Cycads were newcomers in the Mesozoic, but they flourished greatly in the hot and wet climate. Cycads can be characterized by their thick stems, their fan-like leaves, and their remarkable resemblance to palms. But cycads are not the same as palms. Palms are flowering plants whose seeds are covered in shells. Cycads do not have covered seeds, and so they declined when the climate got cooler and the flowers emerged in the Cenozoic.