from less than a meter to as much as about 9 meters in length (nearly 30 ft), making them among the largest Paleozoic invertebrates (1)(2)  
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The Endocerida are [[Nautiloidea|nautiloid]] cephalopods from the Ordovician that extend into the Silurian, characterized by the presence of conical structures in the siphuncle known as endocones and by a phragmocone free of organic deposits. Endocerid shells are longiconic and breviconic orthocones and endogastric cyrtocones. Sutures are generally transverse, septa close spaced. The siphuncle is invariably ventral, though not necessarily on the venter; tubular with straight or concave segments and generally large in proportion to the phragmocone, reaching as much as about half its diameter. In size endocerid shells vary in size from less than a meter to as much as about 9 meters in length (nearly 30 ft), making them among the largest Paleozoic invertebrates (1)(2)  
== Derivation==
== Derivation==
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Endocerids are derived from the ellesmeroceriatid ''[[ Pachendoceras]]'' through the ancestral ''[[Proendoceras]]'' at or just after the end of the [[Gasconadian]] ([[Tremadocian]] ) stage at the beginning of the Ordovican, by a supression of the endosiphuncluar diaphragms and development of endocones.(2)(3)
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Endocerids are derived from the ellesmeroceratid ''[[Pachendoceras]]'' through the ancestral ''[[Proendoceras]]'' at or just after the end of the [[Gasconadian]] ([[Tremadocian]]) stage at the beginning of the Ordovician, by a suppression of the endosiphuncular diaphragms and development of endocones.(2)(3)
== The Endocerid Siphuncle ==
== The Endocerid Siphuncle ==
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The basic endocerid siphuncle has been introduced. Siphuncle segments in the Endocerida are straight, sinuous, or concave, and never expanded into the chambers as with the comtemporary [[Actinocerida|actinocerids]] and [[Discosorida|discosorids]]. Septal necks may be orthochoanitic, short; holochoanitic, reaching close to the previous septum; or macrochoanitic, reaching beyond the previous septum   Connecting rings are thick and often show layering. With concave segments, connecting rings are thickened inwardly and leave depressions or concavities in siphuncle nternal molds.(1)(2)  
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The basic endocerid siphuncle has been introduced. Siphuncle segments in the Endocerida are straight, sinuous, or concave, and never expanded into the chambers as with the comtemporary [[Actinocerida|actinocerids]] and [[Discosorida|discosorids]]. Septal necks may be orthochoanitic, short; holochoanitic, reaching close to the previous septum; or macrochoanitic, reaching beyond the previous septumConnecting rings are thick and often show layering. With concave segments, connecting rings are thickened inwardly and leave depressions or concavities in siphuncle internal molds.(1)(2)  
Endocones, with apeces pointing aft (adapically) develop in the after or apical portion of the siphuncle where they produce a balancing weight that compensates for the visceral mass forward in the body chamber and remaining portion of the siphuncle. This helps keep the shell (and animal) in a horizonal orientation during life. Endocones are typically penetrated by a narrow axial tube, the endosiphotube which often contains irregular diaphragms, reminiscent of the ellesmeroceratid ancestors.(1)(2)
Endocones, with apeces pointing aft (adapically) develop in the after or apical portion of the siphuncle where they produce a balancing weight that compensates for the visceral mass forward in the body chamber and remaining portion of the siphuncle. This helps keep the shell (and animal) in a horizonal orientation during life. Endocones are typically penetrated by a narrow axial tube, the endosiphotube which often contains irregular diaphragms, reminiscent of the ellesmeroceratid ancestors.(1)(2)
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In some, such as the endogastric [[Piloceratidae]] and essentially orthoconic [[Endoceratidae]] the endosiphuncle, comprising the structures within, is simple. In others, such as the [[Manchuroceratidae]], [[Emmonsoceratidae]]. [[Najacertidae]], and strange [[Allotrioceratidae]] the endosiphuncle is complex with additional structures. Principal among these are longitudial processs or partitions that project inward from the ventral or dorsal, and sometimes lateral margin of the siphuncle, about which the endocones form;  thin, dark, blade-like features bisecting primary structures; and what look like fine, often symmetrically postitioned, tubules. (1)(2)  
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In some, such as the endogastric [[Piloceratidae]] and essentially orthoconic [[Endoceratidae]] the endosiphuncle, comprising the structures within, is simple. In others, such as the [[Manchuroceratidae]], [[Emmonsoceratidae]], and [[Najacertidae]], the endosiphuncle is complex with additional structures. Principal among these are longitudial processs or partitions that project inward from the ventral or dorsal, and sometimes lateral margin of the siphuncle, about which the endocones form;  thin, dark, blade-like features bisecting primary structures; and what look like fine, often symmetrically positioned, tubules. (1)(2) Inclusion of the [[Allotrioceratidae]] is suspect.
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Two suborders proposed by Flower in 1958 (4), the Proterocameroceratina and the Endoceratina are generally recognized, although not in all taxonomies. The Proterocameroceratina are the earliest and include such genera as ''Proendoceras'', ''Clitendoceras'', ''Mcqueenoceras'' and ''Proterocameroceras'' The Proterocamerocerina gave rise in the early Middle Canadian (early Arenig) to the Endoceratina, possibly from ''Clitendoceras'', and to derived families within the suborder with more complex siphuncle interiors. In contrast the Endoceratina, although including the largest endocerids, remained internally simple.
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Two suborders proposed by Flower in 1958 (4), the [[Proterocameroceratina]] and the [[Endoceratina]] are generally recognized, although not in all taxonomies. The Proterocameroceratina are the earliest and include such genera as ''Proendoceras'', ''Clitendoceras'', ''Mcqueenoceras'' and ''Proterocameroceras'' The Proterocameroceraiina gave rise in the early Middle Canadian (early Arenig) to the Endoceratina, possibly from ''Clitendoceras'', and to derived families within the suborder with more complex siphuncle interiors. In contrast the Endoceratina, although including the largest endocerids, remained internally simple.
==References==
==References==
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*[4] Flower, R.H. 1958; Some Chazyan and Mohawkian Endoceratida, Journal of Paleontolgy V.32, no.2, pp59-62, May 1958
*[4] Flower, R.H. 1958; Some Chazyan and Mohawkian Endoceratida, Journal of Paleontolgy V.32, no.2, pp59-62, May 1958
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Credits:
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JM 2/20/09
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[[user:john M|John M]] Feb 20, 2009
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:: corrections & revisions July 15, 2014

Latest revision as of 22:31, 15 July 2014

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