Great site for fossil hunters
One of the better sites along these lines:
Mars Fossils, Pseudofossils or Problematica?
The author is probably more skeptical about these being real biogenic fossils than most. He got that way by studying them long and hard. If these are fossils, which is not certain but possible, then it is likely these organisms evolved during the watery epoch of Mars hundreds of millions of years ago. In that case the Opportunity Rover seemed to have stumbled upon a marine reef of hard-shelled but eroded organism debris.