Relative Dating uses the Law of Superposition, which states that in an undisturbed sequence of rocks, the oldest layers are at the bottom and the youngest are at the top.
Biostratigraphy relies on the presence of index fossils to correlate rock layers across different geographic locations, allowing scientists to synchronize the relative ages of distant formations.
Absolute Dating (Radiometric Dating) measures the decay of radioactive isotopes (e.g., Carbon-14 or Uranium-Lead) to determine the exact numerical age of a rock or fossil in years.
Half-life is the constant rate at which a parent isotope decays into a stable daughter product; by measuring the ratio of parent to daughter, the time elapsed since the mineral formed can be calculated.
| Feature | Body Fossil | Trace Fossil |
|---|---|---|
| Source | Physical remains of the organism | Evidence of the organism's behavior |
| Examples | Teeth, bones, shells, wood | Footprints, burrows, nests, dung |
| Information | Anatomy, size, species ID | Movement, diet, social behavior |
Mold vs. Cast: A mold is a negative 'imprint' or hole left in the rock, whereas a cast is a positive '3D model' formed when that hole is filled with new material.
Relative vs. Absolute Dating: Relative dating tells you if a fossil is older or younger than another, while absolute dating provides a specific age in years (e.g., 65 million years old).
Identify Index Fossils: When presented with multiple stratigraphic columns, look for the fossil that appears in only one layer per column but is present in all columns; this is your marker for a specific time horizon.
Check for Unconformities: Always look for wavy lines in diagrams that indicate erosion or missing time; these 'gaps' in the fossil record can lead to incorrect relative dating if ignored.
Verify Decay Ratios: In radiometric dating problems, remember that after one half-life, 50% remains; after two, 25% remains. Always check if the question asks for the amount of 'parent' remaining or 'daughter' produced.
Sanity Check: If a question involves soft-bodied organisms (like jellyfish), remember they are rarely fossilized; if they are found, it implies exceptional conditions like an anaerobic (oxygen-free) environment.
The 'Complete Record' Fallacy: Students often assume the fossil record is a complete history of life. In reality, it is highly biased toward organisms with hard parts and those living in sedimentary environments (like oceans or river deltas).
Confusing Age of Rock vs. Fossil: In sedimentary rocks, the fossils are roughly the same age as the sediment. However, you cannot usually use radiometric dating directly on sedimentary rock; you must date nearby igneous layers (ash falls or lava flows) to 'bracket' the fossil's age.
Trace Fossils and Identity: It is often impossible to link a trace fossil (like a footprint) to a specific species of body fossil; scientists usually assign them to broader groups based on size and gait.
