| Bennettitales | |
|---|---|
 | |
| Restoration of a member ofWilliamsoniaceae byThérèse Ekblom | |
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| Life restoration ofCycadeoidea (Cycadeoidaceae) | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Spermatophytes |
| Order: | †Bennettitales Engler, 1892 |
| Families | |
Bennettitales (also known ascycadeoids) is an extinctorder ofseed plants that first appeared in thePermian period and became extinct in most areas toward the end of theCretaceous. Bennettitales were amongst the most common seed plants of theMesozoic, and had morphologies includingshrub andcycad-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 likelypollinated by insects.[1]
Although certainlygymnospermssensu 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 toangiosperms (flowering plants) andgnetophytes (a rare and unusual group of modern gymnosperms), forming a broader group known asAnthophyta. Molecular data contradicts this, with gnetophytes found to be much more genetically similar toconifers. The exact position of Bennettitales remains uncertain.
Bennettitales are divided into two families,Cycadeoidaceae andWilliamsoniaceae, which have distinctgrowth habits. Cycadeoidaceae had stout,cycad-like trunks with bisporangiate (containing bothmegaspores andmicrospores)strobili (cones) serving as theirreproductive structures. Williamsoniaceae either had bisporangiate or monosporangiate cones, and distinctly slender and branching woody trunks.[2] The Williamsoniaceae grew as woody shrubs with adivaricate branching habit, similar to that ofBanksia.[3] It has been suggested that Williamsoniaceae are aparaphyletic (not containing all descendants of a common ancestor) assemblage of all Bennettitales that do not belong to the Cycadeoidaceae.[3]
In general, bennettitalean leaves are attached to the stem with a helical (corkscrew) arrangement. Some leaves (most species ofNilssoniopteris, etc.) are narrow, solitary blades with a smooth-edged ("entire") margin.[4] Most leaf morphotypes (Pterophyllum,Ptilophyllum,Zamites,Otozamites, etc.) arepinnate (feather-shaped), with many small leaf segments attached to a central shaft. Others (Anomozamites, a few species ofNilssoniopteris) are incompletely pinnate (sawtooth-shaped) and transitional between these two end members. One unusual leaf form,Eoginkgoites, even approaches apalmate appearance similar to early species ofGinkgo.[5]
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 thecuticle can be assigned to either group with confidence. Thestomata of bennettitaleans are described assyndetocheilic. This means that the main pairedguard cells develop from the same mother cells as the subsidiary cells which surround them. This contrasts with thehaplocheilic 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.[6]
Like other gymnosperms, bennettitalean reproductive inflorescences come in the form ofcones, which producepollen andovules (unfertilized seeds). The cones have a thick centralreceptacle surrounded by simple, helically-arranged fertile and infertile structures. Tissue at the base of the cone forms layers of scale-like or petal-likebracts to protect the radiating inner structures. Some authors refer to bennettitalean cones as "flowers", though they are not equivalent to trueangiosperm flowers. Pollen is often enclosed in pairedsynangia (pollen sacs). The synangia lie on the adaxial (inner) edge of pollen-bearing leaf-like structures known asmicrosporophylls. This contrasts with cycads, all of which lack discrete synangia and bear pollen on the abaxial (outer) surface of their microsporophylls.[7]
Many bennettitaleans arebisporangiate, 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, infertileinterseminal 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.[7]
Most bennettitaleans in the familyWilliamsoniaceae are insteadmonosporangiate, 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.[7]
Seeds are dicotyledonous (possess twoembryonic leaves), with a central embryo surrounded by three layers: the thin megagametophyte, the slightly thickernucellus, and the protectiveintegument. The upper tip of the seed is tapered and opens through a thin and often extendedmicropyle. A long, narrow micropyle extending out of the seed is superficially similar to the condition in livinggnetophytes. 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 thechalaza (base of the seed), unlike cycads or gnetophytes, where the layers are fused for much of their height.[7]
Cycadeoidaceans have been suggested to have been self-pollinating, with their stems and cones buried underground,[2][8] although it has alternatively been proposed that they were pollinated by beetles.[9] The flower-like williamsoniacean male reproductive structureWeltrichia is associated with the female reproductive structureWilliamsonia, though it is uncertain whether the parent plants weremonoecious (male and female reproductive structures being present on the same plant) ordioecious (where each plant has only one gender of reproductive organ).Weltrichia was likely primarilywind-pollinated, with some species possibly pollinated by beetles.[10]
