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A chance encounter between determined fishermen and a great white shark off the Tuscan coast in 1666 sparked a chain of events that would help change humans views of fossils and Earth’s geologic past (Cutler 2003, pp. Nicolas Steno (1638-1686) dissected the head of this shark and realized fossil tongue stones believed to be petrified snake or dragon tongues were actually fossil shark teeth (Prothero 1998, p. One problem still existed, how do fossils become embedded in solid rock?Steno recognized that fossils represent organisms that became buried in sediment, which later turned into rock.For example, the decay of potassium-40 to argon-40 is used to date rocks older than 20,000 years, and the decay of uranium-238 to lead-206 is used for rocks older than 1 million years.Radiocarbon dating measures radioactive isotopes in once-living organic material instead of rock, using the decay of carbon-14 to nitrogen-14.Sedimentary rock is made of particles derived from other rocks, so measuring isotopes would date the original rock material, not the sediments they have ended up in.

These break down over time in a process scientists call radioactive decay.

Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter.

Each isotope is identified with what is called a ‘mass number’.

Brongniart was the first to use fossils to date rock strata., which states that geologic events are caused by natural processes, many of which are operating in our own time.

Put another way, the natural laws that we know about in the present have been constant over the geologic past.

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