Fossils are records of past life preserved in rock, amber, ice, or other materials. In paleontology, dinosaur bones, teeth, eggs, footprints, and skin impressions help scientists reconstruct how ancient organisms lived and changed over time. These remains matter because they provide physical evidence that life on Earth has not stayed the same.
Layer by layer, the fossil record shows patterns of extinction, adaptation, and the appearance of new forms.
Key Facts
- Older fossils are usually found in deeper, undisturbed rock layers because of the law of superposition.
- Relative dating orders fossils by rock layer position, while radiometric dating estimates numerical ages.
- Half-life formula: remaining fraction = (1/2)^n, where n is the number of half-lives.
- Transitional fossils show mixtures of traits that link major groups, such as feathered dinosaurs connecting non-avian dinosaurs and birds.
- Evolution is supported when fossil sequences show gradual changes in anatomy through time, such as limb shape, skull structure, or tooth form.
- Trace fossils, such as footprints and nests, preserve behavior and can show movement, herding, nesting, or predator-prey interactions.
Vocabulary
- Fossil
- A fossil is preserved evidence of an ancient organism, such as a bone, shell, footprint, or imprint.
- Stratigraphy
- Stratigraphy is the study of rock layers and their order to understand the relative ages of fossils and geologic events.
- Transitional fossil
- A transitional fossil has traits that show an evolutionary connection between older and newer groups of organisms.
- Radiometric dating
- Radiometric dating estimates the age of rocks or fossils by measuring the decay of radioactive isotopes.
- Trace fossil
- A trace fossil is preserved evidence of an organism's activity, such as a footprint, burrow, nest, or bite mark.
Common Mistakes to Avoid
- Assuming every fossil is a complete skeleton is wrong because most fossils are fragments, traces, or impressions preserved under special conditions.
- Treating deeper layers as always older is wrong because folding, faulting, erosion, or overturned strata can disturb the original order of rock layers.
- Saying humans evolved from dinosaurs is wrong because humans are mammals, while birds are the living descendants of theropod dinosaurs.
- Expecting evolution to form a straight ladder is wrong because evolution is a branching process with many side branches, extinctions, and shared ancestors.
Practice Questions
- 1 A volcanic ash layer above a dinosaur fossil is dated to 66 million years old, and an ash layer below it is dated to 70 million years old. What age range can scientists assign to the fossil?
- 2 A rock sample contains 25% of its original radioactive isotope. If the isotope has a half-life of 10 million years, how old is the sample?
- 3 A fossil dinosaur has sharp teeth and long claws, but it also has preserved feathers and a wishbone-like structure. Explain how this fossil could support the idea that birds evolved from theropod dinosaurs.