Apparent Ammonite Fossil On Mars
Ancient Organisms Sported Spiral Shells
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Opportunity continues to produce amazing images and the trek toward Erebus Crater has yielded more geysers and mudholes on the way.  This particular shot from Sol 619 shows a very muddy area and what appears to be the shell of an ammonite, a large cephalopod that is similar to the cuttlefish and the nautilus here on Earth.
These are ammonite fossils.  You can see the distinct spiral pattern of each fossil here.  The only living relatives of the ammonites today are the nautilus and the cuttlefish.
Here is a pearly nautilus as photographed by Nick Dakin.

Few of these animals survived the great extinction events that killed dinosaurs and other species which had been very common all over the world before then.
On sol 619, Opportunity imaged this muddy region of Meridiani Planum.  Here is a fairly faithful color image of the area, along with some other details that are not readily apparent without magnification.  Image is from L2-L5-L7 filters.

Click the image for the larger version.  The original image data is here at the NASA/JPL web site.
This is a false color image of the same area, created from the L2 and L7 filters and using synthetic green.  I have left the color saturation rich for ease of spotting the features.

Click the image for the larger version.  The original image data is here at the NASA/JPL web site.

Look at the rock near the lower left of center- and just about 30 degrees from the tip of the rover solar panel.  That is the one in question.

Magnified and left at saturated color, we can see in this stereo view that the rock does indeed show a spiral form and is consistent in three dimensions.  It even shows the "knobby" structure of the suspected shell, known as "sutures".  These exactly match the form of an ammonite shell as seen in the examples at the top of this page.

Here is the marked up version of the rock.  The spiral shape is clear.  The rock itself also shows the same coloration as the other fossils, the "hematite" blue, which appears gray in the color corrected images.

While this is not proof that this is an ammonite shell, it is in context with all the other fossils- a marine creature that was common at the tine that these other organisms were alive.

Given that Meridiani Planum was once a very large ocean on Mars, and that it slowly dried up, producing the many thousands of square kilometers of mud polygons we see today, and that this was an area rich in life, we can see much supporting evidence for this to be an ammonite or one of its relatives.