Lyginopteridales

The Lyginopteridales are an extinct group of seed plants known from the Paleozoic. They were the first plant fossils to be described as pteridosperms (a polyphyletic group sometimes referred to as "seed ferns") and, thus, the group on which the concept of pteridosperms was first developed; they are the stratigraphically oldest-known pteridosperms, occurring first in late Devonian strata; and they have the most primitive features, most notably in the structure of their ovules. They probably evolved from a group of Late Devonian progymnosperms known as the Aneurophytales, which had large, compound frond-like leaves. The Lyginopteridales became the most abundant group of pteridosperms during Mississippian times, and included both trees 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 (lianescent) 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 Cathaysia and east equatorial Gondwana they persisted into the Late Permian, but subsequently became extinct. Most evidence of the Lyginopteridales suggests that they grew in tropical latitudes of the time, in North America, Europe and China.

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).^{[citation needed]}

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 and then Stamnostoma. 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.^{[citation needed]}

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.

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, 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. 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. The only notable exceptions were a distinctive group of Mississippian-age lyginopteridaleans that had platyspermic ovules and are referred to the fossil family the Eospermaceae.

The Lyginopteridales produced small trilete pre-pollen that superficially resemble the spores of non-seed plants but with a fundamentally gymnosperm-like wall-structure. The pre-pollen was produced by sporangia that formed regular clusters (synangia). The stratigraphically older lyginopteridaleans had trusses of synangia borne on slender axes, which were attached to vegetative fronds; these are referred to the fossil genera Telangium if they are anatomically preserved or Telangiopsis if they occur as adpressions. The more primitive forms of Telangium had sporangial walls that were essentially uniform in thickness. In stratigraphically younger Telangium species, however, the side of each sporangium that formed the outer surface of the synangium tended to be thicker than the inwards-facing wall, suggesting that the sporangia split to release the pre-pollen along this inner wall.^{[citation needed]}

In Pennsylvanian-age lyginopteridalean synangia, the sporangia were usually attached to a pad of tissue that was probably homologous to a pinnule on the vegetative fronds. For instance, Crossotheca synangia bore elongate, "epaulet"-shaped pads, that were arranged in a pinnate pattern in a frond that was wholly or partially fertile. The Feraxotheca synangium in contrast had a much less elongate, almost radial pad.