How to read a dinosaur fossil range

By Sophie Taylor · Published 27 October 2026 · Data as of 2026-10-27 (Paleobiology Database download) · 4 min read

A dinosaur’s fossil range is the stretch of geological time in which its dated fossils have been found. On FossilRange it is written as two numbers of millions of years ago, oldest first. Stegosaurus has the range 154.8–145 Ma: its well-dated records fall in the Kimmeridgian and Tithonian stages of the Late Jurassic. A range is not a lifespan, and it does not date the first or the last animal of the genus. It summarises where the dated rocks are.

The two numbers

Both numbers are stage boundaries taken from the International Chronostratigraphic Chart. For Stegosaurus, 154.8 is the base of the Kimmeridgian and 145 is the top of the Tithonian, which is also the line between the Jurassic and the Cretaceous. Read the range as “from 154.8 to 145 million years ago”. The first number is always the larger one, because it lies further back in time.

Behind the two numbers are individual fossil records. The Paleobiology Database lists 85 records for Stegosaurus. 83 of them are dated closely enough to be placed in one stage by their mid-point age: 10 in the Kimmeridgian and 73 in the Tithonian. The range covers both stages because the dated records still reach into both after the outermost tenth on each side has been set aside.

How a range is worked out

Every genus page uses the same four steps, described in full on the Data & method page:

1. Keep only the records whose age bracket is 12 million years wide or narrower. Records dated to “somewhere in the Late Cretaceous” would stretch a range for no good reason.
2. Take the mid-point age of each remaining record and find the 90th and 10th percentiles. That sets aside the oldest and the youngest tenth.
3. Widen the result to whole stages, so the range starts at the base of a stage and ends at the top of one.
4. Before all this, drop small groups of records that sit more than 20 million years away from the rest, the usual sign of a misdated record or an old synonym.

The last step matters more than it sounds. Nine records once filed as Gyposaurus sinensis, an Early Jurassic name, used to stretch Gryposaurus back to 199.5 million years ago. With them removed its range is 83.6–72.1 Ma, all within the Campanian.

Why so many ranges are identical

Because each end snaps to a stage boundary, genera from the same rocks often end up with exactly the same range. 46 genera of the 99 on this site sit inside a single stage. These are the six most common ranges:

Counted over the 99 genera on FossilRange. Examples are the genera with the most fossil records.

So when Tyrannosaurus and Triceratops both read 72.1–66 Ma, it does not mean they appeared and disappeared together. It means the dated records of both fall in the Maastrichtian, a stage 6.1 million years long.

How long is a typical range?

The median range is 11.5 million years: half of the genera have a shorter one. The shortest are Brontosaurus and Kentrosaurus at 4.2 million years each, and the longest is Megalosaurus at 55.

Range length depends heavily on which stages are involved. The Maastrichtian lasted 6.1 million years, the Campanian 11.5 and the Norian 21.3. A genus known only from the Campanian therefore always shows a longer range than one known only from the Maastrichtian, however rich or poor its fossils are. Compare ranges within the same part of the time scale, and treat a difference of one stage as noise rather than news.

What a range cannot tell you

A range can only be as old as the oldest dated rock that has produced the genus, and only as young as the youngest. If a genus lived in a place or at a time with few fossil-bearing rocks, its range will be shorter than its real time on Earth. That is why “range” is the right word and “lifespan” is not.

A range also says nothing about how long any single animal lived, or how long a species lasted. A genus can hold several species, and the range covers all of their records together; Stegosaurus has 7 accepted species in the database. And a range is a snapshot: new finds, new dates and new identifications move the ends. The figures here reflect the database as downloaded on 2026-10-27.

Reading the chart on a genus page

Each genus page opens with the same chart. The thin coloured ribbon at the top is the whole age of dinosaurs on this site, 251.9 to 66 million years ago, in the official stage colours: purple for the Triassic, blue for the Jurassic, green for the Cretaceous. The amber outline marks the slice that is enlarged below. In the enlargement, the amber bar is the genus itself and the pale bars are the genera most often found in the same fossil collections. Where a genus has no such companions, grey bars show genera of the same age instead.

When a genus is among the 15 highest or lowest on the site for range length, record count or spread across countries, its page also places it on a strip where every line is one of the 99 genera. That is the quickest way to see how unusual it is.

Try it with two genera

The Who lived when tool measures any two ranges against each other. Allosaurus and Stegosaurus share the same 154.8–145 Ma range, and the database lists them together in 53 fossil collections: compare them. For the opposite case, see How far apart in time were famous dinosaurs?

Sources

• The Paleobiology Database (paleobiodb.org), data service v1.2, licensed CC BY 4.0; retrieved 2026-10-27. All counts, ranges and pairs in this article are calculated by FossilRange from that download.
• International Commission on Stratigraphy, International Chronostratigraphic Chart: stage names, base ages and colours.
• FossilRange, Data & method: which genera are included and how ranges are derived.