The figure below is based on a diagram in Chamberlain's paper The Gymnosperms, published in 1935. It has been updated to reflect new information, and the names of representative fossil discoveries have been roughly noted in the time scale. The majority of fossils listed are based on leaf fragments, but some are of cones; these latter fossils are listed within the "trunk" of the Cycadales family tree. Early seed ferns (Cycadofilicales), from which the Bennettitales and the Cycadales sprang, were derived from asexual ferns that had begun to produce both large spores with female gametes and smaller spores with male gametes.

Over time, the female spore and a supply of starch became encased in a hard shell, while the male spore remained small for mobility. Both were produced on the edges of leaf structures. Eventually, all seeds became restricted to the tip or base of the leaf. A remnant of this reproductive leaf design can still be recognized in Cycas today. It has reached it's pinnacle in the hard, protective female cone and male pollen cones of all modern cycad genera.

As leaf structure further diversified, some leaves held seeds or pollen, while others became purely photosynthetic structures. In the beginning, these vegetative leaves were simple--a single sheet of veined tissue stiffened by a stout rachis (as in the modern Bird Nest fern, Asplenium nidus).

Efficiency and mechanical considerations apparently gave an edge to a compound leaf design (which would be less prone to wind damage), because simple leaf designs were relatively short- lived in both the Cycadales and the Bennettitales.