Cycadales

True cycads, both living and extinct belong to the order Cycadales. Cycads range from the Permian to recent times. All members of Cycadales are dioecious (they have male and female plants). Living cycads have cones at the apex of the stem. Pollen cones are small and compact. Male gametes carried by the pollen are flagellated, a primitive characteristic shared with Ginkgo biloba (Taylor, Taylor, and Krings, 2009, p. 706). Leaf traces arise from the vascular cambium and spiral upward around the stem before entering the leaf (girdling leaf traces). Cycadales have a eustele stem. In cross-section a massive central core of pith is surrounded by vascular tissue. An outer layer of bark encases the vascular cambium and pith. Pinnate, frond-like leaves form a rosette at the top of the trunk. The upper stem is clothed in scale leaves, which eventually form a fibrous sheath around the stem. Leaf bases covering the trunk provide a firm outer layer.

Charmorgia and Lyssoxylon are two important fossils found in the Chinle formation of Arizona that belong to Cycadales (Tidwell, 1998, pp 196-197). Many Mesozoic species had slender branching forms. Extant short, squat cycad forms did not appear until the Tertiary (Willis & McElwain, 2002, p. 135).

Cycadeoidales or Bennettitales

The extinct order Cycadeoidales or Bennettitales range from the Triassic to the late Cretaceous. In the U.S. this order is referred to as Cycadeoidales, while in Europe it is known as Bennettitales. Members of this group bear a striking resemblance to true cycads. For many years they were classified as cycads; however, a number of features have been observed that suggest they are a separate taxonomic group. In some classification schemes the cycadeoids are placed in a separate division Cycadeoidophyta.

Cycadeoid or bennittite cones are embedded among leaf bases. The leaf traces pass directly from the vascular cambium to the leaves (Tidwell, 1998, p. 196). Fossil cones suggest that the cycadeoid (bennittite) cones were predominately unisexual early in their evolution and later became bisexual (Willis & McElwain, 2002, p. 137). The fronds of cycadeoids (bennittites) were spirally arranged at the top of branches. Although the leaves of cycadeoids (bennittites) superficially look like cycads there are important differences in leaf epidermal structure. Cycadeoid (bennittite) had eustele stems with a large pith surrounded by a ring of vascular tissue. The exterior of the stem was covered with diamond-shaped leaf bases. One group of cycadeoids had a short barrel-like trunk crowned with large fronds. A second group was more shrub or tree-like. Perhaps the most well known cycadeoid genus is Williamsonia. Williamsonia resembled a shrub or tree and grew to a height of 3 meters. Williamsonia reached its greatest diversity during the Jurassic.

The cones of cycadeoids are remenescent of flowers. Some paleotologists have suggested that the cycadeoids may have a close evolutionary relationship with angiosperms (flowering plants), although current evidence makes this tie unlikely. Both Cycadales and Cycadeoidales most likely evolved from medullosan seed ferns (Willis & McElwain, 2002, pp. 136-137).

Science Olympiad Fossil Event

The 2015 Science Olympiad Fossil List does not include cycads.

Kenrick, P. and Davis, P. (2004). Fossil Plants. Smithsonian Books: Washington.

Taylor, T.N., Taylor E.L. & Krings, M. (2009). Paleobotany: The Biology and Evolution of Fossil Plants [2nd Ed]. New York: Academic Press.

Tidwell, W.D. (1998). Common Fossil Plants of Western North America. [2nd Ed]. Washington: Smithsonian Institution Press.

Willis, K.J. & McElwain, J.C. (2002). The Evolution of Plants. New York: Oxford University Press.