Several groups of Jurassic and Early Cretaceous insects possessed a longproboscis, and it has been suggested that they fed onnectar produced by bennettitalean reproductive structures, such as the bisexual williamsoniacean reproductive structureWilliamsoniella, which had a long, narrow central receptacle which was likely otherwise inaccessible.[11] Early Cretaceous bennettitalean pollen has been found directly associated with a proboscis bearing fly belonging to the extinct familyZhangsolvidae, providing evidence that this family acted as pollinators for the group.[1] 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.[12]
The Cycadeoideaceae (originally "Cycadeoideae") were named by English geologistWilliam Buckland in 1828, from fossil trunks found in Jurassic strata on theIsle of Portland, England, which Buckland gave thegenus nameCycadeoidea. 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.[13] In publications in 1870, Scottish botanistWilliam Carruthers and English paleobotanistWilliam Crawford Williamson described the first known reproductive organs of the Bennettitales from Jurassic strata ofYorkshire and Jurassic-Cretaceous strata of theIsle of Wight and the Isle of Portland.[14][15][16] Caruthers was the first to recognise that Bennettitales had distinct differences from cycads, and established the tribes "Williamsonieae" and "Bennettiteae",[16] with the latter being named after the genusBennettites named by Caruthers in the same publication, the name being in honour of British botanistJohn Joseph Bennett.[14][17] The order Bennettitales was erected by German botanistAdolf Engler in 1892, who recognised the group as separate from the Cycadales.[18]
TheAnthophyte hypothesis erected by Arber and Parking in 1907[19] posited thatangiosperms arose from Bennettitales, as suggested by the wood-like structures and rudimentaryflowers.[2] Based on morphological data, however, Bennettitales were classified as amonophyletic group when paired withGnetales.[20] a study in 2006 suggested that Bennettitales, Angiosperms, andGigantopteridales form aclade based on the presence ofoleanane.[21] Molecular evidence has consistently contradicted the Anthophyte hypothesis, finding that Angiosperms are the sister group to all living gymnosperms, including Gnetales.[22] Some authors have suggested due to similarities between their seed coats, Bennettitales form a clade with the gymnosperm orders ofGnetales andErdtmanithecales, dubbed the "BEG group".[23] However, this proposal has been contested by other authors, who contend that these similarities are only superficial and do not indicate a close relationship.[24] A 2017 phylogeny based on molecular signatures of fossilised cuticles found that Bennettitales were more closely related to theGinkgo+Cycads clade than conifers, and were closely related toNilssonia andPtilozamites.[25]
The oldest confirmed fossils of bennettitaleans are leaves ofNilssoniopteris shanxiensis, a species from the upper part of the UpperShihhotse Formation inShanxi Province,China.[6] This strata is dated to the earlyKungurian stage of the earlyPermian (Cisuralian), around 281 million years ago.[26] Supposed Carboniferous-Permian records ofPterophyllum do not have conclusive bennettitalean affinities or have been reinterpreted as cycad foliage in the form genusPseudoctenis.[27] True Permian records of benettitalean leaves are rare; outside of the Shihhotse Formation they are only found in theLate Permian (likelyChanghsingian)-ageUmm Irna Formation inJordan.[6] This formation is notable for the early occurrence of other Mesozoic-style flora, including the earliest records ofcorystospermalean foliage (Dicroidium).[28] The order Fredlindiales (containing the genusFredlindia) from the Late Triassic of Gondwana appears to be closely related to Bennettitales, but differs from it in some aspects of its reproductive organs.[16]
The bennettitalean fossil record reappeared in theMiddle Triassic, and williamsoniaceans became globally distributed by the end of the period.[29][6] The oldest bennettitalean reproductive structures are smallWilliamsonia "flowers" from the Middle TriassicEsk Formation ofAustralia.[16] While Williamsoniaceae had a global distribution, Cycadeoidaceae appear to have been primarily confined to the western parts ofLaurasia, and are primarily known from the Cretaceous.[16] Bennettitales were widespread and abundant during the Jurassic and Early Cretaceous, however Bennettitales severely declined during the Late Cretaceous, coincident with the rise offlowering plants, being mostly extinct by the end of the period, with the final known remains from the Northern Hemisphere being found in the polar latitudeKakanaut Formation inChukotka, Russia, dating to theMaastrichtian, assignable toPterophyllum.[30] A possible late record has been reported from the earlyOligocene of eastern Australia andTasmania, assignable to the genusPtilophyllum, but no cuticle was preserved, making the referral inconclusive.[31]
Bennettitales is typically considered the sole order in theclassBennettitopsidaEngler (1897) orCycadeoideopsidaScott (1923). Most paleobotanists prefer the two families as used here, though some authors, such as Anderson & Anderson (2007), classify the order via a larger number of families.[33] Anderson & Anderson also classified the ordersFredlindialesAnderson & Anderson (2003)[34] andPentoxylalesPilger & Melchior (1954) within Bennettitopsida.[33]
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