Found this at the war field quarry in Wyoming. It's from the green river formation. Though it is only a midsection partial, I believe it's priscacara. It's actually really well preserved with gill material, scales and skin leftover.
You seem to be showing it upside down to me.
Unless I'm wrong what I'm seeing is oriented like this showing the dorsal and pectoral fins as well as a large gill plate on what appears to be part of quite a large head in proportion to the body.
I've hunted that area & found a couple whole fish. I also found some partials & ones that "exploded" so the remains are fossilized in a chaotic way. It's been yrs. since I've been there. Always wanted to go back. I love Kemmerer. Some big Browns in the reservoir there.