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The Lyginopteridales were the archetypal pteridosperms: They were the first plant fossils to be described as pteridosperms and, thus, the group on which the concept of pteridosperms was first developed;〔Oliver, F. W. & Scott, D. H. (1904). "On the structure of the Palaeozoic seed ''Lagenostoma Lomaxi'', with a statement of the evidence upon which it is referred to ''Lyginodendron''." ''Philosophical Transactions of the Royal Society of London, Series B,'' 197: 193-247.〕 they are the stratigraphically oldest-known pteridosperms, occurring first in late Devonian strata;〔Rothwell, G. W., Scheckler, S. E. & Gillespie, W. H. (1989). "''Elkinsia'' gen. nov., a Late Devonian gymnosperm with cupulate ovules." ''Botanical Gazette,'' 150: 170-189.〕 and they have the most primitive features, most notably in the structure of their ovules.〔Long, A. G. (1959). "On the structure of ''Calymmatotheca kidstoni'' Calder (emended) and ''Genomosperma latens'' gen. et sp. nov. from the Calciferous Sandstone Series of Berwickshire." ''Transactions of the Royal Society of Edinburgh,'' 64: 29-44.〕 They probably evolved from a group of Late Devonian progymnosperms known as the Aneurophytales,〔Rothwell, G. W. & Erwin, D. M. (1987). "Origin of seed plants: an aneurophyte/seed-fern link elaborated." ''American Journal of Botany,'' 74: 970-973.〕 which had large, compound frond-like leaves. The Lyginopteridales became the most abundant group of pteridosperms during Mississippian times, and included both trees〔Long, A. G. (1979). "Observations on the Lower Carboniferous genus ''Pitus'' Witham." ''Transactions of the Royal Society of Edinburgh,'' 70: 111-127.〕 and smaller plants.〔 During early and most of middle Pennsylvanian times the Medullosales took over as the more important of the larger pteridosperms but the Lyginopteridales continued to flourish as climbing (lianesent) and scrambling plants. However, later in Middle Pennsylvanian times the Lyginopteridales went into serious decline, probably being out-competed by the Callistophytales that occupied similar ecological niches but had more sophisticated reproductive strategies. A few species continued into Late Pennsylvanian times, and in China persisted into early Permian (Asselian) times, but then became extinct. Most evidence of the Lyginopteridales suggests that they grew in tropical latitudes of the time, in North America, Europe and China. ==Ovules== As the Lyginopteridales are the earliest-known gymnosperms, and the development of ovules was one of the key innovations that enabled seed plants eventually to dominate land vegetation, the evolution of lyginopteridalean ovules has attracted considerable interest from palaeobotanists. The most important work on these early ovules was by the British palaeobotanist Albert Long, based mainly on early Mississippian, anatomically preserved fossils from Scotland (UK). Some of the earliest ovules (e.g., ''Genomosperma'') consisted of a nucellus (the equivalent of the sporangium wall) surrounded by a sheath of slender axes known as a pre-integument.〔 It is widely thought that this arrangement was derived from an ancestral condition where there was a cluster of sporangium-bearing axes, but where only one eventually retained its megasporangium, the others forming the surrounding pre-integument. Progressively, this sheath of axes became fused to form a continuous integument that surrounded and protected the nucellus, such as in ''Eurystoma''〔Long, A. G. (1960). "On the structure of ''Samaropsis scotica'' Calder (emended) and ''Eurystoma angulare'' gen. et sp. nov., petrified seeds from the Calciferous Sandstone Series of Berwickshire. ''Transactions of the Royal Society of Edinburgh,'' 64: 261-280.〕 and then ''Stamnostoma''.〔Long, A. G. (1960). "''Stamnostoma huttonense'' gen. et sp. nov. - a pteridosperm seed and cupule from the Calciferous Sandstone Series of Berwickshire." ''Transactions of the Royal Society of Edinburgh,'' ''64'': 201-215.