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.

Plants on Earth began life in the water. The earliest plants were aquatic algae, living in the warm seas more than 500 million years ago. By the Silurian and possibly the latest Ordovician (as much as 440 million years ago), plant life on land had begun. While terrestrial plants had more access to sunlight, challenges included supporting their weight and distributing water throughout their tissues. During the Devonian explosion (55 million years, which is brief in geologic time), plants evolved from small, simple forms to a huge variety of larger, complex forms adapted to life on land. Club mosses, horsetails, ferns, and probably sphenopsids originated. The incredible seed, which safely packages the developing plant embryo, also evolved. Plants extended their reach underground with root systems, providing the anchor and water supply to support large trees. Although plants were to undergo many changes over time, by the end of the Devonian (the golden age of plant evolution), the groundwork had been laid for the terrestrial ecosystems of today.