Much of
the history of life has been recorded in fossils- which are themselves
a kind of happy coincidence, literally the accidental casts or reliefs
of ancient plants and animals. Silt, clay, fine-grained sand, and
dissolved minerals such as silica or iron often gradually
replace the
actual once-living parts of buried organisms in casts formed by
surrounding dirt. In time the dirt becomes sedimentary rock, containing
the fossils.
These occur either as positive or negative casts- the former, such as a shark’s tooth or a dinosaur leg bone (see fiigure of Iguanadon at right); the latter, such as the cast of a mollusk shell or the primitive, sponge-like Dickinsonia from 650 million years ago (seen below), leaving in the rock a negative relief or impression (fossa in Latin, hence the term “fossil”).
These occur either as positive or negative casts- the former, such as a shark’s tooth or a dinosaur leg bone (see fiigure of Iguanadon at right); the latter, such as the cast of a mollusk shell or the primitive, sponge-like Dickinsonia from 650 million years ago (seen below), leaving in the rock a negative relief or impression (fossa in Latin, hence the term “fossil”).
