Period· The columns of time

Cretaceous

145 Ma → 66 Ma

The longest period of the Phanerozoic: flowers spread across the land, the world grows hot enough to lose all its ice, and an asteroid closes it in a single day.

What the evidence shows

Its name comes from chalk, and the chalk is made of plankton: single-celled algae wearing armour of calcite plates, numerous enough that their remains build the white cliffs of northern Europe and beds like them on three continents. Between 94 and 83 million years ago there is no permanent ice anywhere, the sea stands at its highest level of the last half billion years, and the temperature gap between the tropics and higher latitudes stays under 14 °C, against about 20 °C today. Flowers become abundant and start to take over, though the date of their origin is the longest-running argument in botany. It ends at 66 million years, when a rock about ten kilometres across strikes the shallow sea off Yucatan in the northern spring, a season readable in the bones of fish killed that same day, while more than a million cubic kilometres of lava are erupting in India across the same boundary.

Bounds and conventions

Its top is among the sharpest boundaries on the chart, an iridium layer traceable worldwide. Its base is not: at 145 million years it still lacks a golden spike. The origin of flowering plants remains genuinely open, with unmistakable fossils from about 125 million years, contested older claims, molecular clocks pointing well into the Jurassic, and a reading of the fossil record itself that now places several families there too. How much of the extinction the Deccan eruptions account for is still argued, since at least one pulse began tens of thousands of years before the impact.

Sources

1. Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
2. Hay (2004), Coccolithophores · Carbonate fluxes and calcareous nannoplankton
3. O'Brien et al. (2017), Earth-Science Reviews · Cretaceous sea-surface temperature evolution: constraints from TEX86 and planktonic foraminiferal oxygen isotopes
4. Silvestro et al. (2021), Nature Ecology & Evolution · Fossil data support a pre-Cretaceous origin of flowering plants
5. During et al. (2022), Nature · The Mesozoic terminated in boreal spring
6. Jenkyns (2010), Geochemistry, Geophysics, Geosystems · Geochemistry of oceanic anoxic events