See pixel location 374,852 (bottom sixth, near center) in NASA photo:

http://marsrovers.jpl.nasa.gov/gallery/all/2/n/067/2N132312350EFF17B8P1944L0M1.HTML

full sized image:

http://marsrovers.jpl.nasa.gov/gallery/all/2/n/067/2N132312350EFF17B8P1944L0M1.JPG

Is that an archaic fish fossle laying there? Here is a clip of the possible fish:

Archaic fish can be worth some money! See:

http://www.texasrockshop.com/fossils03.html

http://www.twoguysfossils.com/fish.htm

http://www.paleodirect.com/fish1.htm