The gymnosperms dominated the Jurassic period. Ginkgoes, conifers, the true cycads, williamsonias and the cycadeoids filled a warm, humid world almost from pole to pole. High sea levels meant many warm inland seas flooding the continents. Mountain ranges were relatively low and an immense double landmass straddling the equator (Gondwana in the south and Laurasia in the north) created a paradise for cycad-like vegetation. The fossil record of leaves, stems and cones indicates that this type of plant proliferated during this time.

The three fossil leaf types attributed to the Mesozoic Cycadales are Nilssonia, Ctenis, and Pseudoctenis. The nilssonians were an undecided lot; some had simple entire leaves, while others adopted the compound leaf first adopted by the majority of ferns back in the Carboniferous. In the end, a compound leaf design won out.

The compound nilssonnian leaf pattern had short, blunt, and wide leaflets similar to Pentoxylon, shown at the lower right. (Some authorities feel that all simple leaves are better grouped with Taeniopteris, a seed fern, and that compound leaves are actually Pseudoctenis.) The leaflets of Nilsonnia are attached to the upper surface of the rachis (as in the modern Lepidozamia and Dioon) and the veins are parallel, as in Macrozamia and Encephalartos. Fossilized stems suggest that the leaves of Nilssonia did not leave leaf bases behind when they dropped away; instead they abscised cleanly leaving a scar (as can be seen on the stems of Pentoxylon at right). Nilsonnia was very different from the cycadale norm and may deserve family rank of its own alongside the Cycadaceae, Stangeriaceae, and the Zamiaceae.

Pseudoctenis spatula, fossilized leaf
Jurassic

Pentoxylon reconstruction by Mark Hallett. This Nilsonnia-type cycadale lived during the Jurassic.