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How Do Scientists Use Fossil Layers to Date Ancient Life Forms?
科学常识延展阅读·独立成篇（2026-D049）
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Geologists study rock layers, called strata, to understand Earth’s history over millions of years.
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Fossils found in lower strata are usually older because sediment builds up gradually over time.
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Index fossils—species that lived briefly but spread widely—help match layers across distant regions.
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Radiometric dating measures radioactive decay in volcanic ash layers above or below fossils.
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By combining fossil evidence with chemical analysis, scientists build timelines of evolutionary change.
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Some layers contain microfossils too small to see without a microscope but rich in clues.
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Disturbances like folding or faulting can complicate interpretation, so multiple lines of evidence are used.
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This method revealed that dinosaurs disappeared about 66 million years ago after an asteroid impact.
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Fossil layering also shows how oxygen levels and ocean chemistry changed during key eras.
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Understanding deep time helps us predict how current environmental shifts might affect future life.