unusual pebble shape

In the "mission success" panorama

http://origin.mars5.jpl.nasa.gov/gal...B009R1_br2.jpg

there is visible a pebble or small stone that has a
very unusual shape: it appears to have two "stalks"
that protrude upwards (rather like the stalks
or antennae of a snail).

One can see the pebble by scrolling about 3/5 of the
way to the right, and about 3/5 of the way down,
in the panorama.

In the high resolution view, one of the stalks can be
seen to be casting a shadow.

Any idea how a stone with this unusual shape could
be formed? Or could it be some part of the rover
airbag that fell off while it was rolling?

PM

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