FOSSIL FERNS.
Osmundaceae.
From the Culm of Silesia, Stur[761] described impressions of sterile fronds which he named Todea Lipoldi on the ground of the similarity of the finely divided pinnules to those of Todea superba and other filmy species of the genus. The type-specimen of Stur (in the Geological Survey Museum, Vienna) affords no information as to sporangial characters and cannot be accepted as an authentic record of a Lower Carboniferous representative of the family. Another more satisfactory but hardly convincing piece of evidence bearing on the presence of Osmundaceae in pre-Permian floras has been adduced by Renault[762], who described petrified sporangia from the Culm beds of Esnost in France as Todeopsis primaeva ([fig. 256], F). These pyriform sporangia are characterised by the presence of a plate of large cells comparable with the subapical group of “annulus” cells in the sporangia of the recent species ([fig. 221]).
Zeiller[763] has published a figure of some sporangia described by Renault from Autun resembling the Osmundaceous type in having a plate of thick-walled cells instead of a true annulus, but the plate is larger than the group of cells in the recent sporangia, and both sporangia and spores are smaller in the fossil. The sporangia from Carboniferous rocks described by Weiss as Sturiella[764] bear some resemblance to those of recent Osmundaceae, but there is no adequate reason for referring them to this family.
The generic name Pteridotheca is employed by Scott as a convenient designation for unassigned petrified sporangia of Palaeozoic age with an annulus and other characters indicating fern-affinity. In the species P. Butterworthi[765] the sporangia are characterised by a group of large cells suggesting comparison with the annulus, or what represents the annulus, in Osmundaceae and Marattiaceae. Scott has also described a sporangium from the Coal-Measures containing germinating spores[766]; the structure is similar to that of recent Osmundaceous sporangia, and it is interesting to note that germinating spores have been observed in the recent species Todea hymenophylloides[767].
Additional evidence of the same kind is afforded by fertile specimens of a quadripinnate fern with deeply dissected oval-lanceolate pinnules described by Zeiller from the Coal-Measures of Heraclea in Asia Minor as Kidstonia heracleensis[768] (fig. 256, E). Carbonised sporangia were found at the base of narrow lobes of the ultimate segments and, as seen in fig. 256, E, the sporangial wall is distinguished by a plate of larger cells occupying a position like that of the “annulus” of recent Osmundaceae. Zeiller regards the sporangia as intermediate between those of Osmundaceae and Schizaeaceae. From the same locality Zeiller describes another frond bearing somewhat similar sporangia as Sphenopteris (Discopteris) Rallii (fig. 256, D)[769]: the term Discopteris was instituted by Stur for fertile fronds referred by him to the Marattiaceae[770].
It is by no means safe to assume that these and such Upper Carboniferous sporangia as Bower[771] compared with those of Todea were borne on plants possessing the anatomical characters of Osmundaceae rather than those of the extinct Palaeozoic family Botryopterideae. This brings us to the important fact, first pointed out by Renault, that the Botryopterideae are essentially generalised ferns exhibiting many points of contact with the Osmundaceae[772]. It is clear that whether or not we are justified in tracing the Osmundaceae as far back as the Lower Carboniferous period, some of the characteristics of the family were already foreshadowed in rocks of this age.
Through a fortunate accident of preservation, unequivocal evidence of the existence of Osmundaceae in the Palaeozoic era is supplied by the Russian Upper Permian genera Zalesskya and Thamnopteris.
Zalesskya.
This generic title has been instituted by Kidston and Gwynne-Vaughan[773] for two Russian stems of Upper Permian age, one of which was named by Eichwald[774] Chelepteris gracilis, but the probability that the type of the genus Chelepteris is generically distinct from Eichwald’s species necessitated a new designation for the Permian fern.