It is interesting to compare these different forms of Hausmannia with the fronds of recent species of Dipteris represented in [fig. 231]. The more deeply dissected type, such as H. dichotoma, closely resembles D. Lobbiana or D. quinquefurcata, while the more or less entire fossil leaves ([fig. 278], E, F and [fig. 289]) are very like the somewhat unusual form of Dipteris conjugata shown in [fig. 231], B, p. 297.
Other species of the genus are recorded from Liassic rocks of Steierdorf[978] (Hungary) and of Bornholm[979]. Nathorst[980] has described a small Rhaetic species from Scania: a French Permian plant described by Zeiller[981] and compared by him with H. dichotoma, may be a Palaeozoic example of this Dipteris-like genus.
Some segments of leaves from the Eocene beds (Middle Bagshot) of Bournemouth, and now in the British Museum, described by Gardner and Ettingshausen[982] as Podoloma polypodioides, bear a close resemblance in the venation to the lamina of Dipteris conjugata.
CHAPTER XXII.
Marattiales (Fossil).
The discovery of Pteridosperms has necessarily led to a considerable modification of the views formerly held that existing genera of Marattiaceae represent survivors of a group which occupied a dominant position in the forests of the Coal age. Mr Arber writes:—“The evidence, formerly regarded as beyond suspicion, that the eusporangiate ferns formed a dominant feature of the vegetation of the Palaeozoic period, has been undermined, more especially by the remarkable discovery of the male organs of Lyginodendron by Mr Kidston. At best we can only now regard them as a subsidiary group in that epoch in the past history of the vegetable kingdom[983].” Dr Scott expresses himself in terms slightly more favourable to the view that the Marattiaceae represent the aristocracy among the Filicales. He says:—“We now have to seek laboriously for evidence, which formerly seemed to lie open to us on all hands. I believe, however, that such careful investigation will result in the resuscitation of the Palaeozoic ferns as a considerable, though not as a dominant group[984].” Zeiller’s faith[985] in the prospect of Marattiaceous ferns retaining their position as prominent members of Palaeozoic floras, though shaken, is not extinguished: he recognises that they played a subordinate part.
Reference has already been made to the impossibility of determining whether Palaeozoic fern-like fronds may be legitimately retained in the Filicales, or whether they must be removed into the ever widening territory of the Pteridosperms. The difficulty is that the evidence of reproductive organs is very far from decisive. In the absence of the female reproductive organs, the seeds, we cannot in most cases be certain whether the small sporangium-like bodies on fertile pinnules are true fern sporangia or the microsporangia of a heterosporous pteridosperm. What is usually called an exannulate fern sporangium, such as we have in Angiopteris and in many Palaeozoic plants, has no distinguishing features which can be used as a decisive test. The microsporophylls of the Mesozoic Bennettitales produced their spores in sporangial compartments grouped in synangia like those of recent Marattiaceae; and in the case of Crossotheca, a type of frond always regarded as Marattiaceous until Kidston[986] proved it to be the microsporophyll of Lyginodendron, we have a striking instance of the futility of making dogmatic assertions as to the filicinean nature of what look like true fern sporangia. In all probability Dr Kidston’s surmise that the supposed fern sporangia known as Dactylotheca, Renaultia, Urnatopteris are the microsporangia of Pteridosperms will be proved correct[987]. The question is how many of the supposed Marattiaceous sporangia must be assigned to Pteridosperms? There is, however, no reasonable doubt that true Marattiaceae formed a part of the Upper Carboniferous flora. All that can be attempted in the following pages is to describe briefly some of the numerous types of sporangia recognised on Palaeozoic fern-like foliage, leaving to the future the task of deciding how many of them can be accepted as those of ferns. It is impossible to avoid overlapping and some repetition in the sections dealing with true Ferns and with Pteridosperms. The filicinean nature of the stem known as Psaronius (see [page 415]) has not as yet been questioned.
The nomenclature of supposed Marattiaceous species from Carboniferous and Permian rocks is in a state of some confusion owing to a lack of satisfactory distinguishing features between certain types to which different generic names have been assigned. As we have already seen in the case of supposed leptosporangiate sporangia, the interpretation of structural features in petrified or carbonised sporangia does not afford an example of unanimity among palaeobotanical experts.