Probably changes in climate combined with competition from groups that were faster growing, such as the Pinophyta and angiosperms. They have a woody trunk that is covered with persistent leaf bases and terminates in a tuft of pinnately compound leaves. The trunk is sometimes very short, scarcely emerging above ground level, but in Macrozamia is known to reach 18m. Most species do not grow more than about 2 m tall. Those that grow in Florida and Alabama are the almost subterranean kind.
Cycads grow extraordinarily slowly.
One specimen of Dioon is thought to be approximately 1,000 years old. I could not find its height, but it is less than 18m. The anatomy of the trunk is distinctive. The pith and cortex contain large amounts of starch. The vascular strands to the leaf go round the stem for a considerable distance before turning up and entering the leaf.