Organisms.
Fossils found in newer layers of Earth's crust tend to be similar to present-day organisms.
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The principle of fossil succession states that fossils found in younger layers of rock tend to be more similar to current species, while fossils in older layers differ more. This is because newer fossils represent species that have evolved and survived to the present day, while older fossils may belong to species that have become extinct. This pattern helps scientists understand the evolution and biodiversity of life on Earth.