Temporal range: Triassic to Oligocene
|A Cycadeoid, showing an "inflorescence" in the top-right|
Bennettitales (also known as cycadeoids) is an extinct order of seed plants that first appeared in the Triassic period and became extinct in most areas toward the end of the Cretaceous (i.e. they existed around 252 to 66 million years ago), although some Bennettitales appear to have survived into Oligocene times in Tasmania and eastern Australia. The taxon comprises two groups, the Cycadeoidaceae, represented by Cycadeoidea and Monanthesia, and the Williamsoniaceae including Williamsonia, Williamsoniella, Wielandella and Ischnophyton which had slender, branching trunks and either bisporangiate or monosporangiate strobili.
Originally, Bennettitales were thought to be cycads for their strap-like leaves, however their flower-like reproductive parts separate the two groups. The respective groups within Bennettitales (Cycadeoidaceae and Williamsoniaceae) each have their own unique traits. Cycadeoidaceae had stout trunks and bisporangiate strobili(cones serving as their reproductive structures) and Williamsoniaceae can have either bisporangiate or monosporangiate cones, and distinctly slender and branching wood-like trunks. These bisporangiate cones consist of layers of protective bracts, a curved microsporophyll, and an ovulate receptacle. The bisporangiate cones remained closed at maturation, and most likely were obligate self-fertilizers.
In general, Bennettitales have leaves attached adaxially (toward the stem) with a distinct midrib. Veins stem from the midrib at an approximately 90 degree angle. They have tough cuticles as well, as they were able to withstand high sulfur dioxide levels in the Triassic/Jurassic period for a sizeable amount of time. Some were characterized by thick trunks and pinnately compound leaves that bore a superficial resemblance to those of cycads, differing primarily in stomatal arrangement.
Bennetittales were first identified by Engler in 1892 as separate from Cycadales, and then further were differentiated into the two groups Cycadeoidaceae and Williamsoniaceae by Caruthers. The first major hypothesis developed was the Euanthial hypothesis established by Arber and Parking in 1907. This hypothesis posited that angiosperms arose from Bennettitales, as evidenced by the wood-like structures and rudimentary flowers. This theory placed them among the anthophytes, leading it to be known more commonly as the Anthophyte hypothesis. Based on morphological data, however, Bennettitales were classified as a monophyletic group when paired with Gnetales. Genetic data showed that modern extant seed-plants form their own monophyletic group, excluding Bennettitales.Modern theory suggests that Bennettitales, Angiosperms, and Gigantoteridales form a clade based on the presence of oleanane. Recent evidence from examining phase-contrast X-rays of gymnosperm seeds suggested that the Euanthial hypothesis is supported. This is still a hotly debated topic. Mostly, it is understood that by morphological data, Euanthial hypothesis is supported but modern cladisitic tests suggest otherwise. Uncovering information about this extinct group is still far from finished, as new species are being discovered such as Nilssoniopteris binggouensis in 2014 and Anomozamites sanjiaocunensis in 2015.
Bennettitales are also linked to the diversification of insects due to their flower-like reproductive parts. Specifically, the origin of insect mouth parts is connected to Bennettitales and Gnetales.
Fossil leaf of Zamites mariposana from the Jurassic.
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