EVOLUTION OF THE PSILOPHYTE PALEOFLORA

A century has now elapsed since Sir John William Dawson described the nude plant, Psilophyton, from the Devonian rocks of Gaspe (1859). Comprising an exceedingly ancient and simply constructed plant (fig. 1), it was the first fossil member of the ancient phylum Psilophyta to be discovered. At first the idea that it was a fossil plant was received with considerable scepticism but by the turn of the century, owing to the discovery of additional Devonian plants, it was generally conceded to be an authentic land plant. During the succeeding three decades numerous Devonian land plants of diverse character were described from rocks on all the continents. The problems of evolution posed by this earliest land vegetation-the Psilophyte Paleoflorahave been especially difficult to resolve. In particular, divergent views have been expressed with respect to their probable antiquity, their phylogenetic relations and their rate of evolution. Evidence afforded by more recent discoveries of new land plants in Devonian, Silurian and Cambrian rocks makes it possible to view these problems with better perspective. To understand the nature of the issues to be discussed, it is desirable first to recall certain salient features of the earliest land flora. The oldest land plants now known are from the Early Cambrian of the Baltic region (Reissinger, 1939; Naumova, 1949), where approximately 20 sporetypes of very simple construction in terms of size, shape, and sculpture are recorded. Spores of somewhat more complex character occur in the Middle and Late Cambrian of Kashmir, Spiti, and the Salt Range in India (Ghosh and Bose, 1950; 1952; Ghosh, Sen and Bose, 1951; Jacob et al., 1953a; 1953b), and in eastern Sweden (Darrah, 1937). Approximately 60 Cambrian spore-genera are now on record, all discovered largely during the past decade. These spores may be regarded as reproductive cells of land plants because they are cutinized. Further, at least some of the Cambrian plants were vascular since woody fragments showing scalariform tracheids, and tracheids with simple and bordered pits, occur in association with the spores in India. Most importantly, several phyletic lines appear to be represented, though their affinities in terms of modern types are difficult to judge. Naumova suggested that the spores from the Baltic region represent both Bryophyta and Pteridophyta. In the latter group she noted that one of the types is similar to spores of the Calamariaceae of Carboniferous age. The Indian workers (Jacob et al., 1953b) report that their fossils apparently represent 6 different groups of vascular plants, chiefly primitive pteridophytes (including equisetalean types), probably seed ferns, and possibly primitive gymnosperms. That the phylum Lepidophyta was already established by the Middle Cambrian is shown by the occurrence of Aldanophyton in Siberia (Krystofovich, 1954), which is represented by several stems, one up to 8.5 cm. long, with spirally arranged microphyllous leaves. Remains of land plants have not yet been recorded from Ordovician or Lower to Middle Silurian rocks. However, several genera are known from the Upper Silurian of Australia. Some are of un-