Bennettitales

Bennettitales Temporal range: Triassic to Cretaceous PreЄ Є O S D C P T J K Pg N
A Cycadeoid, showing an "inflorescence" in the top-right
Scientific classification
Kingdom:	Plantae
Superdivision:	Spermatophyta
Division:	Cycadophyta
Subdivision:	Cycadophyta
Class:	Spermatopsida
Order:	Bennettitales †
Families
Cycadeoidaceae Williamsoniaceae

Bennettitales (the 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. Some were characterized by thick trunks and pinnately compound leaves that bore a superficial resemblance to those of cycads, differing primarily in stomatal arrangement.

The taxon comprises two groups, the Cycadeoidaceae, represented by Cycadeoidea and Monanthesia which had stout trunks and bisporangiate strobili (cones serving as their reproductive structures), and the Williamsoniaceae including Williamsonia, Williamsoniella, Wielandella and Ischnophyton which had slender, branching trunks and either bisporangiate or monosporangiate strobili. Bennettitales have been placed among the anthophytes and for some time were considered to be close relatives of the flowering plants on account of their flower-like reproductive structures (a whorl of microsporangiate structures that surround an inner receptacle). However, more detailed morphological studies have shown the hypothetical anthophytes clade to be polyphyletic, with the Bennettitales more closely related to the cycads, ginkgo and conifers than to the angiosperms. Molecular fossil evidence, however, indicates the possibility that Bennettitales and Angiosperms (along with Gigantopteridales) form a clade, based on the presence of oleanane in fossils of these groups.

Taxonomy

The Cycadeoideaceae (originally “Cycadeoideae”) were named by William Buckland in 1828, from fossil trunks found in Jurassic strata on the Isle of Portland, Dorset, which Buckland gave the genus name Cycadeoidea. Buckland provided a description of the family and two species, but failed to give a description of the genus, which has led to Buckland's description of the family being considered invalid by modern taxonomic standards. In publications in 1870, the British botanists William Carruthers and William Crawford Williamson described the first known reproductive organs of the Bennettitales from Jurassic strata of Yorkshire and Jurassic-Cretaceous strata of the Isle of Wight and the Isle of Portland. Caruthers was the first to recognise that Bennettitales had distinct differences from cycads, and established the tribes "Williamsonieae" and "Bennettiteae", with the latter being named after the genus Bennettites named by Caruthers in the same publication, the name being in honour of John Joseph Bennett. The order Bennettitales was erected German botanist Adolf Engler in 1892, who recognised the group as separate from the Cycadales. The Anthophyte hypothesis erected by Arber and Parking in 1907 proposed that angiosperms arose from Bennettitales, as suggested by the wood-like structures and rudimentary flowers.

Evolutionary history

The affinities of Bennettitales to other seed plants remains uncertain. The oldest known remains of Bennettitales are Nilssoniopteris shanxiensis from the upper part of the Upper Shihhotse Formation, Shanxi, China dating to the early Permian (Cisuralian). Permian records of Benettitales are rare, with the only other records being from the Late Permian (likely Changhsingian) aged Umm Irna Formation in Jordan. Bennettitales became widespread during the Triassic, being globally distributed by the end of the period. Bennettitales remained widespread during the Jurassic and Early Cretaceous, however Bennettitales severely declined during the Late Cretaceous, coincident with the rise of flowering plants, 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.

Gallery

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  A fossilised Bennettitales, squashed by later compression.

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  Side view. Note embedded "flowers".

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  A flower-like structure.

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  Fossil leaf of Zamites mariposana from the Jurassic.

External links

• Gymnosperms
• Lecture Notes on Mesozoic Mesophytic Communities
• Weblinks to articles about the Bennettitales