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Bennettitales (also known as cycadeoids) is an extinct order of Spermatophyte that first appeared in the Permian period and became extinct in most areas toward the end of the Cretaceous. Bennettitales were amongst the most common seed plants of the Mesozoic, and had morphologies including shrub and cycad-like forms. The foliage of bennettitaleans is superficially nearly indistinguishable from that of cycads, but they are distinguished from cycads by their more complex flower-like reproductive organs, at least some of which were likely pollinated by insects.
Although certainly sensu lato (cone-bearing seed plants), the relationships of bennettitaleans to other seed plants is debated. Their general resemblance to cycads is contradicted by numerous more subtle features of their reproductive systems and leaf structure. Some authors have linked bennettitaleans to Flowering plant (flowering plants) and Gnetophyta (a rare and unusual group of modern gymnosperms), forming a broader group known as Anthophyta. Molecular data contradicts this, with gnetophytes found to be much more genetically similar to . The exact position of Bennettitales remains uncertain.
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 Plant cuticle can be assigned to either group with confidence. The of bennettitaleans are described as syndetocheilic. This means that the main paired 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.
Most bennettitaleans in the family Williamsoniaceae are instead monosporangiate, with separate pollen and ovule-producing (unisexual) cones on the same plant. The ovule-producing (female) cones ( Williamsonia, etc .) are similar to mature bisporangiate cones, with interseminal scales and ovule-tipped sporophylls enclosed by bracts. Pollen-producing (male) cones ( Weltrichia, etc .), on the other hand, feature an exposed crown of tapering microsporophylls with adaxial rows of synangia. The microsporophylls may host a single linear row of paired synangia, or instead synangia arranged in a pinnate (feather-shaped) pattern.
are dicotyledonous (possess two Cotyledon), with a central embryo surrounded by three layers: the thin megagametophyte, the slightly thicker nucellus, and the protective integument. The upper tip of the seed is tapered and opens through a thin and often extended micropyle. A long, narrow micropyle extending out of the seed is superficially similar to the condition in living Gnetophyta. Once the seed is fertilized, the micropyle is sealed by a plug-shaped extension of the nucellus. Unlike living gymnosperms, the tip of the nucellus lacks a pollen chamber (receptacle for stored pollen). The integument is dense and thick, with many layers of differentiated cells. This contrasts with the thin, biseriate (two cell-layer) integument of gnetophytes. Bennettitaleans also lack another gnetophyte-like trait: a sheath of fused bracteoles enveloping the seed. Most integument cells are not unusual in size or shape. However, near the micropyle the innermost layer of integument cells become radially-oriented and elongated, partially closing in on the micropyle. The nucellus and integument are unfused above the chalaza (base of the seed), unlike cycads or gnetophytes, where the layers are fused for much of their height.
Cycadeoidaceans have been suggested to have been self-pollinating, with their stems and cones buried underground, (2025). 9780253001993, Indiana University Press. ISBN 9780253001993 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 Monoecy (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. The interseminal scales of Bennettitales ovulate cones may have become fleshy at maturity, which could have potentially made then attractive to wild animals that served as seed dispersers.
The bennettitalean fossil record reappeared in the Middle Triassic, and williamsoniaceans became globally distributed by the end of the period. The oldest bennettitalean reproductive structures are small Williamsonia "flowers" from the Middle Triassic Esk Formation of Australia. While Williamsoniaceae had a global distribution, Cycadeoidaceae appear to have been primarily confined to the western parts of Laurasia, and are primarily known from the Cretaceous. Bennettitales were widespread and abundant during the Jurassic and Early Cretaceous, however Bennettitales severely declined during the Late Cretaceous, coincident with the rise of , being mostly extinct by the end of the period, with the final known remains from the Northern Hemisphere being found in the polar latitude Kakanaut Formation in Chukotka, Russia, dating to the Maastrichtian, assignable to Pterophyllum. A possible late record has been reported from the early Oligocene of eastern Australia and Tasmania, assignable to the genus Ptilophyllum, but no cuticle was preserved, making the referral inconclusive.
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