Large cephalopod from Graf

[tombk]

I was collecting isorthoceras cephalopods at Graf Iowa (Elgin Member of the Maquoketa Formation, upper Ordovician). I found one partial specimen that in cross-section is about 8-9 times larger than any other I’ve found there.
 

This first pic is from the field. (Don’t worry, the next photos have a scale cube in them!)

The large specimen is about 8.9 cm by 5 cm. You can see cross sections of the usual-size isorthoceras in the rock (filled and infilled), and they’re closer to 1 cm across. 

Unfortunately, the specimen is fairly fragile. I’ve already had to glue together six pieces it fell apart into, and I haven’t been able to remove it from the matrix any further to get a better look at its sides. 
 

I’m wondering whether it’s an unusually large isorthoceras or a different animal altogether. Thoughts? 

[connorp]

I don't see a siphuncle... Not sure what it is. Neat find.

[Al Dente]

It could be Endoceras. Endoceras, Spyroceras, and Beloitoceras are found in the same beds as the Isorthoceras.

 

 

[Tidgy's Dad]

Hate to say it, but maybe just a concretion......................

[Rockwood]

55 minutes ago, Tidgy's Dad said:

Hate to say it, but maybe just a concretion......................

One would need to explain the other three similar shapes next to it. I would see that as a caution too.