This looks like the negative impression of a tiny Stegosaurus. I saw it a while back but couldn’t find it again, until now! The egg isn’t very developed. It might have died or maybe wasn’t fertilized properly. Anyway, somehow the little body of the Stegosaurus was pressed into the egg top. We can see the back plates!!!
If it weren’t for the back plates, I wouldn’t have noticed at all. It is 2.25″ x 1.6″ x .5″
I found it in Glen Rose, TX while digging on private property, with permission! It is right by Dinosaur Valley State Park.