Cephalopod or not?

[Jan Lester]

I’ve had this forever, I think I found it in Middle TN. I initially thought it was a cephalopod, but I know more now, and I don’t see any septum(s). What say ye?

Thanks!

[Rockwood]

Not

[Jan Lester]

I didn’t post a side elevation of this. Anybody have an idea what it is? I want to give it to some little boys who are VERY interested in fossils. 

[Rockwood]

I think it may be a clast that was somehow displaced from a degraded shell bed. It's not much more than a guess though. 

[Jan Lester]

Thanks!

[Jan Lester]

So I showed my son this rock and how I was prepping it, and @Rockwood’s opinion that this was a clast. Drew promptly told me this was his fossil rock (guess I won’t give it to little boys!) and showed me the septums (septae?). The lines he highlighted are actually mineralized with quartz, unlike the surrounding parts of the cephalopod. He did have the advantage of looking closely at it for years. 

[Rockwood]

Gee. I'd like to go along, but I'd be lying. I take a dim view on just about all forms of lying to young people.

[ynot]

It looks more like a chert nondual than a shell of any type.

the supposed septum lines are not properly aligned for septum.

[Jan Lester]

@Rockwood This is his opinion, I would never ask you to lie. Otherwise I wouldn’t ask for your opinion! I don’t think it’s chert, when the limestone is scraped away there are tons of quartz crystals. Now I’m curious where it came from, he’s pretty sure HE found it. I’ll let him show it to one of his profs and see what they think. I’m also curious if those are brachiopods (I’m thinking bivalves); I guess our new Geology dept head is a brachiopod expert or something, so she can tell him. Our area is mostly Ordovician, not many bivalves. Thanks for looking!

[cngodles]

It has features of pinnids, a family of bivalves, but there isn't enough evidence to say for sure. Members of the Pinnidae don't reach back into the Ordovician, according to the PBDB. But they've been wrong before. They do say they appear in the Silurian, an adjacent younger period.

 

The cross-section from your photos is very curious. I imagine the hinge on the left side (dorsal margin) and the flared opening (ventral margin) on the right.

 

These examples below are probably from a different age.

Figures 1-4 from Die Versteinerungen des Deutschen Zechsteinge-birges (The fossils of the German Zechsteinge Mountains)

 

Edited November 21, 2022 by cngodles