Question: Who was the first person to use fossils for dating rock layers?
Disambiguated questions and answers: question: Who was the first person to use fossils for dating rock layers theoretically?
Who was the first person to use fossils for dating rock layers in large-scale practical application?
answer: Nicolaus Stenonius
Niels Steensen
Nicolaus Steno
Nicholas Steno
William Smith
William 'Strata' Smith
Natural Questions answer: William Smith (from the Wikipedia article "Relative dating")