Yes, Ron, I follow you. But how can you be certain, absolutely certain, that there are no seashells on Mars? And how can you be certain that the images in the picture are not, indeed, seashells (fossilized or otherwise)?

Whence cometh thy certainty on the matter? ;-) You say "common sense". I say "knee jerk assumption".

You note that I am neither assuming that they are seashells or that they are not. I don't believe I'm in a position to make such assumptions, and I don't believe you are either.