NSR Mosasaur ? Bone but which part?!?

[shel67]

 I found this nice chunk of mosasaur (?) bone over the weekend at the North Sulphur River in Ladonia, Texas.  It has beautiful black enamel on each side that is very smooth and shiny. It has a bowl shaped curve to it on one side.  I was thinking it was a rib chunk, but I wasn’t sure with it having the curve. What do you all think?  Thank you in advance! 

[JohnBrewer]

Not sure you could even identify it as mosasaur! Can’t help but it doesn’t look ribby to me. Maybe @JarrodB can help. 

[JarrodB]

Based on color & texture I would say fish skull fragment. 

[LordTrilobite]

I disagree, the structure does not look like fish to me. A few sides look pretty worn, so IDing it will be hard because of that. But the bone structure does look like reptile.

[All Day Rocks]

Looks to be a mosasaur part in my opinion. As far as what, it's hard to say exactly.

[shel67]

I have a small mosasaur rib chunk that was IDed, and it very similar to this one, hence my first thought that it was from the same type of reptile. I’m such a newbie at this, so I really appreciate all of the input! 

[jpc]

Maybe a well worn tail vertebra...?

[M Harvey]

It's an worn mosasaur atlas.  Part of the occipital complex at the base of the skull.