Seed ferns sounds like a contradiction, since modern ferns reproduce with spores, not seeds. It took scientists awhile to understand seed ferns, which flourished in the Carboniferous (about 325 million years ago). Common in wetlands, including coal swamps, they were first classified as ferns because of their large, divided leaves (fronds). The fronds of some seed ferns were huge (7 m, nearly 25 feet) in length, though most were 1-5 meters. In the late 1800s, to the surprise of many, paleobotanists pieced together evidence that they made seeds (sexual reproductive structures). It turns out that seed ferns are more closely related to flowering plants than to ferns. Like flowering plants, seed ferns made pollen, adapted to be spread by wind and maybe insects. Seed ferns diversified during the early Mesozoic, giving rise to new groups. Some had reproductive structures like those of flowering plants and suggesting insect pollinated. By the later Cretaceous (65 million years ago), however, seed ferns began to decline and gradually disappeared from Earth.

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.