Oh great lj overmind:
NASA is out with a new photo of a so-called "Mars Flower".
What is it? Is it a fossil (e.g., a shell left by some ancient creature) or a non-fossiliferous rock inclusion?
Without even doing any chemical/geological analysis, I vote for non-fossil, not left by a living organism.
Why? I've done a fair amount of fossil hunting (cephalopods and brachiopods in Wisconsin, diplodocus in Wyoming, cup corals in New York, etc. etc.). I very rarely see a rock containing a single fossil in isolation. Why? Because of math. Organisms are arranged in a pyramid - with many common animals at the bottom, and the rarer ones (e.g., predators) at the top. The most commonly found fossils are of the most common organisms. And these are often found in clusters, clumps or groups:
Either they find one space that's really great for growing and thus grow together in a clump (perhaps a founder individual and all its offspring) or else they are washed together by floods or currents after they die. Without knowing anything about fossil life, you can look at the above picture and tell that (a) there are a lot of shells all clumped together; and (b) most of them are one type; and (c) there are a few here and there of a different type.
Walk out on any beach and you'll find a million shell? Chances are these are dominated by one or two types. Given enough time, these will turn to fossils.
Thus, I think it's pretty safe to say that if we were to find fossils on Mars, the first ones we would find are likely to be really common organisms, and we are likely to find a LOT of them all in clusters.
We've had a lot of probes go to Mars to a lot of different locations and they've looked at a lot of rocks and NONE of them have seen macroscopic (big enough to be seen with the naked eye) fossils (or living creatures). Even if there were intelligent Martians alive and hiding from our rovers and landers, the stuff that they live on would be evident. (For example, maybe Bilbo Baggins is stealthy enough to escape human notice, but it would be impossible for him to hide all his grasslands and fruit trees.)
Oh great lj overmind, with this knowledge, is it safe to really conclude for certain that Mars never had macroscopic life (because we would have seen it by now)?
And thus this "Mars flower" is not a fossil, but just a (very interesting) rock?