| Cycadophyta Fossil range: Early Permian–Recent | |
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 A scene from the Late Carnian. In the immediate foreground is the cycad Pseudoctenis (with palm-like leaves), and to the left of Stagonolepis another cycad, Leptocycas (resembling a small palm). | |
| Scientific classification
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Kingdom |
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Division |
Cycadophyta |
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Class |
Cycadopsida |
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Order |
Cycadales |
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Cycads are a group of seed plants characterized by a large crown of compound leaves and a stout trunk. They are evergreen, gymnospermous, dioecious plants having large pinnately compound leaves. They are frequently confused with and mistaken for palms or ferns, but are related to neither, belonging to the division Cycadophyta.
Origins[]
The cycad fossil record dates to the Early Permian, 280 mya. There is controversy over older cycad fossils that date to the Late Carboniferous period, 300–325 mya. One of the first colonizers of terrestrial habitats, this clade probably diversified extensively within its first few million years, although the extent to which it radiated is unknown because relatively few fossil specimens have been found. The regions to which cycads are restricted probably indicate their former distribution on the supercontinents Laurasia and Gondwana.
The family Stangeriaceae (named for Dr. William Stanger, 1812(?)–1854), consisting of only three extant species, is thought to be of Gondwanan origin as fossils have been found in Lower Cretaceous deposits in Argentina, dating to 70–135 mya. Zamiaceae is more diverse, with a fossil record extending from the Middle Triassic to the Eocene (54–200 mya) in North and South America, Europe, Australia, and Antarctica, implying that the family was present before the break-up of Pangea. Cycadaceae is thought to be an early offshoot from other cycads, with fossils from Eocene deposits (38–54 mya) in Japan and China, indicating that this family originated in Laurasia. Cycas is the only genus in the family and contains 99 species, the most of any cycad genus. Molecular data has recently shown that Cycas species in Australasia and the east coast of Africa are recent arrivals, suggesting that adaptive radiation may have occurred. The current distribution of cycads may be due to radiations from a few ancestral types sequestered on Laurasia and Gondwana, or could be explained by genetic drift following the separation of already evolved genera. Both explanations account for the strict endemism across present continental lines.
References[]
- De Luca, Paulo (1990) "A Historical Perspective on Cycads from Antiquity to the Present", In: Stevenson, D. (Ed.) The Biology, Structure, and Systematics of the Cycadales, Memoirs of the New York Botanical Garden, 57, p. 1-7, ISBN 0-89327-350-3