No idea!

[Rocksandrocks]

No idea what this fossil is. Found in Northwest Arkansas, where, I believe, it is part of the Boone Formation, Mississippian Period. 
 

I’ve checked my fossil ID book, and every scallop-shaped fossil I see has vertical ridges, rather than horizontal.

 

The fossil is just under 2 inches wide (5.08 cm) and one inch tall (2.54 cm). At its deepest point, it has a depth of 1/2 an inch (1.27 cm). It has a deep ridge in the center and a circular spot at its deepest point (seen most clearly in the second to last photo).

 

Thanks for your help! 
 

Addendum: Just discovered that the circular spot was some kind of sediment that washed away when I was trying to clean that area of the fossil. Under the sediment was a slightly deeper cavity, making the depth of that spot closer to 3/4 of an inch. Added two new pictures with the sediment washed away.

Edited April 7, 2023 by Rocksandrocks

[ClearLake]

That is an odd one.  Very interesting looking.  I'm not at all certain, but I'll throw out some sort of a odd cross section through a cephalopod but I'm still trying to work out the geometry in my head.  

[PRLE]

Looks like trilobite pygidium, but I am far from an expert.  

[Rockwood]

Crinoid holdfast ?

[Mochaccino]

 

These flat, curved layers strongly remind me of steinkern fossils of Gonioceras, which is an unusual flat-shelled nautiloid cephalopod restricted to the Ordovician. Here are a couple specimens in my collection, which are from Wisconsin.

 

 

Here are some diagrams showing what the organism might have looked like in life:

 

The "chamber" steinkerns look a bit narrow, but that could just be because your section is from the very tip of the shell. If I'm looking at this correctly, the wavy lines in the surrounding rock may be part of the chambers as well, and if so the way they curve back out towards the edges is consistent with Gonioceras:

 

 

Granted, Gonioceras is restricted to the Ordovician, so if your specimen is Mississippian there's some inconsistency here. There's the possibility that you are mistaken and the locality for your fossil is indeed Ordovician-aged, which would make Gonioceras or something similar more plausible. There do exist a few other similarly flat-shelled forms, such as this Lambeoceras from the Ordovician of Illinois:

 

https://www.esconi.org/esconi_earth_science_club/2021/07/fossil-friday-66-galena-cephalopods-fossil-fossilfriday-illinois-galena-cephalopod.html

 

Edited April 7, 2023 by Mochaccino

[Rocksandrocks]

!!! 
 

Apparently this could be Ordovician!! (Just found an academic paper that identifies the exact place I found this as an outcropping of Ordovician.) Will spend time later today learning everything I can about Gonioceras. 
 

Thanks, everyone, for your replies! Very exciting. Love this.

[Mochaccino]

3 hours ago, Rocksandrocks said:

!!! 
 

Apparently this could be Ordovician!! (Just found an academic paper that identifies the exact place I found this as an outcropping of Ordovician.) Will spend time later today learning everything I can about Gonioceras. 
 

Thanks, everyone, for your replies! Very exciting. Love this.

Cool, if Ordovician that makes something like Gonioceras much more plausible

[Mark Kmiecik]

Nice find!