Cycads are an order of seed plants, Cycadales, famous for their stout trunks and compound leaves, tropical and subtropical distribution, and association with the Mesozoic era and the dinosaurs. Prior to the rise of flowering plants about 80 million years ago, cycads and their relatives, the conifers, were the dominant plants on Earth. Cycads are one of four divisions of gymnosperms (non-flowering seed bearing plants). The other divisions are conifers, ginkgo, and gnetophytes. Gymnosperms have "naked seeds" in contrast to angiosperms, whose seeds are protected by a carpal. Cycads are often confused with ferns and palms, though they are related to neither.
The evolution of cycads has been dated to the Early Permian, about 280 million years ago, though there is the possibility that some cycads existed in the Carboniferous, as early as 325 million years ago. They quickly gained a worldwide distribution, being found on all the large continents that existed at the time. Cycads, being adapted to subtropical and tropical conditions, flourished for the next 200 million years, as the temperature of the entire planet was balmy and quite uniform. It would not have been unusual to see cycads growing in what is today northern Europe or the northern United States.
Cycads are unusual in that pollen fossils have shown that they were once far more diverse than they are today. There are only about 305 extant species of cycads, in contrast to the thousands that likely existed when the division of plants was at their peak. Significant genetic differences between the three major surviving lineages of cycads also points to their old genetic diversity. Cycads tend to be endemic to their respective continents -- American cycads are different than African cycads, which are in turn different than southeast Asian cycads. There is a substantial amount of disagreement on how many cycads species truly exist, because some distinct species are capable of interbreeding, raising questions about the conventional concept of a biological species.
