Replacement of Fossils

You might think that getting a shell or bone or wood chunk safely buried is the tough part for fossilization, that once something's entombed in sediment it's all smooth sailing. Burial is certainly important, but it's not the end of the story. A lot of things can happen between deposition and exposure. Pore spaces are filled with new minerals. Existing minerals are replaced. Entire structures can be replaced, then lost. These changes all fall under diagenesis. What exactly happens depends on things like the physical and chemical structure of the object in question, temperature and pressure of burial, and the chemical composition of the fluids in the sediment. Denser fossils like teeth are less vulnerable to changes than more porous materials. The form of calcium carbonate known as aragonite is less stable than calcite. Many different minerals and mineraloids can get involved in the fun; for example, there are opalized fossils and pyritized fossils.

Bivalve mold and internal cast (steinkern). Not pictured: bivalve shell.

Because silica and carbonate minerals are so abundant at typical surface and near-surface temperatures and pressures, they are the minerals most frequently involved. In Minnesota, we generally get dolomitization. This is somewhat inconvenient, because dolomitization has a tendency to destroy fossils, and even when it doesn't, it usually leaves behind molds and casts that aren't as crisp as the original. It's a bit like replacing the Venus de Milo or Michelangelo's David with nothing but 2x4 Lego bricks; you'll notice a difference. Dolomitic replacement may give a fossil a quirky sparkly appearance thanks to the dolomite rhombs, but that's about the only plus. Pervasive dolomitization is why many fossils in the Platteville are gray with a sugary appearance: you're actually looking at a natural mold or cast of the original in dolomite.

Sometimes diagenesis gives you exotic, spectacular fossils, and sometimes it gives you dolomite.

Although once in a while you get something to write home about; this is a nautiloid in the Science Museum of Minnesota collections with its internal structures replaced.

I was inspired to write a note about this topic by a different kind of replacement. Someone reviewing one of my work projects commented on a type of replacement seen in some of the fossils, consisting of circular mineralizations. They informed me this was a kind of silicification known as beekite. This immediately twigged my memory banks, because I'd also seen it in photos of fossils from other work projects. Like dolomitization, it's not exactly faithful reproduction, although it can be aesthetically pleasing.

A beekitized (beekitified?) lower Permian brachiopod, central Kansas.