By the Book

Most textbook definitions of "fossil" refer to some variant of "any sign or indication of ancient life".  This covers a lot of territory, from bones and seashells, to chunks of wood and leaf impressions, to worm burrows and animal footprints.  Even globs of feces count as fossils, if they are old enough.  Fossils often are found enclosed in sedimentary rocks (sandstone, limestone, shale, etc.); if instead of being lodged in the rock they are found lying on the surface of the ground they often can be assumed to have come from whatever sedimentary rock formation happens to be located nearby.  (But not always - clearly under some circumstances fossils can be transported a long way from where they were originally buried.  Ancient or modern streams are often involved.)  In the Park many of our older sedimentary formations contain shells (clams, snails, etc.), suggesting a marine or lacustrine (lake) environment; younger rocks carry bones of land vertebrates, pointing to a profound change in topography and climate.  These younger rocks also retain footprints of interesting land animals, most of which are now absent from the Park.  Fossil wood is common.  No dinosaurs, though - our rocks are too young.

Photograph from Anza-Borrego Desert Paleontology Society Archive

How Old?

How old must a thing must be before it can be labeled a "fossil"? There is no hard-and-fast rule, but clearly anything more than a few thousand years old is a legitimate fossil.  Paleontology professors have been known to say that "if it stinks, it belongs to biology; if not, it's ours".  Obviously this doesn't work very well for seashells or leaf impressions.  As members of the Paleo Society we learn how to make this distinction (fossil/not-fossil) using tests involving matches, our noses and/or our tongues.  (Refer to the course notes for the fascinating details.)  Our Park fossils are very much more complicated biologically than the earliest fossils found elsewhere, and very, very much younger - all are less than about 20 million years old.  Our much-prized large vertebrate fossils date from only the last few million years.​

Photograph by Jeannie Johnstone, Anza-Borrego Desert Paleontology Society archives

Fossilized

So what does the common term "to fossilize" mean? In everyday use, it applies mainly to bones and "petrified wood".  In this sense the process of "fossilization" implies an extremely slow molecule-by-molecule replacement of the original material of the bone or tree-fragment by inorganic minerals derived from groundwater.  And so, for instance, much petrified wood now consists of silica (SiO2), with none of the original organic material remaining - although the process of replacement may leave the internal structures of the wood intact and easily recognizable.  To fossilize in this sense requires a very long time, as well as a "matrix" (the rock containing the fossil) that will permit mineral-carrying groundwater to pass through it. This in turn requires holes between the grains composing the rock (called pore spaces) that interconnect, giving the rock "permeability".  Many Park sedimentary rocks have ample porosity and permeability, but have not been exposed to mineral-charged groundwater. Also, many of our fossil bones are so young that the time factor comes into play.  Thus most Park fossils consist of the original material itself, in many cases badly weakened and corroded.  Collecting and preserving such objects - still fossils, whether completely "fossilized" or not - provides many interesting challenges. 

---

​Myrl Beck, Anza-Borrego Desert Paleontology Society, 2008