Solved! I’m gonna try to get some DNA pulled from this bad boy and grow me one in a petri dish. If I don’t check back in here, you might want to call these guys. 
Can you tell us the location where it was found? That would give us the opportunity to look up the age of the rocks it could have come from, and narrow down the options.
My guess: the head of a crinoid - a sea lily.
It was found in Dexter, Michigan.
This is my favorite feature of this thing.
It appears to be the end/bottom part of the segmented shape
The segmented view is one reason I thought it might be a crinoid head. The calyx, or whatever they call that part of the animal that connects the body with the stem looks something like that, in large crinoids I’ve seen. I imagine the geology of Dexter is not too far from the geology of the far south suburbs of Chicago, where there’s an enormous open-faced limestone quarry with, I believe, animals about that size. The age of the rocks may be similar.
I did wonder if it could be a crinoid, but couldn’t find any very similar examples. Another possibility is some kind of brachiopod - these often look like bivalve molluscs (but are not closely related)
There is something bi-valve looking about this thing (The second pic in that original group I posted was a side view that is kind of clam-shaped)
Thought that, too, and I’ve seen brachiopods about that size in that same formation, too.
Close. I think it’s part of the calyx (head) of a blastoid.
The thing about crinoid calyces is that, as far as I know, they were made up of rows of nondescript calcite plates which frequently disassociated upon death. The arms attached at the top, and collected food and passed it down to the mouth, which was located at the top of the calyx. There is a brief discussion of crinoid morphology here.
Blastoids, on the other hand, were built on much the same idea-- a head on a stalk, with arms attached to the head, but the implementation was somewhat different. The head was built of far fewer plates which were solidly attached to each other. There were five ambulacra that ran down the length of the blastoid’s calyx, one on each side. They were basically feeding grooves, what the blastoid’s tube feet came out of. The grooves and lines of holes that you can see on your fossil bear a strong resemblance to these ambulacra, as you can see here.
I think your fossil is embedded in a larger rock which has also been highly eroded-- look how spherical the rock is, and how little relief there is on the fossil itself. Judging from the way the ambulacra taper, I’d guess that in this picture, you’re looking at the top of the calyx.
Bingo! Blastoids, all right.
It looks to me like it’s way too big to be a blastoid. They’re usually only about an inch across and I believe the biggest they come is about 3 inches.
I think it probably is a crinoid, either a well-preserved calyx or maybe a holdfast. I think the part that looks bivalvey is a red herring. The roundness may just be from the mineralized nodule in which the fossil was preserved and the odd texturing and big crack are just from weathering.
Are you sure? If you watch the video, it looks like the fossil is about as long as two joints of the OP’s fingers, which on me, at least, would be 2-2.5 in. This paper has a blastoid size chart for one particular species of blastoid, and the largest ones seemed to be about 4.5 cm by 3 or 3.5 cm, which would still be about right.
I found out that this was found next to a pond in Perrysburg, OH, which is a bit south of where I originally thought it was found.
I’ve been looking at pics of crinoids, and it IS possible that it has some feeding-arm like lines coming out from the top of the calyx part, if that is indeed what it is.
It’s just weird that the shape of the rock around it, including the split, is all so symmetrical.