Top part of skull preteen or a shell of some sea creature?

[GypsyJr]

I was walking my dogs this afternoon (EST) and I noticed a rock that was kind of rusty looking but spherical from the angle I was looking. I picked it up for some reason and when I flipped it over I saw some fossils I've seen before and a couple I haven't. The more I looked at it I thought this maybe part of a skull that sea life made a home in possibly?  There is one fossil that almost loos like it still has color but that maybe the hue off the marbleized fossil I guess too. The fossil is very heavy and round almost oval shape aside from one section that is maybe missing? Also looks like a very small  piece broke off or was scrapped off. You can see the white spot where this may have happened in the side view pics.  

 

I couldn't find my tape measure sorry. I used a AAA battery in a bunch of pics since I'm pretty sure that's a world wide standard. You can also refer to a 500ml Round bottom Pyrex flask. It's not a perfect fit but it would be very close to filling the bottom half of the flask.

[GypsyJr]

The thickness is to the fill line of a 500ml pyrex round bottom flask or the length of a AAA battery

[thelivingdead531]

I think the circular bits are parts of a crinoid stem. I also see a partial brachiopod. The whole thing doesn’t look like a human skull plate to me though (and that’s good news ). 

[Kane]

If found in the Toronto area, it is not likely to encounter vertebrate fossils as much of the underlying geology is too old (upper Ordovician in age).  Apart from the crinoid bits correctly identified above, I am also seeing some typical angular calcite crystals and at least evidence of one brachiopod impression. There may be erratics in your area, suggesting the rock (if not found in a natural formation) could be anywhere from the Ordovician to the Devonian, potentially.