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Unusual Orthoconic cephalopod from the lower Mississippian


howard_l

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I have been working with Mississippian age nautiloids in Kentucky for the past 40 years.  Years ago I found two very complete specimens in the Nancy Member of the Borden Formation in a creek east of Morehead, Kentucky.  The one was a small but very complete Orthoconic nautiloid of the genus Michelinoceras sp. which is commonly found in that area.  The other was found very close to the other and at the time I thought it was another Michelinoceras sp. Recently I was looking at it and saw the shape was much different.   Michelinoceras is very conical and the shape is very round to slightly oval with the Siphuncle in the center.   The unknown specimen is also orthoconic and shows the living chamber and upper shell is just lightly oval similar to Michelinoceras sp. but the end of the shell thins on the sides to the shape of a Baculite shape.   The specimen is highly replaced with calcite and the siphuncle is hard too determine.  What may be the siphuncle is located at the upper part of what I think is the venter.  If I am correct it would be a Bactritida type of nautiloid that were common in the Devonian.   

 

mich.jpg.0390cb99e796ae09ce6f1b87781ec2b4.jpg

 

I have researched the treatise as well as many papers on Mississippian age cephalopods I can find.   I am hoping someone may have ran across a similar specimen.   The first and second picture is of the Michelinoceras sp. specimen.  Approx. 4 inches long and typical Michelinoceras specimens.  The rest of the picture are of the unknown specimen, the red circle is the area where I believe the siphuncle is located. 

 

Picture1,kiiio.jpg

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Tue Jul 20 19-22-48mark.jpg

Tue Jul 20 19-26-19mark.jpg

Tue Jul 20 19-27-53mark.jpg

Tue Jul 20 19-22-48.jpg

Tue Jul 20 19-26-19.jpg

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Can't help with ID, but I don't think there is enough evidence there to say that is the siphuncle.

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I agree with @westcoast. I can see what COULD BE the siphuncle, but, I see similar patterns on the cut surface that mimic the circular spot suggested to be a siphuncle.

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I agree, but it was closest thing that resembled one, it may be to replaced to find one..  I am hoping the shape will lead to a potential identification 

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