Talk:Actinocerida

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Under Ecology I question the conclustions regarding depth and buoyancy control. I'm familiar with Kroger's work but have yet to read his 2009 paper, referenced, beyond the online abstract.

Actinocerids were equatorial, confined as far as we know to low latitudes. They seem to be confined to paleo-tropical continental and insular carbonate and siliciclastic shelves. How far down the continental slope they may have ventured is conjectural.I doubt their maxiumum depth range was more than about 250 meters (800ft) and probably less. Their common habitat may have even been within the photic zone (normally less than 100m.

Westermann*gives a maximum depth for the Actinoceroidea of 50 -150 m ( to abt 480 ft) or 1/3 the theoretical implosion depth based on septal curvature and thickness. I'm not sure I agree with the conclusions but have no basis for argument. If they simply were able to reach their theoretical implosion depth they could have attained close to 450m (abt 1400 ft)

Like all cephalopods, as far as we know, actinocerids were predatory and yes they undoubtedly alternated between mobility and rest. I consider them to have been nekto-benthic, living at or close to the sea floor, and to have been stalkers and ambush predators. They were not swimmers like fish, cuttlefish, and squid but were entirely mobile, able to retreat or change vantage position.

The statement regarding their ability to "control their buoyancy to a greater degree than their contemporaries" is unclear. Does this mean more rapidly, in quicker response, or does it mean more precisely with finer adjustnents. Strikes me that this too is conjectural. Also the reason for the conclusion is not given. Perhaps the thin, expanded connecting rings and parispatium did give them a greater buoyancy control than had by some contemporataries at certain times especially those with thick-ringed holochoantic siphuncles.

Westermann, G. E. G.: Abstract: Strength of concave septa and depth limits of fossil cephalopods , Lethaia Vol 6 no.4, pp 383-493, pub online 9 Oct 2007

John (J.H.McDonnell) 2/12/09 —Preceding unsigned comment added by J.H.McDonnell (talk • contribs) 18:43, 12 February 2009 (UTC)[reply]

Regarding ranges and events, wouldn't it be better to use terms generally more familiar such as middle Ordovician. Kroger's (apparent) use of Dapingian may be perfectly alright , even required, in peer review journals, but it is probably beyond the general knowledge of the broad Wikipedia audience. Anyone interested in details or current trends can go beyond the needs of the article and seek further information.

By the way the Dapingian is the early or lower Middle Ordovician, equivalent to the Whiterockian (White Rock stage) and is given a range of 472 -478 m.y.a. The Dapingian (=Whiterockian = lower Mohawkian) follows the Cassinian stage of the Canadian (Early Ordovician) epoch.

John ; J.H.McDonnell talk 2/12/09