Introduction to the Bennettitales

the cycadeoids

Syndetocheilic stomata in the anthophytes

The bennettitaleans (note informal name rather than the order-level Bennettitales) were originally mistaken for cycads because of their cycad-like growth form (Figure 10.9). However, the reproductive structures of these plants and syndetocheilic stomata place them soundly in the anthophyte clade.

Bennettitaleans first appear in the Triassic and became extinct toward the end of the Cretaceous. Two major groups are recognized: the cycadeoids with stout trunks and bisporangiate reproductive structures, and relatives of Williamsonia and Williamsoniella, which have slender, branching trunks and either bisporangiate or monosporangiate strobili.

Both groups had woody stems that were heavily armored with persistent leaf bases, much like modern cycads. Both produced entire simple leaves or pinnate foliage (VG 2:3), also like modern cycads. Indeed, the groups differ only in the details of stomatal morphology. This difference, when visible, permits unequivocal segregation of Mesozoic cycad and bennettitalean foliage, such as Nilssonia (VG 2:4). Note that this foliage form genus has also been linked to true cycads. Other foliage-types interpreted as benettitalean include , Anomozamites (VG 2:5) and Zamites (VG 2:6)

At one point, the bennettitaleans were put forward as possible flowering plant ancestors because of their flower-like bisexual reproductive structures (Figure 10.10). In Williamsoniella, for example, ovules alternate with interseminal scales on an ovulate receptacle at the center of several organ whorls (Figure 10.10a). This reminded some workers of the angiosperm carpel; to others, the complex alternation of interseminal scales and ovule-cupule seemed too derived to be the ancestral state of the angiosperm carpel. The next whorl out was occupied by microsporophylls bearing pollen sacs. This, in turn was surrounded by several whorls of bracts that many homologize with petals and sepals in flowering plants.

Figure 10.9: (A) Reconstuction of Williamsonia sewardiana with spirally arranged leaf scars. (B) Cycadeoidea trunk and foliage drawn for comparison of proportions.

Other bennettitaleans bore monosporangiate reproductive structures. Ovulate structures, such as Williamsonia, featured ovulate receptacles, interseminal scales and surrounding bracts, but lacked microsporophylls (Figure 10.10b)(VG 2:7). Pollen organs were composed of a whorl of microsporophylls bearing synangia (Figure 10.10c) (VG 2:8). Note the homoplasy in this character -- bisexual reproductive structures -- that is supposed to be characteristic of the anthophyte clade.

More recently, some phylogenetic studies have concluded that the group is a close relative of the angiosperms along with the gnetophytes and Pentoxylon in a group dubbed the anthophytes. This conclusion about bennettitalean relationships is founded upon a series of shared characteristics, including structure of the xylem (water-conducting cells), pollen-producing structures, pollen shape, and seed development. Though there is not a consensus among evolutionary biologists about the nature of the relationships, the same cluster of plant groups has surfaced in several analyses.

Bennettitalean cones Figure 10.10: Examples of bennettitalean reproductive structures. (A) Williamsoniella bisexual reproductive structure. Note microsporophylls with microsporangia embedded within. (B) Williamsonia ovulate structure. (C) Weltrichia pollen-bearing structure with detail of microsporophyll showing attached synangia.

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