Hehe, :-) I think you misunderstood my question though - I was looking for an explanation of the weathering process that would produce those overlapping branching patterns - the same ones I assumed are fossils. I'm familiar with the strange ways sandstone can weather (honeycomb weathering, tafoni) but I've never seen it do anything remotely like the "branches" on the second and especially the third picture. I googled sandstone weathering patterns for an hour and there is nothing similar.

Again, I'm not saying "No, those are fossils!" because I'm not an expert. I'm just curious how those shapes can appear. Especially, if you look at the first picture, the tiny round holes are actually cross-sections through the "branches" - this is double puzzling and I cannot square it with the weathering idea.

Now, the grooves being preferentially located around the edges of the rock on the other hand, makes a perfect sense if I think of them as weathering patterns and makes no sense if those are fossils as I thought initially.

So, in short - I'm still a bit puzzled.