Archaeopteris spp. (progymnosperm tree)
The First Modern Tree
Archaeopteris spp. account for slightly more than half of the identifiable plant fossils collected at the floodplain pond facies of Red Hill; the pre-fern Rhacophyton accounted for most of the rest. Typically, these fossils were relatively intact abscised branches that were probably blown by the wind into the floodplain pond from elsewhere in the floodplain. Larger woody fossils (i.e., trunks or large branches) have yet to be collected, but one likely and several suggestive root casts have been found. The fossils collected at Red Hill are assignable to four species based on leaf morphology. Most belong to either Archaeopteris macilenta or A. hibernica. Two other species, A. halliana and A. obtusa, were much less frequent.
The co-dominance of Archaeopteris and Rhacophyton at Red Hill is typical of Late Devonian floodplain localities from the Catskill Delta (New York, Pennsylvania, Virginia, West Virginia). Archaeopteris was also common to abundant in some near-marine Catskill localities, primarily as sea-drifted logs (form genus Callixylon). In addition, Archaeopteris-dominated forests are common in Late Devonian localities from elsewhere in Euramerica (North America and Western Europe), Gondwana (Africa, Antarctica, Australia and South America), China and Siberia. It has been found at paleolatitudes ranging from equatorial to sub-polar. From its first appearance in the middle Frasnian Archaeopteris quickly became an important and typically dominant component of the flora. Indeed, it became the lynchpin of the first true forests. Archaeopteris remained paramount until the end of the Devonian, at which time it mysteriously became extinct.
The dramatic success of Archaeopteris can be attributed to several significant evolutionary innovations. It was essentially the first modern tree. Trunk diameters in excess of 1 m and estimated heights of up to 30 m have been reported. It has long been known that the wood of Archaeopteris (form genus Callixylon) resembled that of modern conifers. But little was known about the trees development until the results of a recent studies led by Brigitte Meyer-Berthaud and Stephen Scheckler were published. They discovered that Archaeopteris grew in much the same way as do modern trees.
Until the advent of Archaeopteris, bushy and arborescent plants grew only at the tip of their main axes. Branches formed by the forking of the tips, but if a tip died, then growth would cease. In contrast, Archaeopteris had lateral buds in its trunk and branches. This meant that growth would continue whether the axial growth tip was intact or not. Combined with a bi-directional (bifacial) cambium (a feature found in seed plants and some other vascular plants in which a ring of growing tissue that produces wood both in toward the center and out away from the center), Archaeopteris became a truly perennial plant. Indeed, some individuals apparently lived for at least 40-50 years.
The ability to be a long-lived and actively growing plant enabled Archaeopteris to greatly increase in size. This increase in size was facilitated in part by the continued growth of branch bases to form reinforcing structures that resisted breakage. The increase in size and longevity is also associated with the development of a massive root system that would have been useful for structural support and the uptake of nutrients and water. Prior to the arrival of Archaeopteris, root systems rarely went deeper than 10 to 20 cm, but depths in excess of 1 m have been reported for this tree. Moreover, its root exhibited perennial root growth and the repeated production of lateral rootlets. The enhanced penetration of soils by its root system appears to have had a profound impact on pedogenesis (the development of soils) during the Late Devonian.
Perennial growth in Archaeopteris may have facilitated another innovation, webbed and planar leaves. Other Late Devonian plants typically had their primary photosynthetic surfaces on either the narrow leaf-like microphylls of lycopsids or a jumble of finely divided branches. Some, such as the ancient "fern" Rhacophyton, had the finer branches arranged in flattened "fronds". This flattened or planar arrangement is a relatively efficient structure with which to intercept light. A partial to complete planar arrangement of ultimate branches is also evident in Archaeopteris. Indeed, the frond-like appearance of many fossil specimens led early investigators to conclude that it was either a fern or a fern-like plant.
