“An Example of Remarkably Well-Preserved fossils”

 

Sometimes creatures are preserved astonishingly well that it boggles the mind!  From Dinosaur skin impressions, to frozen mammoths and glass insect fossils, the world of fossil preservation is incredibly bewildering!  And such was the case with the whole, unfilled, baby Arc Clams, Anadara trilineata, shown here.  Preserved so well, without a gap between the two valves of the shell, no ocean sediment filled into the inner area of the shell and the shells are “empty”.  These shells were likely moved very little from their original burial spot, so as to preserve both valves together.  Almost universally, fossil clams are discovered with sediment-filled interiors, meaning they were moved around on the bottom of the sea floor before being buried.  Even clams that are preserved “in-situ” or in life-position are usually preserved with sediment filling inside the two valves, making these clams extremely exceptionally, uber rare.  Additionally, almost universally the surrounding rock exerts a degree of pressure on a fossil and can even break, squish, or completely pulverize the organism.  

These Arc Clams seem to have avoided that.  Clams can live either in the sediment, or infaunal, or on the sediment, or epifaunal, at the bottom of the ocean; the two most common lifestyles–there are others.  This particular species lived just below the sediment-water interface.  Reconstructing the ancient lifestyle of this clam was the focus of one of my earliest research papers as a young paleontologist at U.C. Berkeley back in 1984.  I was able to use the commensal marine Polychaete worm Polydora, which leaves a scar on the shell of Anadara, as a proxy measure to reconstruct the lifestyle orientation of these ancient fossils from 4 million years ago.  Commensalism is when two creatures share an ecological relationship that neither harms (parasitism) nor benefits (symbiosis) the two species involved.  The link to the manuscript, and some of the diagrams used, appear below.

Thompson, W., 1984.  Anadara from the Purisima Formation along the Monterey Bay

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