〕 These earliest ovules had the apical part of the nucellus exposed, from which there was a projection known as a lagenostome (sometimes also called a salpinx) that facilitated capture of the pollen and directed it down to the pollen chamber above the megagametophyte. In later, Pennsylvanian-age ovules such as ''Lagenostoma''〔 the nucellus became almost entirely encased by and fused to the integument, leaving just the small distal opening in the integument known as the micropyle through which pollen passed. Nevertheless, most lyginopteridalean ovules retained a lagenostome, despite its function in pollen capture having been replaced by the micropyle. As with all seed-plants, the lyginopteridalean ovules had just one functional seed megaspore within the nucellus. In some, however, three other, aborted megaspores were still present that together with the functional megaspore represented the remains of what would have been the tetrad of megaspores in the ancestral pre-seed plants.〔Schabilion, J. T. & Brotzman, N. C. (1979). "A tetrahedral megaspore arrangement in a seed fern ovule of Pennsylvania age." ''American Journal of Botany,'' 66: 744-745.〕 Most if not all lyginopteridalean ovules were borne in an outer protective sheath of tissue known as a cupule (there are some lyginopteridalean ovules that have been reported without a cupule, but these may simply have been shed from the cupule before being fossilised). In Late Devonian and early Mississippian species, the cupule contained several ovules,〔〔Long, A. G. (1961). "''Tristichia ovensi'' gen. et sp. nov., a protostelic Lower Carboniferous pteridosperm from Berwickshire and East Lothian, with an account of some associated seeds and cupules. ''Transactions of the Royal Society of Edinburgh,'' 64: 477-489.〕〔Long, A. G. (1965). "On the cupule structure of ''Eurystoma angulare''. ''Transactions of the Royal Society of Edinburgh,'' 66: 111-128.〕〔Long, A. G. (1966). "Some Lower Carboniferous fructifications from Berwickshire, together with a theoretical account of the evolution of ovules, cupules and carpels." ''Transactions of the Royal Society of Edinburgh,'' 66: 345-375.〕〔Long, A. G. (1969). "''Eurystoma trigona'' sp. nov., a pteridosperm ovule borne on a frond of ''Alcicornopteris'' Kidston. ''Transactions of the Royal Society of Edinburgh,'' 68: 171-182.〕〔Long, A. G. (1975). "Further observations on some Lower Carboniferous seeds and cupules." ''Transactions of the Royal Society of Edinburgh,'' 69: 267-293.〕〔Long, A. G. (1977). "Some Lower Carboniferous pteridosperm cupules bearing ovules and microsporangia." ''Transactions of the Royal Society of Edinburgh,'' 70: 1-11.〕 but by Pennsylvanian times there was normally just a single ovule per cupule.〔 A number of cases have been found of cupules occurring in clusters at the ends of branching axes.〔Long, A. G. (1963). "Some specimens of ''Lyginorachis papilio'' Kidston associated with stems of ''Pitys''." ''Transactions of the Royal Society of Edinburgh,'' 65: 211-224.〕 Many palaeobotanists now interpret these clusters of cupulate organs as fertile fronds, in which the cupulate tissue was derived from the laminate part of the frond that surrounded the ovule and thereby provided added protection for it. Most lyginopteridalean ovules were radiospermic.〔Long, A. G. (1959). "On the structure of ''Calymmatotheca kidstoni'' Calder (emended) and ''Genomosperma latens'' gen. et sp. nov. from the Calciferous Sandstone Series of Berwickshire. ''Transactions of the Royal Society of Edinburgh,'' 64: 29-44, pls. 1-4.〕 The only notable exceptions were a distinctive group of Mississippian-age lyginopteridaleams that had platyspermic ovules and are referred to the fossil family the Eospermaceae.〔〔〔Long, A. G. (1961). On the structure of ''Deltasperma fouldenense'' gen. et sp. nov., and ''Camptosperma berniciense'' gen. et sp. nov., petrified seeds from the Calciferous Sandstone Series of Berwickshire. ''Transactions of the Royal Society of Edinburgh,'' 64: 281-295.〕〔Long, A. G. (1961). Some pteridosperm seeds from the Calciferous Sandstone Series of Berwickshire. ''Transactions of the Royal Society of Edinburgh,'' 64: 401-419.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Lyginopteridales」の詳細全文を読む スポンサード リンク
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