Horsetails are rare today, but flourished on Earth during the Carboniferous (about 325 million years ago), and remained important in many kinds of ecosystems into the early Mesozoic. Throughout their history, horsetails have preferred environments with disturbance, such as flooding and burial of vegetation in sediment. In contrast to the small horsetails of today, Carboniferous and Permian horsetails in the genus Calamites could be 10 meters tall (33 feet) with large leaves. Tree-like in shape, they formed the lower canopy (club mosses were even taller) of dense forests. Calamites, like modern horsetails, had segmented stems with circles (whorls) of branchess and leaves at the segment junctures (nodes). The stems of calamites had a hollow central area surrounded by a cylinder of woody tissue (picture a cardboard tube). Because these plants frequently lived in bars within streams, or on stream banks, they were often buried in floods. Their stems are often found still in place in ancient river deposits, as stem casts.

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.