Relatives of the club mosses were among the first vascular plants (with specialized water conducting cells). Dating to at least the early Devonian (about 400 million years ago), club mosses dominated many ancient landscapes, particularly swamps and other wetlands. Tree-form (arborescent) club mosses grew fast and reached heights of 30 meters (100 feet). Their crowns leafed out last, most of their lives being spent as poles covered with leaves. The formation of the crown and cone-like reproductive organs signaled the end of the plant's life. Root systems of arborescent club mosses were long and shallow, extending out tens of feet from the trunk. They frequently fossilized, thanks to their hollow centers filling with sediment, acting as a 3-dimensional mold. Warming and drying pulses from the later Carboniferous through the Jurassic caused a series of extinctions, eliminating arborescent club mosses, but leaving smaller, herbaceous ones like what we see today. The coal we use is largely made up of giant club moss remains.

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.