[Boundless]

> But seriously, imagine if they found some indisputable
> fossil in a photo, like a fish head, that could NOT be
> explained by geologic processes.

This would be an alien fossil.
Would it be that easy to conclusively identify it as such?

The gravimetric, solar/thermal and chemical environment
on Mar, even when it might have supported water-based
carbon-based life, would have been very different from
anything at anytime on Earth.

Of course, if all inanimate processes can be ruled out...

[ElkGroveDan]

Well I used the fish as an example, but clearly there are fossil shapes that could excede the boundaries of geologic explanation.

I don't think it would be unreasonble to imagine multiple appendages or fins in an aqueous environment, or perhaps some sort of spinal structure, assuming life on the alien location had enough time to evolve that far.