Actinocerida

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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)   
'''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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==Origin and Range.==
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==Origin and Range==
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Actinocerids have their beginning near the end of the Canadian (late Arenigian) and came into fruition in the Whiterockian (early Middle Ordovician ). They diversified quickly and were at their greatest abundance during the Middle and Late Ordovician, then declined in the early Silurian and became a minor constituent of cephalopod faunas through the Devonian. Carboniferous forms previously identified as actinocerids have been reassigned to the Pseudorthocerida. (Flower 1857, 1968, 1976, Teichert 1964)  
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Actinocerids have their beginning near the end of the Canadian epoch (late Arenigian) and became plentiful during the Whiterockian epoch (early Middle Ordovician ). They diversified quickly and were at their greatest abundance during the Middle and Late Ordovician, then declined in the early Silurian and became a minor constituent of cephalopod faunas through the Devonian. Carboniferous forms previously identified as actinocerids have been reassigned to the Pseudorthocerida. (Flower 1857, 1968, 1976, Teichert 1964) As such, it is doubtful that they survived the extinction event at the transition from the Devonian to the Carboniferous.
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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) Protactincerida 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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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) [[Protactincerida]] 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 ==
== Siphuncle Details ==

Revision as of 18:28, 3 January 2009

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