Fossil in round rock?

[KWalla]

My daughter found this in a dry creek bed near our house. I was using muriatic acid to clean it up and noticed it was fizzing and pulled it out and saw the formation. There’s a black and red tar on the back that didn’t come off in the acid and it’s 3.5” x 3.25”. We have found a lot of arrowheads, native American artifacts and what we believe are fossils (teeth, bones, coral) but this one stood out the most. Does anyone know if this could possibly be a fossil? And if so, what kind? 

[Fin Lover]

This looks geologic to me.

[Rockwood]

No. These are definitely fossils. It's hard to tell for certain, but I think tabulate coral. Possibly bryozoan, or both. 

[Mark Kmiecik]

Fragments of marine creatures.