Cycads, with frond-like leaves and barrel-like trunks, look like palm trees, although they are not closely related. Cycads are ancient, first appearing fossilized in the early Permian (almost 300 million years ago). They became so abundant in the Mesozoic that it is called the age of cycads, reaching their heyday during the Jurassic. While cycad fossils tend to be plant fragments, paleobotanists have pieced together evidence that fossil cycads were similar to modern ones. Straight, thick trunks with scaly bark were topped by crowns of long, thin leaves. Tree forms as tall as 18 meters (60 feet) provided habitat for dinosaurs. However, their slow growth, thick leaves, tough construction, and toxic chemicals probably deterred many herbivorous dinosaurs from eating them. Cycads declined during the Cenozoic, perhaps among the many plant groups outcompeted by faster-growing flowering plants and conifers. Today, most cycads have a restricted geographic distribution, confined to one continent, or even to one region of a continent, in patches of what was once a more continuous distribution.