Using the Who lived when tool
The Who lived when tool answers two questions from the same data as the rest of FossilRange: which of the 99 genera have a fossil range covering a chosen moment, and how far apart in time two genera are. For the first, type a number of millions of years ago between 251.9 and 66, or drag the slider. For the second, choose two genera and read the overlap or the gap.
Picking a moment
The box takes any value from 251.9, the start of the Triassic, to 66, the end of the Cretaceous, to one decimal place. The slider covers the same span with older times on the left, matching every chart on the site. The page opens at 150 million years ago, which falls in the Kimmeridgian: 19 genera on this site have ranges covering it, from Cetiosaurus to Torvosaurus.
Here is what the tool returns every 10 million years:
| Ma | Stage | Genera | Best-recorded |
|---|---|---|---|
| 250 | Olenekian | 0 | none |
| 240 | Ladinian | 1 | Thecodontosaurus |
| 230 | Carnian | 1 | Thecodontosaurus |
| 220 | Norian | 1 | Thecodontosaurus |
| 210 | Norian | 1 | Thecodontosaurus |
| 200 | Hettangian | 5 | Lufengosaurus, Coelophysis |
| 190 | Pliensbachian | 1 | Dilophosaurus |
| 180 | Toarcian | 0 | none |
| 170 | Bajocian | 0 | none |
| 160 | Oxfordian | 4 | Megalosaurus, Cetiosaurus |
| 150 | Kimmeridgian | 19 | Allosaurus, Camarasaurus |
| 140 | Berriasian | 3 | Megalosaurus, Cetiosaurus |
| 130 | Hauterivian | 8 | Iguanodon, Megalosaurus |
| 120 | Aptian | 9 | Tenontosaurus, Psittacosaurus |
| 110 | Albian | 2 | Psittacosaurus, Carcharodontosaurus |
| 100 | Cenomanian | 4 | Carcharodontosaurus, Spinosaurus |
| 90 | Turonian | 1 | Alectrosaurus |
| 80 | Campanian | 39 | Saurornitholestes, Troodon |
| 70 | Maastrichtian | 32 | Triceratops, Saurornitholestes |
Each moment links to the tool with that value filled in. “Best-recorded” are the genera with the most fossil records among those alive.
Moments at 250, 180 and 170 million years ago return no genus at all. That does not mean no dinosaurs lived then. It means none of the genera on this site, which needs 12 records to include a genus, has dated fossils in those stages.
Why the answer jumps in steps
Ranges on this site run from stage boundary to stage boundary, so the answer only changes when the moment crosses into a new stage. Move from 72.2 to 72.0 million years ago and the count goes from 39 to 32, because 72.1 is the boundary between the Campanian and the Maastrichtian. Anywhere inside one stage gives the same list. The shaded column on the chart shows the stage you are in, and the sentence above the chart names it with its start and end.
The step pattern is honest about the data. A smooth curve would suggest that the ages of the fossils are known more precisely than they are. The guide to reading a fossil range explains how each range is built from individual records.
Reading the chart
Each bar is one genus, ordered from the oldest start to the youngest. The bar runs from the old end of its range to the young end, and the numbers beside it give both in millions of years. The amber line is your moment. Names are links: tap one to open that genus’s page, with its map, rock formations and the genera it was found with. Under the chart the same genera are listed as plain links, which is easier to scan on a phone.
Comparing two genera
The second tool, lower on the page, takes two genera and gives one of three answers.
- Overlap. Allosaurus and Stegosaurus have the same range, so they overlap by 9.8 million years, all of the shorter range. The database lists them together in 53 fossil collections.
- Meet. Velociraptor ends at 72.1 million years ago and Tyrannosaurus begins there. The tool reports that the ranges meet and that stage-level data cannot settle whether the two overlapped.
- Gap. Stegosaurus and Tyrannosaurus are 72.9 million years apart. The tool also counts the genera on this site that lived wholly in the gap: 35 of them.
For overlapping pairs, the facts row also says whether the two genera appear in the same fossil collections. This uses each genus’s eight most frequent companions. When the two genera have no country in common, the tool says so, because then they cannot share a collection.
What the tool does not do
It does not estimate sizes, populations or where animals roamed. It does not fill gaps with guesses: a genus appears only for the stages its dated records support. And it does not cover all dinosaurs, only the 99 genera that pass the site’s inclusion rules on the Data & method page.
Linking to a result
The page address updates as you use the tools, so a result can be bookmarked or shared. /when/?t=150 opens a moment, and /when/?a=stegosaurus&b=tyrannosaurus#compare opens a pair. Every genus page has two links into the tool: one compares that genus with another, and one shows who else was alive at the middle of its range.
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.