I think they could be fossils. I've always been of the opinion that the most successful macroscopic life-forms on Mars would be spherical, and have a thick skin. This would allow them to be pressurized. An impervious skin and a high internal pressure would both restrict sublimation, making it easier to contain water in a liquid form, essential for biological processes.
That's why I've always been of the opinion that the "berries" are biological in origin. They fit the bill, and they don't look like any non-biological concretions that I've seen.
Apart from the "berries", there have been several pictures of larger objects that appeared to be hollow shells, with holes in them. These too are fossils, in my opinion. The spheres here could be "berries" or they could be smaller relatives of those shells.
Now the mark of a successful lifeform is ubiquity, or near-ubiquity. On Earth, for instance, grasses and trees are found almost everywhere. If the berries were just confined to a couple of locations, then that might suggest they were just concretions, or something else non-biological. But if it turns out that they occur almost everywhere, I don't think it's credible that near-identical non-biological concretions would occur almost everywhere on Mars.
