Some of the Palaeozoic species of Sphenopteris probably represent the fronds of true ferns, but others are known to have been borne by Pteridosperms. S. Hoeninghausi ([fig. 290], C, p. 399) is the foliage of Lyginodendron, and Scott[1415] speaks of three species, S. dissecta, S. elegans, and S. Linkii as the leaves of Heterangium. Grand’Eury[1416] has recorded the occurrence in French Coal-Measures of seeds in association with other Sphenopteroid fronds.
Mariopteris, Diplotmema, Palmatopteris.
The discovery of sporangia on the fronds of several Palaeozoic species of Sphenopteris and Pecopteris has led to the institution of new generic names, which indicate an advance in knowledge beyond the stage implied by the use of those provisional designations based solely on the form and venation of the pinnules. Other names have been created by authors in place of Sphenopteris and Pecopteris on the ground that a striking feature in the mode of branching of fronds is sufficiently important to justify generic recognition even in the absence of fertile specimens. As examples of designations based primarily on the branch-system of compound leaves, the genera Mariopteris, Diplotmema, and Palmatopteris may be briefly considered ([fig. 354] A–C). Dr Kidston[1417] is of opinion that the creation of new genera for purely vegetative characters of fronds is of no real advantage, and he prefers to retain the older provisional names for species known only in the sterile condition. On the other hand, if we are sufficiently familiar with specimens large enough to enable us to recognise a well-defined morphological character, it may serve a useful purpose to employ a generic designation for features which may have a phylogenetic value. A comparative examination of Palaeozoic, Mesozoic, and recent compound fronds, including both Pteridosperms and true ferns, brings to light certain distinguishing features characteristic of the older types which, as Potonié maintains[1418], point to the derivation of the pinnate habit from a primitive dichotomous system of branching. For a more complete discussion of this question reference should be made to Potonié’s suggestive papers. Among recent ferns Matonia and Dipteris, two survivals from the past, afford instances of fronds with a branching system of the dichotomous type.
Similarly, in Gleichenia, Lygodium, and more rarely in species of Polypodiaceae (e.g. Davallia aculeata, [fig. 232]) dichotomy is a striking feature of the fronds. In the great majority of recent ferns the fronds have assumed a pinnate habit. Among Palaeozoic fern-like fronds dichotomous branching of the main rachis and of the pinnae is much more common. Potonié draws attention to several other features which distinguish Palaeozoic fronds from the majority of later species: the frequent occurrence of pinnules borne directly on the main rachis ([fig. 354], D), and of modified pinnules or Aphlebiae on the rachis and petiole, are characters to which he attributes an evolutionary significance. The main point is that a comparative examination of leaf-form affords evidence in favour of the view that the modern type of frond, with its naked rachis bearing two rows of pinnae, has been derived from a less specialised type in which the distinction between the parts of the leaf is much less evident. The primitive leaf was probably a dichotomously branched axis provided with a continuous lamina which eventually became broken up into separate lobes or pinnules.
As the dichotomy of the frond became less regular, a pinnate habit was acquired, as is clearly seen in many Palaeozoic types which constitute connecting links between forked and pinnate fronds ([fig. 354], D). The Aphlebiae may be remnants of the once-continuous lamina on the petiole, and the normal pinnules borne on the rachis may be regarded as the attributes of fronds in which the division of physiological labour had not reached the stage which characterises the leaves of recent ferns.
Mariopteris.
This name, which is due to Zeiller[1419], is applied by him to Palaeozoic fronds characterised by a double bifurcation of the rachis of the primary pinnae. Mariopteris muricata (= Pecopteris muricata Schloth.) may be taken as the type of the genus. This species is common in the Lower and Middle Coal-Measures of Britain and rare in the Upper Coal-Measures[1420]. It is described by Kidston[1421] as one of the most polymorphic and widely distributed Coal-Measure species. The pinnules as seen in [fig. 364], B, are of the Sphenopteroid type. No fertile specimens are known, but it is significant that Grand’Eury[1422] has recorded the association of Mariopteris muricata and seeds.
The main rachis gives off alternate naked branches, each of which bifurcates at its apex into two short naked axes, and these are again forked, the ultimate branches having the form of bipinnate pinnae provided with large Sphenopteroid pinnules ([fig. 354], B). Zeiller includes in Mariopteris some species which Stur[1423] referred to his genus Diplotmema. Possibly some of the Palaeozoic fronds with a zigzag rachis may have been climbers like Lygodium.
Fig. 354.