The swamps that were turned into coal over time grew at the end of this
era

Precambrian

Paleozoic

Mesozoic

Cenozoic

Paleozoic

Explanation:

The Paleozoic Era is a geologic era that lasted from 541 to 251.902 million years ago. This is divided into six geologic periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The swamps that were turned into coal over time grew during the Carboniferous period, which expands from 358.9 million years ago to 298.9 million years ago. The period is well-known because many coal beds were formed during that time period. During the Carboniferous, terrestrial animals were well-established. It ended with a minor extinction event known as the Carboniferous rainforest collapse.

brave new world, aldous huxley (1932)