Questions about some Ordovician nautiloid cephapod fossils.

[ntloux]

Members of the Fossil Forum have been very helpful with previous submissions.  I would appreciate comments/suggestions concerning some nautiloid cephalopod fossils that I have.  Image 1 is typical of the Ordovician nautiloid cephalopod fossils found in the Cincinnatian region.  Given the bryozoan encrustations, it clearly remained on the sediment surface for a period of time.  One source suggests that you can only identify the species by slicing them lengthwise and examining the siphuncle; is this true?  Images 2a and 2b are images of a significantly larger Ordovician  nautiloid cephalopod that I have tentatively identified as a Cameroceras both due to its larger size and the very large siphuncle illustrated in Image 2b.  Image 3 is also of a larger nautiloid cephalopod that appears to be a mold.  If one assumes that the piece on the right side is part of the same fossil then it displays a quite small diameter siphuncle.  The fossil in Image 3 does however appear to contain part of the living chamber.   Lastly, Image 4 is of a nice endoceras Ordovician cephalopod from Ontario.  This fossil appears to have been buried in fine grain sediments so it may very well have perished from suffocation during a sediment resuspension event.  There does appear to be a layer on the surface.  This brings me to the question: Did paleozoic cephalopod shells have a skin?

[Ludwigia]

I'm not so sure, but that last one might just as well be a Treptoceras. They are quite common in that provenance. Sorry I can't help with the others.

[ntloux]

Thank you for the suggestion.  I was relying on the folks in Ontario.  I do have a treptoceras duceri specimen from the Cincinnati area.

[Ludwigia]

7 minutes ago, ntloux said:

I was relying on the folks in Ontario

 

I hail from Brampton and have even collected on the Humber River myself.

[FossilDAWG]

I also suspect your Humber River specimen is a Treptoceras; T. crebriseptum is quite common there.  Treptoceras is an actinocerid with a fairly small bead-like siphuncle.  Cameroceras is the endocerid you can find there; endocerids had a massive siphuncle made of cone-shaped endocones nested together.  Endoceras and other relatives are quite often found as just the siphuncle, as the cameral chambers tended to be more fragile and often were broken away.

 

I'm not sure what you mean by "do nautilod cephalopods have a skin"?  They had an outer shell, and of course inner septa separating the chambers (camerae) and a siphuncle.  Very commonly in the calcareous shale that is so prominent in the Cincinnatti area, southern Ontario (Verulam Formation for example), and elsewhere, the chemistry of the sediment tended to dissolve aragonite but not calcite.  So molluscs in general are found as internal molds showing the sediment-filled interior but no shell.  There are some exceptions, such as the snail Cyclonema that had a more calcitic shell so it is usually found intact.  Brachiopods, corals, bryozoans etc had calcite shells so they are preserved with the shell.  Similarly many Cretaceous chalk deposits have lots of oysters (calcite shells) but the other bivalves and the gastropods are no-where to be seen; there were there at one time, but their shells dissolved long ago.  In some places you can find good internal molds where clay or phosphate filled the interior before the shell dissolved.

 

My Treptoceras from Toronto preserve the shell, and your specimen has preserved septa and some of the shell surface (the dark material), though most of it is broken away (not dissolved).  That's common where the fossil is preserved in a solid matrix such as limestone, as it tend to break between the shell and the interior, exposing the septa and leaving much of the shell stuck to the matrix.

 

Don

[ntloux]

Don,  Thank you very much for the information.  It is very helpful  The last specimen was likely buried quite quickly in fine grained sediment.  However, portions do have a thin coating which made me wonder if the shell had some type of a skin. Alternatively, it could presumably result from the decay process (e.g., a mold or fungus coating).

[Kane]

It would not be uncommon to find bryozoan colonies having grown on marine debris in this material. If you see tiny pinprick holes on the covering of the shell, those might be the zooecia of a bryozoan colony.

[ntloux]

Thank you for the information.