The Pseudoborniales represents a unique order that is known from relatively few localities and, to date, is represented by a single species, Pseudobornia ursina. In 1894, the noted paleobotanist A. G. Nathorst (FIG. 10.3) collected a number of Late Devonian fossils from Bear Island (Bj0rn0ya), south of Spitsbergen. Included in the collection were a number of interesting compression and impression specimens consisting of axes bearing whorled, lateral appendages. The only other fossils that have been referred to this group were discovered in Devonian rocks in northeastern Alaska (Mamay, 1962). Since the original description by Nathorst, additional specimens have provided useful information about many previously unknown features of these plants.
Pseudobornia ursina was monopodially branched and believed to have been 15-20m tall (Schweitzer, 1967) (FIG. 10.4 ). The largest axis measures 60 cm in diameter and is
thought to have represented the basal portion of the plant. It is not known what the underground parts were like, but the basal region has been reconstructed as a rhizome bearing roots. The primary stem of P ursina consists of nodes
separated by internodes ~80cm long (FIG. 10.5). In the basal regions of the plant, each node produces one or two first-order branches up to 3 m long and ~10cm in diameter. Second-order branches are produced in a decussate arrangement (FIG. 10.5); each of these laterals bears axes in a pseudodistichous arrangement.
Leaves (sometimes called meiophylls in older treatments, e.g., Benson, 1921) are produced only from the branches of the ultimate order and are arranged in superimposed whorls of four leaves each. A single leaf consists of an elongated, twice-dichotomized petiole to which are attached four leaflets. Each leaflet is up to 6 cm long and about 1 cm wide and consists of a lamina that is highly dissected along the margin. Nothing is known about the venation, but the configuration of the lamina suggests a dichotomous system of terete strands.
Fertile branches up to 30 cm long are produced at the distal ends of primary branches. Each fertile unit consists of whorled bracts and sporangiophores, with the tips of the spo-rangiophores upturned and divided into two segments. Each sporangiophore produces ~30 sporangia.
Nothing is known about the anatomy of P ursina, although Schweitzer (1967) has suggested that the axes were hollow except at the nodes, where nodal diaphragms served to strengthen the stems. Mosbrugger (1990) suggested that the hollow stem of P ursina, would have reduced the maximum bending stress in the periphery and at the base of the trunk, with only a minimum investment of cells; thus, this plant may have grown faster than plants with solid trunks. Moreover, the hollow stem may have served as an aeration system for the roots. This would have been particularly advantageous for this plant, as it lived in swamps in which the root systems were (temporarily) inundated.
The affinities of the Pseudoborniales continue to remain obscure. The morphology of the vegetative organs is like that in the Equisetales, whereas the reproductive parts are more similar to members of the Sphenophyllales.
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