The cycadeoids  resemble the cycads in their growth habit and morphology.  However, the strobili of monoecious species are very complex and include both ovulate and staminate sporophylls in a flower-like arrangement.  Because of the similarities between a flower and the bisexual strobili of the cycadeoids, Tudge (2000) and the Tree of Life Project indicate them as sisters to the gnetophytes and flowering plants.  Pearson (1995) indicates a relationship between the cycadeoids and the gnetophytes, but includes them in a line separate from the flowering plants.  They have a fossil history that extends from the Permian through the Cretaceous, so they overlapped with both groups.  In fact, the cycadeoids exhibited high abundance and diversity throughout the Mesozoic.

Description of the phylum and the taxonomic system follow Bold et al. (1987) and Bierhorst (1971).

I. PHYLUM CHARACTERISTICS

A. Structure

Habit: The cycadeoids are cycad-like seed plants with barrel-shaped stems and large frond-like leaves. All plants are extinct.

Pollen: Pollen grains are similar to those of the Cycads.

Microstrobilus: Microsporophylls are often fused at the base; technically a simple strobilus the strobili often bore both ovulate and pollen-bearing structures.

Seeds: Appear to be similar to those of Cycads.

Megastrobilus: In bisporangiate cones, ovules occur on a central receptacle; the structure suggests either a high degree of self-pollination or a high degree of animal-assisted pollination.

Stems: Very similar to those of the Cycads.

Leaves: Large and frond-like.

Roots: I do not know.

C. Ecology: All plants are extinct. Their fossil history dates from the late Permian to the Cretaceous. They were very abundant during the Mesozoic.

II. TAXONOMY: As I have defined this phylum, the CYCADEOIDOPHYTA has a single class, CYCADEOIDOPSIDA, with 2 single orders.

ORDER CYCADEOIDALES

These plants have a fossil history which ranges from the Permian to the Cretaceous. They are very cycad-like in their growth habit and stem structure. The leaf traces are numerous and c-shaped.

example: Cycadeoidea.

ORDER WILLIAMSONIALES

These plants have a fossil history which ranges from the early to mid Mesozoic (mainly in the Jurassic). They are tree-like. Many have unisexual cones.

example: Williamsonia

revvised: 04/15/2003