Fossils on Mars? We can't know, as MERs are geologist rovers, not paleontologist.
But, if fossils actually exist on Mars, it would be very hard to find them: fossils are usually buried, and they are also rare, as fossil formation is a very low-probability event.
- Fossil formation is very rare (link1, link2)
- If fossils form, they are usually buried
- MERs have not been designed to find fossils, but to study rocks, so they can't identify fossils
- The area covered by MERs is very little, just a few kilometers: it's very difficult to find a fossil, and it is even more difficult that MERs landed in an area with so many fossils as many people say!
- A single photo of a rock does not allow any exact identification, due to lack of third dimension; two different photos of the saem object help, but it isn't still enough, due to strange effect of light & shadows; at least 4 or more images of the same place, taken at different times of the Sol, are needed to at least suppose that a rock is not just a rock
- If a probable-fossil needs to be enhanced, turned, stressed and underlined to allow other people to actually see it, then probably it is not a fossil, it is just you that are seeing things...
All people interested on MER images study are strongly encouraged to consider these things before "deciding" they found another fossil on Mars.... [-(