From their first discovery, fossils had been interpreted as stones that fell from the sky, tricks of the devil, or objects that grew in the rocks in which they are found.
Nicholas Steno’s work with modern shark teeth convinced him that the objects we recognize as fossil shark teeth looked like shark teeth because they were shark teeth, and that they must have been buried in mud or that was now dry land.
Steno also reasoned that if the fossil were a structure that had grown within solid rock, its shape would have been distorted by the enclosing rock. The pristine condition of the fossil shark teeth indicated instead that the tooth must have been buried in soft sediments which hardened later.
Steno’s realization that objects entombed within rock, like fossils or pebbles in a conglomerate were formed before the rock itself was deposited. This was another way to distinguish the relative order of events and is called the principle of included fragments.
Simply put, this principle states that the included fragment (be it a fossil in limestone or a rounded pebble in a conglomerate, shown above) is older than the rock that encloses it.
Image and more information available here.
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