Nautiloid?

[MPH]

Hi guys. Looking for help on this one. Found it this weekend in a west central Indiana creek. Largely glacial till although this was found not far from an outcrop of bedrock (Early carb.) and the matrix of this specimen appears similar to the bedrock matrix. I’m thinking nautiloid. If so, thoughts on genus? Was thinking vestinautilus, but there appears to be more suture lines on this specimen then on the picture in my field guide. A few of the pieces are “removable” thanks to me not realizing what I had when I first picked it up. In the end though, helps with seeing the various aspects a bit more clearly. Can’t see umbilicus. Blue line in one of the pics is pointing to what I think is the siphuncle. Measurement marks are In cm. 

 

Thanks for your help

[Tidgy's Dad]

Not sure what it is, but I don't think those are suture lines, that's external shell ornamentation.

[Ludwigia]

I'm thinking it could also be a gastropod. I would expect to see typical sutures on the steinkern if it was a nautiloid. Could you show us a pic of the other side and maybe also a view of the venter.

[MPH]

Yeah I wondered about gastropod too. So hard to tell sometimes. Was thinking maybe straparollus maybe. Just didn’t know if size matched up. 

[BobWill]

Yes, if you could possibly expose enough of the other side to determine if it is truly planispiral or not. Few gastropods that are will be that flattened and even straparollus is described as pseudelanispiral.

[MPH]

I’ll work on exposing the underside (which is covered in matrix currently). Does this pic of it help with determining its planispirality (did I make that word up? )

[Ludwigia]

Thanks for that photo. To my eyes that's definitely a gastropod and Bob Will is certainly on the right track, since he's quite familiar with the fauna from this period.

[MPH]

Any thoughts on genus based on shell pattern, size, the way it’s coiled, and age?

[BobWill]

Thanks for the new photo. Of course when you say "underside" that would be the more exposed side since it's upside-down in the picture. Looking at the cross-section I don't think this is the Straparollus (now known as Ampiscapha)  that we find in the Texas Pennsylvanian since it has more rounded whorl cross-sections and ours are more shouldered or angulated. It may be another form since they can appear both ways but I am not familiar with those. It is also much larger than the ones I see.

[MPH]

Oh that’s interesting, Bob. So when describing gastropods, if not planispiral, is there a ventral/dorsal aspect one can describe? Does looking down into spiral, so to speak, mean you are looking ventral or dorsal? Superior/inferior? If not planispiral, is there a way to differentiate top and bottom? Or even to know whether it “carried” vertically or horizontally (my field guide makes euomphalus appear to be carried horizontally, for instance)? Reading about the nautiloids/ammonoids, it seems ventral is more towards the “front” of the whorl when describing the siphuncle. Appreciate your wisdom, all. 

[BobWill]

3 hours ago, MPH said:

Oh that’s interesting, Bob. So when describing gastropods, if not planispiral, is there a ventral/dorsal aspect one can describe? Does looking down into spiral, so to speak, mean you are looking ventral or dorsal? Superior/inferior? If not planispiral, is there a way to differentiate top and bottom? Or even to know whether it “carried” vertically or horizontally (my field guide makes euomphalus appear to be carried horizontally, for instance)? Reading about the nautiloids/ammonoids, it seems ventral is more towards the “front” of the whorl when describing the siphuncle. Appreciate your wisdom, all. 

My "wisdom" should be considered as my interpretation only

For coiled cephalopods I believe the "front" would be the aperture and the ventral is the outer edge of the whorl so it would also coil around with the shell. The siphuncle is usually described as more or less ventral or central indicating it's position inside the whorl as it passes through the septa.

 

I'll leave it to someone else to distinguish dorsal from ventral for a gastropod but it is my understanding that for figures and descriptions most authors place the initial part of the shell or apex (posterior) upward and the aperture facing the observer but some European paleontologists have oriented the apex down. To further complicate orientation, in life most gastropods carry the tip of their spire slightly upward and backward and to the right with the aperture down.

[MPH]

Thank you! Learning so much.