This was found on Sol 049 by Opportunity with the microscopic imager in Meridiani Planum.  It is a small rolled up trilobite.  See more information here. The original image is here at the NASA/JPL web site.
This was found on Sol 555 by Opportunity with the microscopic imager in Meridiani Planum.  It is also a rolled up trilobite.  See more information here. The original image is here at the NASA/JPL web site.
Finally, this was found on Sol 708 by Opportunity using the microscopic imager in Meridiani Planum.  It is a third identical rolled up trilobite.  A page on this find will be posted shortly. The original image is here at the NASA/JPL web site.

All three objects are bilaterally symmetrical (meaning they have a right and left "mirror" half) and have the same lobes.  Each object is the same size.  Each object's lobes are the same scale relative to each other.  In all cases, faint corrugations or ridges can be seen at right angles to the central lobe.

With so many identical complex objects being found again and again, how can anyone deny that some process other than geology is responsible?  This meets the so-called Knoll criterion as it shows that some incredibly improbable process is at work, producing forms that geology alone cannot manage.  These are fossils of ancient organisms that lived in the Martian seas many millions of years ago.