The Cretaceous is named after chalk, and forests then grew near the poles
For some 77 million years, the longest period of the past half-billion years, Earth ran warm and mostly ice-free. Shallow seas flooded continents, forests reached the poles, and tiny shelled plankton rained down to form thick beds of chalk, the Latin creta, which gave the Cretaceous its name.
The Cretaceous ran from about 143.1 to 66 million years ago, the third and final period of the Mesozoic and the longest of the entire Phanerozoic. Its standard abbreviation is K, from the German Kreide, chalk. The Belgian geologist Jean d'Omalius d'Halloy first set it apart as a separate period in 1822, calling it the Terrain Crétacé after rock layers of the Paris Basin, where upper layers are packed with chalk made mostly from coccoliths, the calcium carbonate plates of marine microorganisms. Alcide d'Orbigny later carved the French sequence into stages, and today twelve stages of European origin are used around the world.
The warmth pushed global sea levels high, spreading shallow inland seas crowded with marine reptiles, coiled ammonites and reef-forming rudists. Dinosaurs still ruled the land. There is some evidence of short glaciations in the cooler first half, but overall there was little ice anywhere.
Much of the modern living world took shape then. Flowering plants appeared early in the period and spread so fast that they dominated by its close, while many once-common gymnosperm groups faded out. Early relatives of placental and marsupial mammals emerged, the first members of the modern bird lineage appeared near the end, and teleost fish diversified, including the Acanthomorpha, their richest subgroup.
Oddly, geologists cannot say precisely where the period begins. The Jurassic–Cretaceous boundary is the only system boundary still lacking an agreed reference section, because fossil markers are regional and no sharp isotope shifts exist to anchor it; the first appearance of Calpionella alpina, a planktonic protist with an urn-shaped shell, has been proposed. The ending, by contrast, is razor sharp: a worldwide iridium-rich layer dated to 66.043 million years ago, tied to the Chicxulub impact, which may have struck at the end of a gradual decline in biodiversity during the Maastrichtian.
Source: Cretaceous
