'' showing divaricate branching habit
, a williamsoniaceous bennettitalean known from a whole plant, showing Ptilophyllum-type'' leaves attached to a stem Bennettitales are divided into two families,
Cycadeoidaceae and
Williamsoniaceae, which have distinct
growth habits. Cycadeoidaceae had stout,
cycad-like trunks with bisporangiate (containing both
megaspores and
microspores)
strobili (cones) serving as their
reproductive structures. Williamsoniaceae either had bisporangiate or monosporangiate cones, and distinctly slender and branching woody trunks. The Williamsoniaceae grew as woody shrubs with a
divaricate branching habit, similar to that of
Banksia. It has been suggested that Williamsoniaceae are a
paraphyletic (not containing all descendants of a common ancestor) assemblage of all Bennettitales that do not belong to the Cycadeoidaceae. Most leaf morphotypes (
Pterophyllum,
Ptilophyllum,
Zamites,
Otozamites, etc.) are
pinnate (feather-shaped), with many small leaf segments attached to a central shaft. Others (
Anomozamites, a few species of
Nilssoniopteris) are incompletely pinnate (sawtooth-shaped) and transitional between these two end members. One unusual leaf form,
Eoginkgoites, even approaches a
palmate appearance similar to early species of
Ginkgo. The foliage of bennettitaleans resembles that of cycads to such an extent that the foliage of the two groups cannot be reliably distinguished based on gross morphology alone. However, fossil foliage which preserves the
cuticle can be assigned to either group with confidence. The
stomata of bennettitaleans are described as
syndetocheilic. This means that the main paired
guard cells develop from the same mother cells as the subsidiary cells which surround them. This contrasts with the
haplocheilic stomata of cycads and conifers. In haplocheilic stomata, the ring of subsidiary cells are not derived from the same original structures as the guard cells. This fundamental difference is the main way to differentiate bennettitalean and cycad foliage. Many bennettitaleans are
bisporangiate, where the pollen and ovules are hosted on the same (bisexual or hermaphrodite) cone. Cavities filled with curved synangia-bearing microsporophylls are encased by thin radiating structures, including thick, infertile
interseminal scales and fertile sporophylls with ovules at their tips. The presence of ovules at the tips of sporophylls, rather than the tips of stems, is a major difference between the cones of bennettitaleans and gnetophytes. As the cone is fertilized and matures, the microsporophylls wither away and the ovules transform into seeds. although it has alternatively been proposed that they were pollinated by beetles. The flower-like williamsoniacean male reproductive structure
Weltrichia is associated with the female reproductive structure
Williamsonia, though it is uncertain whether the parent plants were
monoecious (male and female reproductive structures being present on the same plant) or
dioecious (where each plant has only one gender of reproductive organ).
Weltrichia was likely primarily
wind-pollinated, with some species possibly pollinated by beetles. Several groups of Jurassic and Early Cretaceous insects possessed a long
proboscis, and it has been suggested that they fed on
nectar produced by bennettitalean reproductive structures, such as the bisexual williamsoniacean reproductive structure
Williamsoniella, which had a long, narrow central receptacle which was likely otherwise inaccessible. Early Cretaceous bennettitalean pollen has been found directly associated with a proboscis bearing fly belonging to the extinct family
Zhangsolvidae, providing evidence that this family acted as pollinators for the group. == Taxonomy ==