Scientists usually do not get a dinosaur’s date by measuring the bone itself. They work out the age of the rock layer containing the fossil, using the order of geological layers, fossils found in them, and numerical ages from suitable rocks nearby. Bone growth lines can answer a separate question: how many years the animal had lived before it died.
How do scientists know the age of a dinosaur fossil?
Most fossils are found in sedimentary rocks such as sandstone, limestone, and shale. These rocks are not usually dated directly with the isotope methods used for igneous rocks. Instead, scientists first determine where the fossil-bearing layer sits in the local rock sequence, then connect that sequence to geological materials that can be dated.
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First, establish the order of the layers
In an undisturbed sequence of sedimentary rocks, lower layers were deposited before the layers above them. Geologists use this principle of superposition alongside evidence of folding, tilting, erosion, and contacts between rock units. That field context matters: layers may have been disturbed, so their present position alone does not always tell the whole story.
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Fossils help establish relative ages, too. If the same recognizable fossil types occur in separate locations, geologists can use them to compare and correlate layers. This tells them which layers are older or younger, even before assigning a numerical age.
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Then, connect the sequence to a numerical clock
Radiometric dating measures the proportions of radioactive parent isotopes and their daughter products in suitable minerals or rocks. Because radioactive isotopes decay at known rates, those measurements can estimate when the dated material formed. The method and sample must be appropriate to the geological event being dated; a numerical result is not automatically the age of every nearby object.
Where a fossil layer lies between datable igneous layers, those dated rocks can bracket its age. A dated layer below the fossil horizon formed earlier and can constrain the fossil layer’s minimum age; a dated layer above formed later and can constrain the other side of the bracket, provided geologists understand the local stratigraphic relationships. USGS describes the general approach as finding an isotopically datable rock together with rocks that can be assigned relative ages from their fossils: Fossils, Rocks, and Time: The Numeric Time Scale.
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What each dating method actually tells you
| Evidence or method | What is examined | What it can tell you |
|---|---|---|
| Stratigraphy (relative dating) | The fossil layer’s position and relationships within a sequence of rocks | Whether the layer is older or younger than related layers; it does not by itself provide a numerical age. |
| Fossil correlation | Fossil types in layers at the same or different locations | Whether layers can be compared and placed in a relative sequence; numerical ages require additional evidence. |
| Radiometric dating | Parent and daughter isotopes in a suitable mineral or rock associated with the sequence | An estimated numerical age for the dated material, which can constrain the fossil-bearing layer. |
| Bone growth lines | Annual growth lines in a thin section of fossil bone | An estimate of the animal’s years of growth before death, not the geological age of the enclosing rock. |
| Radiocarbon (carbon-14) dating | Carbon in recently living material | Useful for remains from roughly the past 50,000 years, not dinosaur-age fossils. |
Why carbon-14 cannot date dinosaur fossils
Carbon-14 has a half-life of 5,730 years, and radiocarbon dating is useful only within roughly the past 50,000 years, according to the USGS explanation of radiometric dating. Non-bird dinosaurs lived tens of millions of years ago, so too little carbon-14 remains for this method to determine their fossil ages. Scientists do not routinely carbon-date dinosaur bones; they date suitable geological materials and interpret those results in the context of the fossil-bearing rocks.
Does “How old is that dinosaur?” mean the animal or the rock?
It can mean either. A fossil’s geological age refers to when the remains were deposited or preserved, inferred from their place in the rock record and constraints from associated materials. The animal’s age at death is a different question.
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For some dinosaurs, paleontologists can examine a thin section of bone and count annual growth lines. That can estimate how many years the animal grew before dying and help reconstruct its growth pattern. Those lines do not independently date the surrounding sedimentary formation.
How precise is a dinosaur fossil’s date?
A fossil’s age is an interpretation supported by several kinds of evidence, not an exact birthday read from a bone. Scientists consider the field setting, stratigraphic relationships, fossils in the sequence, and whether the dated rock or mineral is suitable for the question. Where feasible, they may use more than one analytical method to check a result. Numerical estimates have uncertainty and can be revised as measurements or geological interpretations improve.
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Direct radiometric dating of dinosaur bone has been proposed, but it should not be treated as the routine method. A 2012 comment on a claim involving dinosaur bones from New Mexico questioned the stratigraphic interpretation and whether the published analytical descriptions were sufficient to assess alteration and recrystallization. That critique concerns a specific case; it does not establish that every attempt to date bone directly is invalid. The standard approach remains to establish the fossil’s geological context and date suitable associated materials.
Did all dinosaurs live at the same time?
No. Non-bird dinosaurs lived approximately 245–66 million years ago, across the Mesozoic Era, according to the Natural History Museum, London. That span covers the Triassic, Jurassic, and Cretaceous periods; it does not mean every dinosaur species coexisted. The USGS notes that Stegosaurus was extinct approximately 80 million years before Tyrannosaurus appeared: Did all the dinosaurs live together, and at the same time?
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