How Can Scientists Tell How Old a Dinosaur Was?

A fossil bone can hold growth clues, but counting its lines is not the same as reading a birthday.

Suggested Grades 3–4274 core-passage wordsUpdated 2026-09-18

What you’ll practiseSeparate a visible observation from an age estimate and recognise missing evidence.

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Illustration only. These are not measurements or annual marks from a real fossil.

A record inside the bone

A dinosaur fossil has no birth certificate. Its bones may still contain clues about its life, including how it grew. Scientists study very thin slices of some fossil bones under microscopes.

Bone was living tissue when the animal was alive. It could grow and change. Faster and slower periods of growth sometimes left marks in its microscopic structure.^[1]

Lines need an explanation

Some repeated marks are called lines of arrested growth. They record pauses in growth. When researchers have good reasons to interpret a sequence as annual, that sequence can help them estimate the animal’s age.

The idea is similar to reading tree rings, but it is not a rule that every visible line in every dinosaur bone equals one birthday. Scientists have to identify the marks and consider how they formed.^[2]

Missing pages in the record

Growing bone can remodel its inner layers, removing earlier marks. Damage can destroy part of a fossil. Different bones from one skeleton may preserve different amounts of information.

That means seven visible marks do not automatically prove an age of exactly seven years. A careful report distinguishes the observation—seven marks are visible—from the interpretation of the animal’s age.^[2]

Growth is more than an age

The distance between marks can also provide clues about how much bone was added between them. Comparing several individuals helps researchers construct models of growth through life. A model is an explanation based on evidence, not a photograph of the past.

Tyrannosaurus rex is still being studied this way. Researchers can revise growth estimates when they examine additional bones or interpret marks differently. It is better to ask which study supports an estimate than to memorise one age as a permanent limit for the whole species.^[2]

Word Lab

Use the meaning, hear the word, then try it in your own sentence.

estimate

verb

To calculate an approximate value using available information.

Scientists estimate age from several clues.

observation

noun

Something noticed or measured.

Seven marks are visible is an observation.

interpretation

noun

An explanation of what evidence means.

The age estimate is an interpretation of the marks.

microscopic

adjective

Too small to see clearly without magnification, or relating to a microscope.

They examined the microscopic structure of the bone.

preserve

verb

To keep something from being lost or destroyed.

The fossil did not preserve every growth mark.

Illustrative data for this exercise—not measurements from a real fossil.
Age interval	Bone added
8–9 years	10 mm
9–10 years	12 mm
10–11 years	13 mm
11–12 years	5 mm

Put it in your own words

Explain why seven visible growth marks do not, on their own, establish an exact age of seven years.

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Check your own explanation

Describe what was observed.
Mention possible missing marks.
Explain that the timing of the marks needs interpretation.

This is self-review, not automatic marking.

Compare with a model response

Seven marks are visible, but some early marks could have been removed. Researchers also need evidence that the marks represent annual growth before converting the count into an age.

Sources and revision notes

Source check: 2026-09-18. Grade guidance is an editorial suggestion, not a standardised reading score.

What changed in this edition?

Removed a fixed T. rex lifespan claim and an unqualified growth-rate figure. The practice table is now explicitly labelled as illustrative, not real fossil data.