The scientific study of rock layers and geological time periods using fossils and the remains of living organisms to determine age and sequence.
From 'bio-' (life) + 'stratigraphy' (study of rock layers, from Latin 'stratum'). This field combines paleontology with geology to use life forms as time markers.
Before we had radioactive dating, paleontologists figured out the age of rocks just by looking at fossils—each layer of rock had characteristic organisms, creating a biological timeline stretching back billions of years with no radioactive elements needed!
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