Deep time
How we know how old a fossil is
6 min read
The short version
Nobody dates the bone itself. They date the rock layers around it — usually volcanic ash, which contains crystals with a built-in atomic clock. The fossil must be younger than the layer below and older than the layer above, which is why ages are always given as a range.
Layers first
Sedimentary rock accumulates from the bottom up, so a layer is younger than everything beneath it. Establishing that order across a region is the foundation everything else hangs on.
A dinosaur found between two datable layers is bracketed by them — say, younger than 68.0 Ma and older than 66.8 Ma.
The atomic clock
Volcanic ash contains zircon crystals that trap uranium and exclude lead when they form. Uranium decays to lead at a known rate, so the ratio gives the crystal's age.
Modern uranium-lead work can resolve a 66-million-year-old ash bed to a precision of tens of thousands of years — better than 0.1%.
Dating by fossils
Many dinosaur-bearing sequences contain no datable ash at all. There, age comes from index fossils — pollen, marine microfossils, ammonites — species with short lifespans and wide distributions whose ranges are already calibrated elsewhere.
This is why DinoAtlas records an occurrence's age as a maximum and minimum in millions of years, tied to a named international stage rather than a single number.
Common questions
- Can carbon dating be used on dinosaurs?
- No. Radiocarbon only works back to roughly 50,000 years; dinosaur fossils are thousands of times older.
- Why is a fossil's age given as a range?
- The dated material is the rock above and below, so the fossil is bracketed between two ages rather than pinned to one.
- How accurate are these dates?
- The best uranium-lead ages on Mesozoic ash beds carry uncertainties of a few tens of thousands of years — under 0.1% of the age.