How do we know how old a dinosaur is?
Numbers like "66 million years ago" aren't guesses. They come from reading the order of rock layers and from natural clocks in radioactive minerals.
We don't date the fossil itself
Most dinosaur fossils cannot be dated directly. Instead, scientists date the rock layers the fossil is buried in, using two methods together: relative dating and absolute dating.[1][4]
Relative dating: reading the layers
The basic rule is the "law of superposition": unless the layers have been overturned, lower layers formed before the layers above them, so fossils in lower layers are older than fossils higher up.[2]
Scientists also use "index fossils", such as ammonites or pollen and spores: organisms that were widespread but lived only for a short time. Finding the same index fossil in two places shows the rocks are of similar age. This tells us what is older than what, but not yet a number of years.[5]
Absolute dating: radioactive clocks
Some minerals, such as zircon in volcanic ash, contain uranium that decays into lead at a constant rate. When the mineral crystallizes, the clock starts ticking. Measuring the ratio of uranium to lead (the U–Pb method) tells how many millions of years ago the mineral formed.[1]
If volcanic ash layers are found both above and below the layer with the fossil, they give an "age bracket": the dinosaur lived somewhere between those two dates.[1][4]
The well-known carbon-14 method cannot date dinosaurs. It only works for things up to about 50,000 years old, while dinosaurs are tens to hundreds of millions of years old.[6]
An example from Thailand
Isanosaurus, a long-necked dinosaur from northeastern Thailand, was once thought to have lived in the Late Triassic, which would have made it one of the oldest sauropods in the world. In 2026, U–Pb dating of zircon showed that the rocks containing Isanosaurus formed in the Early Jurassic, a good example of how more precise dating can change what we thought we knew.[3]
Sources and further reading
More articles
- How dinosaur footprints form
- Why we say "dinosaur fossils", not "dinosaur bones"
- The first dinosaur fossils (in the world and in Thailand)
- How dinosaurs get their names
- How many Thai dinosaurs are there, and where were they found?
- Are birds really dinosaurs?
- The day the dinosaurs disappeared
- Animals people think are dinosaurs, but aren't
- Jurassic Park: how much of it is true?
- How do we know what colour dinosaurs were?
- Dinosaur eggs and babies
- How did long-necked dinosaurs get so big?