Actinocerida

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== General Description – what are Actinocerids==
== General Description – what are Actinocerids==
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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.  
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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) 
==Origin and Range.==
==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
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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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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.
 
== Siphuncle Details ==
== 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.  
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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)
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== Classification ==
== Classification ==
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== Evolution==
== Evolution==
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The Middle and Late Ordovician actinocerids are derived from the Wutinoceratidae, an early family from the Whiterock stage. The relationship to the Georginidae is uncertain  ''Polydesmia'' once cited as the ancestral form now seems to be a minor derivation.  
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The Middle and Late Ordovician actinocerids are derived from the Wutinoceratidae, an early family from the Whiterock stage. The relationship to the Georginidae is uncertain  ''Polydesmia'' once cited as the ancestral form now seems to be a minor derivation. (Flower 1968, 1976)
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The wutinocerids (Wutinoceratidae) gave rise to the Armenoceratidae near the end of the Whiterockian through ''Wutinoceras'' and to the Ormoceratidae in the Chazyan through ''Adamsoceras''. ''Cyrtonybyoceras'' is not known to have left any descendants.  ''Wutinoceras'' may also be ancestral to ''Polydesmia'' and thus ''Ordosoceras'' found only in Asia.  
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The wutinocerids (Wutinoceratidae) gave rise to the Armenoceratidae near the end of the Whiterockian through ''Wutinoceras'' and to the Ormoceratidae in the Chazyan through ''Adamsoceras''. ''Cyrtonybyoceras'' is not known to have left any descendants.  ''Wutinoceras'' may also be ancestral to ''Polydesmia'' and thus ''Ordosoceras'' found only in Asia. (ibid)
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The armenocerids,based on internal details of the siphuncle, through an early ''Armenoceras'', gave rise to the Gonioceratidae in the Chazyan. The Gonioceratidae are represented solely by the genus ''Gonioceras'' which is  restricted to the Middle Ordovician. 
 
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The armenocerids may also have given rise to the Actinoceratidae thru an early ''Nybyoceras'', derived in turn from an early ''Armenoceras''. Alternatively the Actinoceratidae may be derived from ''Polydesmia'' through ''Ordosoceras''.  
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The armenocerids,based on internal details of the siphuncle, through an early ''Armenoceras'', gave rise to the Gonioceratidae in the Chazyan. The Gonioceratidae are represented solely by the genus ''Gonioceras'' which is  restricted to the Middle Ordovician. (ibid)
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The actinoceratids gave rise to ''Lambeoceras'' of the Red River faunas and to ''Huronia'' through ''Kochoceras''. ''Lambeoceras'' is the sole genus of the Lambeocertidae. ''Huronia'' and ''Huroniella'' join to form the Huroniadae  
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The armenocerids may also have given rise to the Actinoceratidae thru an early ''Nybyoceras'', derived in turn from an early ''Armenoceras''. Alternatively the Actinoceratidae may be derived from ''Polydesmia'' through ''Ordosoceras''.(ibid)
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The actinoceratids gave rise to ''Lambeoceras'' of the Red River faunas and to ''Huronia'' through ''Kochoceras''. ''Lambeoceras'' is the sole genus of the Lambeocertidae. ''Huronia'' and ''Huroniella'' join to form the Huroniadae. (ibid)
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==Biology and Lifestyle==
==Biology and Lifestyle==
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Nothing is known of their soft part anatomy, although some conjecture can be made based on their being cephalopods. They probably had a hyponome (funnel)for jet swimming. Their central nervous system was probably fairly well developed as were their senses. But whether their eyes had lenses like modern squid and octopus or were like the pin-hole-camera eyes of ''Nautilus'' is unknown. They probably had 10 arms, like ''Michelinoceras'', but no direct evidence has yet been found.  
Nothing is known of their soft part anatomy, although some conjecture can be made based on their being cephalopods. They probably had a hyponome (funnel)for jet swimming. Their central nervous system was probably fairly well developed as were their senses. But whether their eyes had lenses like modern squid and octopus or were like the pin-hole-camera eyes of ''Nautilus'' is unknown. They probably had 10 arms, like ''Michelinoceras'', but no direct evidence has yet been found.  
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==References==
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* Flower, R.H. 1957; Studies of the Actinocerida, Memoir 2, New Mexioc Bureau of Mines and Mineral Resources, Socorro,                                NM.
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* Flower, R.H. 1968; The First great Expansion of the Actinoceroids, Mem 19 Part I, NMBM&MR, Socorro.
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* Flower, R.H. 1976, New AmericanWutinoceratidae with Review of Actinoceroid Occcurrences in Eastern Hemisphere, Mem 28, Part I, NMBM&MR, Socorro
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* Teichert, Curt 1964, Actinoceratoidea, in the Treatise on Invertebrate Paleontology, Vol K                                      (Nautiloidea). Geol Soc of America and University of Kansas Press; pp K190– K216
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* Teichert, Curt 1988, Main Features of Cephalopod Evolution Ch2 in The Mollusca Vol 12, Paleontolgy and Neontology of Cephalopods, Clarke and Treuman Eds, Academic Press.
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* Wade, M, 1988; Nautiloids and theri descendants: cephalopod classification in 1986, in Mem 44, New Mexioc Bureau of Mines and Mineral Resources, Socorro. NM
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John McDonnell
John McDonnell

Revision as of 16:51, 3 January 2009

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