Actinocerida

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(added systematic paleontology at top)
(revised general description as morphologic description, include siphuncle details there in)
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== General Description ==
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== Morphologic Description ==
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'''Actinocrids''' are generally straight shelled cephalopods with a ventral siphuncle composed of expanded segments in which internal deposits form that contain a canal system and in which there is a seam along the inner side of the connecting rings known as a parispatium. Connecting rings are typically thin and septal necks are cytrochoanitic (outwardly curved) to recumbent. Shells vary is cross section from almost circular to very broad with acutly angles lateral edges. Siphuncles vary in position from resting on the ventral margin to slightly below the center. Cameral deposits, especially in the apical portion, are common.(Flower 1957, Teichert 1964)   
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'''Actinocrids''' are generally straight shelled cephalopods with siphuncles composed of expanded segments in which internal deposits formed contain a canal system which connect to a seam along the inner side of the connecting rings known as a parispatium. Siphuncles vary in position from resting on the ventral margin to slightly below the center. Connecting rings are typically thin and septal necks are cytrochoanitic (outwardly curved) to recumbent. Shells vary is cross section from almost circular to very broad with acutly angles lateral edges. Cameral deposits, especially in the apical portion, are common.(Flower 1957, Teichert 1964)   
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=== Siphuncle Details ===
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The internal deposits found in the siphuncle form at the septal foramina and grow forward and back to meet about halfway in each segment but in the mid portion do not attach to connecting ring, leaving instead a space or seam which is the parispatium. The parispatium is connected to a central canal system that runs the length of the siphuncle by a system of radial canals. The radial canals are found in different configurations that have been used in determining different families and genera. Parispatia have not been identified for sure in all actinocerid specimens and have also been noted in some pseudorthocerids. (Flower 1957, Teichert 1964)
==Origin and Range==
==Origin and Range==
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The origin of the actinocerids remains uncertain. The Georginidae found in Northern Australia in the Cassinian Coolbah Formation and in China are the earliest known and may be the ancestral stock for the rest of the order. Their derivation remains a mystery. ''[[Bathmoceras]]'', a late Canadian ellesmerocerid and only representative of the Bathmoceratidae, has been suggested as ancestral based on the lobate inward projections of the connecting rings. An origin in the Trempealeauan (Late Cambrian) [[Protactinocerida]] has been suggested, but since no Protactinocerida are known to have crossed into the Ordovician, this seems unlikely. (Flower 1968, 1976; Wade 1988)
The origin of the actinocerids remains uncertain. The Georginidae found in Northern Australia in the Cassinian Coolbah Formation and in China are the earliest known and may be the ancestral stock for the rest of the order. Their derivation remains a mystery. ''[[Bathmoceras]]'', a late Canadian ellesmerocerid and only representative of the Bathmoceratidae, has been suggested as ancestral based on the lobate inward projections of the connecting rings. An origin in the Trempealeauan (Late Cambrian) [[Protactinocerida]] has been suggested, but since no Protactinocerida are known to have crossed into the Ordovician, this seems unlikely. (Flower 1968, 1976; Wade 1988)
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== Siphuncle Details ==
 
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The internal deposits found in the siphuncle form at the septal foramina and grow forward and back to meet about halfway in each segment but in the mid portion do not attach to connecting ring, leaving instead a space or seam which is the parispatium. The parispatium is connected to a central canal system that runs the length of the siphuncle by a system of radial canals. The radial canals are found in different configurations that have been used in determining different families and genera. Parispatia have not been identified for sure in all actinocerid specimens and have also been noted in some pseudorthocerids. (Flower 1957, Teichert 1964, 1988)
 

Revision as of 13:39, 8 December 2012

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