Ferns are the second most diverse plant group on Earth today, following only the flowering plants in number of species. The origin of ferns is not well understood, though. Weedy, fern-like plants appear in the latest Devonian, specialized to colonize disturbed sites such as volcanic landscapes. The oldest, definite fern fossils are from the early Carboniferous (about 350 million years ago). By the middle Carboniferous, all major fern growth forms were present: trees, vines, and ground cover, mostly specialized to exploit disturbances. Among these plants, however, were tree ferns up to 10 meters (32 feet) tall that lived in swampy wetlands, and whose remains fossilized into coal. Many of these early groups of ferns went extinct in the Paleozoic but were eventually replaced by new forms, many of which have survived to the present day. A huge diversification of ferns happened during the Cretaceous (150 million years ago,) and new kinds of ferns have continued to evolve into modern times.

The great biogeographer Wladimir Peter Koppen once said that plants are crystalized visible climate. He had studied the distribution of modern plants, but there is no reason to believe that ancient plants were not equally sensitive to climate. Indicators of paleoclimate, such as rainfall and surface temperature, can be found in the chemistry of fossil plants and the rocks that surround them. The form of the fossils themselves can also reveal a great deal about climate. For example, plants have tiny openings on their leaves (stomata) through which they absorb CO2 and release oxygen. More stomata occur in low CO2 atmospheres, and fewer in high CO2 environments. Some woody plants have growth rings, showing the alternation of favorable and unfavorable conditions. Leaf shapes can also act as thermometers. Leaves with serrated edges (toothed margins) are more common in cooler climates, whereas smooth-edged leaves dominate in warmer climates. By studying modern forests, and applying the findings to extinct plant communities, past climate conditions can be inferred. Changes in fossil plant assemblages mirror changes in global climate over time.