In addition to planation, the leaves of Archaeopteris exhibit varying degrees of webbing (i.e., the presence of leafy tissues between fine branches or veins). The extent of webbing differs considerably among leaf species of Archaeopteris. For example, the distal margins of the incompletely webbed leaves of A. macilenta are deeply incised, while those of the completely webbed leaves of A. halliana, A. hibernica and A. obtusa are smooth. Its not clear whether the leaf species of Archaeopteris actually represent different biological species or are in fact different leaf variations in one or more species. Also, technically speaking, the leaves are not true leaves. The planated tissues of Archaeopteris attach directly to the woody branch, whereas true leaves are attached via a specialized organ, the stem.
Planated and webbed leaves could improve light interception for Archaeopteris, but they could also increase moisture stress. As the leaves captured CO2 from the air, they would loose water through evaporation. This loss could be compensated for by improved water uptake in the roots, but the plant would dry out if the soil became too dry. One solution to water stress is to shed the leaves during the dry season. Because Archaeopteris had perennial growth buds, it could afford to shed its leaves and then regrow them at a later time. Although it seems very plausible, we dont know for sure whether Archaeopteris shed its leaves to avoid seasonal drought. But we do know that it was deciduous. Rather than shed individual leaves, however, it shed leafy branch systems that consisted of two orders of smaller branches and the leaves themselves. These systems attached to the main branches at specialized abscission organs and it is these structures that constitute the most commonly fossilized portions of the tree.
Although Archaeopteris exhibited numerous unique features in its vegetative organs (leaves, perennial branching, deciduous habit, root system, conifer-like wood), it was relatively conventional in its reproduction. As with most of its contemporary vascular plants, Archaeopteris produced spores rather than seeds. However, it exhibited what is generally considered to be an advanced system of spore production called heterospory. Heterosporous plants produce two sets of specialized spores: female megaspores and male microspores. This condition, which has evolved independently in several lineages (e.g., barinophytes, stauropterian "ferns" and arborescent lycopsids), is widely believed to be a precursor to seed reproduction. The sporangia of Archaeopteris are located on two or more rows modified ultimate branches. They can be seen at the bottom of the fossil image of Archaeopteris hibernica to the right.
The history of our understanding of this plant is illustrative of one of the major problems facing paleobotanists: Plant fossils are almost always fragmentary, and the larger the plant, the more difficult it is to relate one fragment to another. Consequently, paleobotanists resort to form taxa (i.e., names are given to distinctive fragments). In the case of Archaeopteris, the leaves and their attached ultimate branches were assigned to the form genus Archaeopteris while the fragments of trunks and major branches were identified as the form genus Callixylon. It was not until Charles S. Beck discovered a museum specimen in which the form genus Archaeopteris as connected to the form genus Callixylon, that these two taxa could be considered a single biological taxon.
In response to his discovery, Beck proposed a new group, the progymnosperms, to accommodate this plant with freely shed spores and the conifer-like wood. Soon, other plants were assigned to the progymnosperms on the basis of their reproduction and stem anatomy. Three groups of progymnosperms are now relatively well known: Aneurophytales, Protopityales and Archaeopteridales. The Aneurophytales (e.g., Aneurophyton, Proteokalon, Rellimia, Tetraxylopteris and Triloboxylon) are Middle to Late Devonian plants that are considered to be the most primitive members. They were shrubby plants with three-dimensional branching. Most were homosporous (i.e., one size of spore) but one genus, Tetraxylopteris, produced spores with a considerable range in sizes. The Propityales is represented by a single genus, Protopitys, from the late Early Carboniferous. The Archaeopteridales is represented by Svalbardia (late Middle Devonian) and Archaeopteris (Late Devonian).
The progymnosperms are widely regarded as the ancestors of seed plants. As a group they share several vegetative (e.g., branching patterns and vascular anatomy) and reproductive ( e.g., clusters of paired sporangia that split longitudinally) features with the more primitive trimerophytes of the Early and Middle Devonian. They also have vegetative features otherwise found only with seed plants (e.g., bifacial vascular cambium) and reproductive features otherwise found only in early gymnosperms (e.g., the cell pattern of sporangial epidermis). Differences of opinion occur, however, as to which group of progymnosperms gave rise to early seed plants and whether early seed plants evolved once or several times from progymnosperm stock.
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