Found this while on a hike with my son. Didn’t even notice what was on/in the back until cleaning it today. Any ideas at all? Could it possibly be a fossil of some sort? Doesn’t look like it to me.
They did it like Back-of-the-Boat said. Fossilization.
That's called a "trace fossil" because it is an internal mold formed from the mineralization of the voids inside the animal; which did not fossilize as well.