Triassic, Jurassic and Cretaceous dinosaurs in numbers
Of the 99 genera on this site, 70 have fossil ranges in the Cretaceous, 31 in the Jurassic and 3 in the Triassic. Those figures add up to 104, more than 99, because 5 genera span a period boundary and count in both. They describe this site’s selection, which takes the genera with at least 12 fossil records in the Paleobiology Database. They are not a census of how many dinosaurs lived in each period.
The three periods side by side
| Period | Ma | Length (My) | Genera | Genera per My |
|---|---|---|---|---|
| Triassic | 251.9–201.4 | 50.5 | 3 | 0.06 |
| Jurassic | 201.4–145 | 56.4 | 31 | 0.55 |
| Cretaceous | 145–66 | 79 | 70 | 0.89 |
Period boundaries from the ICS chart. A genus counts in every period its range touches.
The Cretaceous is the longest of the three, at 79 million years, but length alone does not explain the gap. Per million years the Cretaceous still has more genera here than the Jurassic, and far more than the Triassic. The genera that cross a boundary are Megalosaurus (Jurassic / Cretaceous), Cetiosaurus (Jurassic / Cretaceous), Coelophysis (Triassic / Jurassic), Coelurus (Jurassic / Cretaceous) and Thecodontosaurus (Triassic / Jurassic).
Stage by stage
Split into its 30 stages, the Mesozoic looks very uneven. The Campanian has the most genera on this site, 39. 5 stages have none at all: Induan, Olenekian, Toarcian, Aalenian and Bajocian.
Two clusters stand out. The first is the Late Jurassic, where the Kimmeridgian has 19 genera, most of them from the Morrison Formation of the western United States. The second is the last two stages of the Cretaceous: 52 genera have ranges that reach the Campanian or the Maastrichtian, with 1,816 of the site’s 3,628 fossil records between them. 40 of those 52 genera have records from the United States or Canada.
Fossil records per period
Counting fossil records instead of genera gives the same order, with a wider spread. A record here is one genus identified in one fossil collection, so a genus dug up at many sites contributes many records.
| Period | Records of its genera | Best-recorded genus |
|---|---|---|
| Triassic | 76 | Plateosaurus (36) |
| Jurassic | 1,205 | Allosaurus (147) |
| Cretaceous | 2,488 | Triceratops (166) |
Records of every genus with a range in the period; the 5 boundary-crossing genera count in both periods.
A genus with many records is one that turns up again and again in the field, at many sites or in many layers of the same rock unit. Plateosaurus, the best-recorded Triassic genus here, has 36 records, fewer than 31 genera from the other two periods.
Groups in each period
| Group | Triassic | Jurassic | Cretaceous |
|---|---|---|---|
| Theropods | 1 | 8 | 26 |
| Sauropodomorphs | 2 | 15 | 4 |
| Ornithopods | 0 | 3 | 18 |
| Horned dinosaurs | 0 | 0 | 11 |
| Dome-headed dinosaurs | 0 | 0 | 2 |
| Armoured & plated dinosaurs | 0 | 5 | 9 |
Number of genera of each group with a range in each period; a genus spanning a boundary counts in both.
Sauropodomorphs are the only group with more Jurassic than Cretaceous genera here. All 11 horned dinosaurs and both dome-headed dinosaurs are Cretaceous. Theropods and sauropodomorphs are the only groups present in all three periods on this site.
Why the Triassic looks so empty
Only Plateosaurus, Coelophysis and Thecodontosaurus have Triassic ranges here. The site includes genera with at least 12 fossil records in the database, and only these three genera with Triassic dates reach that number. A Triassic genus known from a few skeletons will not appear, however important it is. The low count is a statement about this site and its threshold, not about the Triassic itself. The Data & method page lists the full inclusion rules.
The same caution applies to the empty stages. A stage with no genera here has no genus with enough dated records in it to pass the threshold. That can mean few fossil-bearing rocks of that age, or rocks that have been studied less, or genera that fall just under the line.
Using the counts
Treat these numbers as a map of where the fossil record on this site is rich. To see who is in each stage, use the stage list on the Who lived when page, or open a period page: Triassic, Jurassic, Cretaceous. 26 genera on this site are purely Jurassic, and the timeline shows all 99 on one chart.
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.