Niels Stenson (Nicholas Steno) was a Danish anatomist responsible for developing a fundamental geologic theory with his "dissertationis prodromus" in 1669. In this work he outlined three of the defining principles of the
science of stratigraphy: the law of superposition, the law of original
horizontality and the law of lateral continuity. Steno became
interested in the study of rock layers (stratigraphy) by chance, when a local duke ordered him to
dissect a large shark local fisherman caught off the coast of Livorno,
Italy. Steno noted that the shark's teeth resembled certain stony
objects found within rock formations, which were referred to at the
time as "tongue stones" or glossopetrae. Steno was the first to
put forth the idea that these glossopetrae were in fact altered teeth
that had belonged to ancient sharks that were buried in the mud long
ago. This idea ran contrary to the common belief that fossils
"grew" inside of rock outcrops, but echoed ideas of contemporary
geologists Robert Hook and John Ray. Steno's work on shark teeth
led him to the question of how solid objects could come to be found
inside another solid object, thus feeding his interest in stratigraphy
and sedimentology. His observations contributed to the
fundamental observations that
bind together the study of geology, which over time provided the
framework that would allow fellow scientists to use fossil evidence to
examine how life evolved on Earth.
http://en.wikipedia.org/wiki/Nicolas_Steno#Contributions_to_paleontology_and_geology