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Cycads

FOSSIL FIELD GUIDE

Cycads

Family: Cycadophyta
Cycadophyta includes 3 families Cycadaceae, Stangeriaceae, and Zamiaceae

Time:
Cretaceous

Place:
Worldwide

In Our Region:
Carlsbad, Point Loma Foundation

Description:
Cycad fossils have been found on every continent, including Antarctica, and some of the larger islands of the world. During the Mesozoic, they were extremely diverse and accounted for about 20% of the world’s flora during the Triassic and Jurassic. In fact, the Jurassic is often referred to as the “Age of Cycads.” Cycads were probably an important food source for many herbivorous animals during the Jurassic Period.

Fossil remains that have been found include leaves, stems, cones and seeds. Although they bear a superficial resemblance to palms, they are a completely distinct group of seed plants, and are nearly unique among living plants in possessing motile sperm cells.

Their seeds are not enclosed within an ovary, such as that found in other plants like apples or coconuts, but are exposed and visible on leaf-like structures, arranged in a cone shape. Because the seeds are exposed in this fashion, cycads are considered part of the gymnosperms, or plants with “naked seeds.”

Ecology:
Today we have 11 living genera, and 19 extinct genera known only from fossils. Cycads have existed for 300 million years! Sometimes described as “living fossils,” they occur in greatly reduced numbers today, and may be close to the end of their tenure on Earth.Cycad fossil

Living cycads exist in several different habitats, such as rainforests and seasonally dry forests, in grasslands and savannas. Some grow at very high elevation. All living species occur within the tropical, subtropical and warm temperate areas of the world.

Cycads are dioecious, which means there are separate male and female plants. Fertile seeds are only found when the male and female plants happen to grow near one another, and mature at the same time, so it is hard for them to reproduce. Germination is also slow. Because they grow very slowly and reproduce irregularly, they are facing extinction. Many are protected by law, because plant collectors have reduced their numbers, as well.

Beetles and bees may have played important role in pollination of cycads. The seeds are large and often brightly colored in reds and yellows, and thus able to attract animals to participate in dispersal of the seeds.

Cycads have an interesting association with bacteria that exist in special roots growing up out of the soil, called coralloid roots, because of their similarity in appearance to coral. The roots allow the cyanobacteria contained within to be exposed to light, so they can photosynthesize. The cycad gathers nitrogen as a nutrient from the cyanobacteria.

People in Africa, Asia, Australia, and other regions have used cycads as a food source, since the central part of the trunk of the plant has a high level of starch. Seeds of various species of cycads are eaten also. However, seeds can be toxic to livestock, and have been noted as a cause of “Guam dementia,” a type of neurological disorder occurring in people of the islands of Guam, Ryukuku and northern Australian, where native people consume large quantities of cycad seeds. Two toxic compounds, cycasin and macrozamin, are unique to cycads and probably evolved as a protective adaptation against grazing by animals.

Further Research:
The relationship of cycads to other living gymnosperms, such as conifers, is in need of further research.

Suggested Reading:
Jones, David L. 1993. Cycads of the World. Chatswood, Australia: Reed Publishers.

Text: Margaret Dykens and Lynett Gillette
Photo: Jim Melli