Mesozoic Cycadeoids

Professor Ralph E. Taggart

Department of Plant Biology and Department of Geological Sciences

Michigan State University

This reconstruction of Williamsonia from the Jurassic of India (Sahni, 1932) looks like a typical cycad - but it isn't! There was a highly diverse group of Mesozoic plants, probably descended from Paleozoic Pteridosperms, that produced foliage that was, in many cases, superficially similar to that of cycads.

The group, variously known as the Bennettitales or Cycadeoidales, is a distinct group of gymnosperms that can be distinguished on the basis of foliage only if cuticle is sufficiently well-preserved to permit study of the leaf stomata.

If reproductive material is preserved, there is no chance of confusing these plants with true cycads. This model of Williamsoniella from the Field Museum in Chicago, shows why this is the case. True cycads produce cones, but these Mesozoic cycad-like plants have unique flower-like reproductive structures, visible at several points in the model. These are not true flowers, but they do make the group a focal point of interest with respect to the origin of the Angiosperms!

One of the most common North American cycadeoids is the genus Cycadeoidea, shown here as reconstructed by Delevoryas (1971). The most common fossils of these plants are petrifactions of the short, barrel-like to sub-spherical trunk which carried a crown of cycad-like leaves. The plants were very common in the Cretaceous of North America but are rare to absent elsewhere.

The trunks are typically covered by old leaf bases and most people who find the fossils think they are a fossil pineapple! The bumps on the trunk above represent "buds" of reproductive structures buries under the surface of the trunk:

One of the embedded Cycadeoidea "buds" (Stewart, 1983). A conical area in the center (or) bears many small ovules while the rest of the structure is a tangle of filaments bearing pollen sacs. Originally it was thought that these were buds that would later emerge to cover the trunk will elaborate, flower-like structures.

Delevoryas was able to demonstrate that these structures never did open and that they were probably self-pollinated with limited potential for seed dispersal. This may account for the fact that the group is endemic to North America and is confined to the Cretaceous.

Unless the group was involved in the origin of the Angiosperms, they have left no descendents and apparently became extinct by the close of the Mesozoic.

Ralph E. Taggart (