Well also consider that even if Earth and Mars are the same age, we can factor in that most models predict the moon slaming into earth during its early formation. If the same event didn't happen until much later in Mars formation, it could be possible Mars had a head start. Granted didnt Phobos slam into Mars too? I don't really know when that was supposed to have happened.

But yeah I'm getting way into speculation here than actually any science.

In your opinion, and assuming life did in fact exist on mars, what are the odds that curiosity or another rover would just stumble upon a fossil out in the open?

Like the rover is driving a long and boom just in frame in a rock or in the Martian soil is a fish fossil or something.

My assumption would be because of wind and sand erosion it's super unlikely. We would have to dig to find anything. But the thought a fossil just showing up in one of these photos makes me so excited. Especially considering where Curiosity is now.