Sol 166 Sea Shell inside Endurance Crater
Small broken shell resembles moon shell
Page Link:
Endurance Crater provides us with excellent material because of the cross section of Martian geology presented there.  We can see the varying layers of sediment revealed and eroded over time, and due to this, we can also see some fossils that are not commonly seen on the surface elsewhere in Meridiani Planum.  This shell is one such example.
In this image, we see a pretty typical scene from the crater wall.  This was taken on Sol 166, when the rover Opportunity was located about the glacier, where groundwater had come from a fracture in the crater wall and frozen as it flowed

The shell is at the lower right in the frame.

The original image is here at the NASA/JPL web site.

  
A total of three images from three viewpoints have been located.  The first pair are perfect stereo data and allow us to create a view so we can confirm the structure is in fact that of a shell and not simply a random bit of stone.

Unfortunately, no other parts of the spectrum are made available.  Only R1 and L7 (near the violet part of the spectrum) were released.

  
A small, preliminary stereo pair (cross eyed type) shows that it is convex, rounded and smooth, with faint markings on the shell's exterior and some sort of internal structure.

The spherules give us a scale of 5 pixels per millimeter, roughly.  This reveals that the shell is roughly 16 millimeters in length, from top to bottom of the image.

  
Given this information, we can gauge the shell against terrestrial ones.  It is small but not unreasonable.  But there also appear to be faint markings on the surface.  Could we increase the contrast and brightness settings to show the markings more clearly?

High contrast stereo view shows markings similar to those seen on terrestrial sea shells.  This is another useful indicator that we are not seeing a random piece of rock, but an orderly structure.  Note the wafer-thin out shell continues around to the upper right and down to the soil.  It is an unblemished perfect arc, just as we might expect for a sea shell.

Now compare the above Martian sea shell to the montage at the left.  It would be right at home next to these moon snails and spiral shells.

Pay close attention to the lower right image that shows the inner detail of the fractured shell.  The conic spiral section is a near perfect match for the center of the Martian shell above.

The shape and interior detail are identical as are the ways these shells break.  No rock will do this; only a thin walled structure with a spiral interior will.

I have assembled some stereo anaglyphs here to make the images accessible to more people.

First anaglyph:  wide field

Second anaglyph:  cropped field