Arion Posted September 20, 2014 Share Posted September 20, 2014 Back in July picked up this orthocone from the Lindsay Formation (Ordovician) in Collingwood, Ontario. I was pleased with it because, though it's far from a perfect specimen, it's not entirely flattened (as is the case with some other orthocones I've found from the Whitby Shale). Still haven't been able to ID it conclusively yet, seems to most closely resemble Actinoceras beloitense (going from a couple of old drawings in publications that predate me by a fair margin). Any thoughts? - Arion Link to comment Share on other sites More sharing options...
FossilDAWG Posted September 21, 2014 Share Posted September 21, 2014 Can we get some idea of the size of the specimen? I don't think it is an Actinoceras, as in the actinocerids the siphuncle is really large and solid, filling much of the interior of the shell. This feature should be obvious in the end view of the specimen, which shows the surface of a septum, but it is absent. Don Link to comment Share on other sites More sharing options...
Arion Posted September 21, 2014 Author Share Posted September 21, 2014 Right sorry, should have included a scale; it's about 1.5 inches long 0.5 inches wide. My ID as A. beloitense was because the phragmocone appears to expand adapically, as in this species (won't let me link to the image, but a quick google image search for A. beloitense will turn up what I'm talking about). Nothing of the internal anatomy appears visible in cross-section; is it possible the siphuncle was infilled with sediment and is obscured in adapertural view? Link to comment Share on other sites More sharing options...
FossilDAWG Posted September 21, 2014 Share Posted September 21, 2014 The size of your specimen is also a counterindication for Actinoceras, as those nautilods are usually much larger. The siphuncle should occupy at least 2/3 of the cross section of the shell, quite unlike your specimen. The actinocerids were large sea floor-living predators, and the deposit-filled siphuncle weighted the shell down to help keep the animal on the sea floor. Also, many of the actinocerids had shells that were markedly oval in cross section, with the siphuncle in contact with the ventral surface. I think you should look at other genera, such as Michelinoceras or Sactoceras, which are more consistent with your specimen in size, whorl section, and small siphuncle. These genera are also more common in the Lindsey, whereas Actinoceras is more typical of the Coboconk and Gull River. Keep in mind that it is often impossible to get a firm ID on specimens such as yours, as details of the siphuncle structure (such as the nature of the connecting rings and any biogenic deposits) and surface ornament are obscured by the manner of preservation as an internal cast. Don Link to comment Share on other sites More sharing options...
Jazzynicks Posted April 19, 2021 Share Posted April 19, 2021 I have a couple of these also and I wish I knew what type of rock this is and if they are for sure orthocone fossils! Link to comment Share on other sites More sharing options...
Ludwigia Posted April 19, 2021 Share Posted April 19, 2021 7 hours ago, Jazzynicks said: I have a couple of these also and I wish I knew what type of rock this is and if they are for sure orthocone fossils! Limestone and yes. 1 Greetings from the Lake of Constance. Roger http://www.steinkern.de/ Link to comment Share on other sites More sharing options...
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