Rock Anomaly in Mars Photo

I know this is just a rock but it sure is oddly shaped. This photo taken by the Mars Curiosity rover has what appears to be a perfectly square shaped rock. Could is it possibly be a fossil of some sort or some other type of object. Here is a link to the photo http://mars.jpl.nasa.gov/msl/multimedia/raw/?rawid=RRA_408244639EDR_F0050926R...