Domain Eucarya or Superkindgom Eukaryota
The cycads or 'tree fern's comprise a class of seed-bearing plants containing 100-200 species in eleven genera and three families. All modern cycads have a large woody (palm-tree-like) trunk with a crown of fernlike pinnately or bipinnately compound leaves at the crown. Unlike palms, all cycads are dioecious and bear their naked seeds in cones on the female plants. As the plant grows, the stubs of old leaves leave traces which ring the plant.
Cycads and cycad-like plants appear in the fossil record from the Permian period through to the present. However, their heyday was the Mesozoic Era, where they and the similar-appearing Bennettitophyta ('Cycadeoids') filled evolutionary niches left by the Permian-Triassic mass extinction to become the Earth's dominant plants on every continent. Thus, the huge plant-eating dinosaurs munched primarily on cycads and cycadeoids. Towards the end of the Mesozoic Era, the Magnoliophyta (flowering plants) began to spread and give these plants some competition.
The asteroid impact at the end of the Cretaceous wiped out the cycadeoids but the cycads survived; the three modern cycad families appeared during the Cenozoic Era. Today, these 'living fossils' are found around the world in a wide belt around the Earth's tropics, in Southeast Asia, Australia, Africa, South America, Central America, Mexico, and southern Japan.
The King Sago Palm (Cycas Revoluta) from
the Ryukyu Islands is the most common cycad you will see planted decoratively. Although a toxic substance, cycasin, pervades all cycads, several species are used as food sources by several cultures in the tropics. Elaborate procedures of beating, washing, and fermenting parts of the tree (most
importantly the starchy trunk) are necessary to remove the toxins. Some of the procedures involve ceremonies to ask the tree's spirit for permission to cut it down.
Fossil cycads have not been given an overall systematic treatment yet, and so some of the fossil genera below may belong to other phyla, such as the Bennettitophyta, the Gingkophyta, or the Pteridospermophyta, the common ancestor of them all. Fossil cycads are classified based upon both cone and leaf fragments; as time passes, some genera may be merged.
Several early Permian-Triassic genera are intermediate between the Cycadaceae and Stangeraceae:
A fossil family, Nilssoniaceae, is placed in the Cycadophyta by some, and in the Gingkophyta by others:
- Suborder Cycadinae
- Suborder Zamiinae
Land Plants Online: Southern Illinois University (Carbondale) College of Science
Royal Botanic Gardens Sydney - The Cycad Pages
The Cycads: Fossils of the Past by Jose Manuel Sanchez de Lorenzo Cáceres
University of California, Berkeley Museum of Paelontology
Introduction to the Cycads: Legacy of the Mesozoic
Brenner, Stephenson, Twigg: Cycads: evolutionary innovations and the role of plant-derived neurotoxins
Trends in Plant Science, 8/9 September 2003