re: Exquisitely argued
And yet my first impression is that we got us a major fossil here, not the first we've seen on Mars, but one of the most intriguing.
Let's say it is a "fossil," not a "rock" at all. But how could we tell the difference? This is an ongoing problem with Mars exploration to date; it's true with our robotic explorers, and it would be true with human explorers:
There is almost no way to be sure about whether a given formation or finding is the result of Mars biology or some other non-biological cause.
But maybe it's just a rock. ;-)
Cheers,
BRFelix