Circular (possibly spherical) Fossil ID Request - Caledon, ON, Canada

[QueenCoeurl]

This mystery fossil was found in a rock pile in the Caledon, ON area, not far from the Niagara Escarpment. 

 

When I split the rock, it was revealed and split into convex and concave halves that then fit together. It is almost perfectly circular, and is 13mm (0.51 inches or just over 1/2 inch) in diameter. There are clearly many other fossils within this rock, but I fear splitting it further and destroying this mystery fossil without ID on it first.

 

I've taken it to a few other hobbyists I know in person, but they has also been unable to ID it, and the best guess I've received is that it some sort of Brachiopod. I remain unsure, as there are no ridges or grooves, and the edge is very smooth. Also the closest looking brachiopod species I could find was the "Obiculoidea" which is normally found in the Texas to Ohio areas.

Since the fossil is "bisected" perpendicular to the rock layers, I've had the thought that it could be a cephalopod (some sort of orthocone?) shell, but can't find good cross section images of such fossils that look the same. However the smooth-ness, central void, and convex & concave split would fit what I see from cephalopod fossils cut open length-wise.

 

Let me know what you guys think!

[Rockwood]

Septum of a straight shelled nautiloid. 

[QueenCoeurl]

2 minutes ago, Rockwood said:

Septum of a straight shelled nautiloid. 

My hunch was right! woo! Thank you.

 

 

Would it make sense to try and remove more of the matrix to reveal the top of it and see how long it is? Or are they fairly fragile?

[Rockwood]

24 minutes ago, QueenCoeurl said:

Would it make sense to try and remove more of the matrix to reveal the top of it and see how long it is? Or are they fairly fragile?

I think "fairly" is an understatement. It might be possible, but it would be difficult. 

[Mark Kmiecik]

I would leave this as it is. These are fairly common. You should be able to find literally "tons" more in the area.