Fig. 257. Cladophlebis denticulata. (From a specimen in the British Museum from the Inferior Oolite rocks of Yorkshire. Slightly reduced.)
A fertile frond from the Molteno (Rhaetic) beds of South Africa referred to Cladophlebis (Todites) Roesserti (Presl)[808] represents in all probability an Osmundaceous fern closely allied to Todites Williamsoni. The same species is described by Zeiller[809] from Rhaetic rocks of Tonkin and very similar types are figured by Leuthardt[810] from Upper Triassic rocks of Basel as Pecopteris Rutimeyeri Heer, and by Fontaine[811] from rocks of the same age in Virginia.
Cladophlebis.
The generic name Cladophlebis was instituted by Brongniart for Mesozoic fern fronds characterised by ultimate segments of linear or more or less falcate form attached to the pinnae by the whole of the base, as in the Palaeozoic genus Pecopteris, possessing a midrib strongly marked at the base and dividing towards the distal end of the lamina into finer branches and giving off secondary forked and arched veins at an acute angle. The term is generally restricted to Mesozoic fern fronds which, on account of the absence or imperfection of fertile pinnae, cannot be safely assigned to a particular family. In the case of the species described below, the evidence in regard to systematic position, though not conclusive, is sufficiently strong to justify its inclusion in the Osmundaceae.
Cladophlebis denticulata Brongniart. Figs. [256], A; [257], [258].
| 1828. | Pecopteris denticulata[812], Brongniart, Prodrome, p. 57; Hist. vég. foss. p. 301, Pl. XCVIII. figs. 1, 2. |
| — | P. Phillipsii, Brongniart, Hist. p. 304, Pl. CIX. fig. 1. |
This species is often confused[813] with Todites Williamsoni. The name Pecopteris whitbiensis has been used by different writers for Jurassic fronds which are undoubtedly specifically distinct: specimens so named by Brongniart should be referred to Todites Williamsoni, while P. whitbiensis of Lindley and Hutton[814] is Brongniart’s Cladophlebis denticulata. It is impossible to determine with accuracy the numerous examples described as Pecopteris whitbiensis, Asplenium whitbiense, Cladophlebis Albertsii (a Wealden species[815]), Asplenium, or Cladophlebis, nebbense[816], etc., from Jurassic and Rhaetic strata. The Cladophlebis denticulata form of frond is one of the commonest in recent ferns; it is represented by such species as Onoclea Struthopteris, Pteris arguta, Sadleria sp., Gleichenia dubia, Alsophila lunulata, Cyathea dealbata, and species of Polypodium. It is, therefore, not surprising to find records of this Mesozoic species from many localities and horizons. All that we can do is to point out what appear to be the most probable cases of identity among the numerous examples of fronds of this type from Mesozoic rocks, particularly Rhaetic and Jurassic, in different parts of the world. The name Cladophlebis denticulata may be employed in a comprehensive sense for fronds showing the following characters:—
Leaf large, bipinnate, with long spreading pinnae borne on a comparatively slender rachis. Pinnules, in nearly all cases, sterile, reaching a length of 3–4cm., acutely pointed, finely denticulate or entire, attached by the whole of the base ([fig. 257]). In the apical region the pinnules become shorter and broader. Venation of the Cladophlebis type ([fig. 256], A). Fertile pinnules rather straighter than the sterile, characterised by linear sori parallel to the lateral veins ([fig. 258]).
In endeavouring to distinguish specifically between fronds showing a general agreement in habit with C. denticulata, special attention should be paid to venation characters, the shape of the pinnules, the relation of the two edges of the lamina to one another, and to the amount of curvature of the whole pinnule. Unless the material is abundant, it is often impossible to distinguish between characters of specific value and others which are the expression of differences in age or of position on a large frond, to say nothing of the well-known variability which is amply illustrated by recent ferns. It is remarkable that very few specimens are known which throw any light on the nature of the fertile pinnae. Fig. 258 represents an impression from the Inferior Oolite rocks of the Yorkshire coast in which the exposed upper surface of the pinnules shows a series of parallel ridges following the course of the lateral veins and no doubt formed by oblong sori on the lower surface. There can be little doubt that the specimen figured by Lindley and Hutton and by others as Pecopteris undans[817] is, as Nathorst suggests, a portion of a fertile frond of C. denticulata. A fertile specimen of a frond resembling in habit C. denticulata, which Fontaine has described from the Jurassic rocks of Oregon as Danaeopsis Storrsii[818], exhibits, as that author points out, a superficial resemblance to the specimen named by Lindley and Hutton Pecopteris undans. There is, however, no adequate reason for referring the American fragment to the Marattiaceae. In the absence of sporangia we cannot speak confidently as to the systematic position of this common type; but there are fairly good grounds for the assertion that some at least of the fronds described under this name are those of Osmundaceae. The English specimen shown in [fig. 258] is very similar to some Indian fossils figured by Feistmantel as Asplenites macrocarpus[819], which are probably identical with Pecopteris australis Morris[820], a fern that is indistinguishable from Cladophlebis denticulata. Renault[821] figured a fertile specimen of the Australian fossil as Todea australis, which agrees very closely with that shown in [fig. 258], and the sporangia figured by the French author are of the Osmundaceous type. Another example of a fertile specimen is afforded by a Rhaetic fern from Franconia, Asplenites ottonis, which is probably identical with Alethopteris Roesserti Presl [= Cladophlebis (Todites) Roesserti], a plant closely resembling Cladophlebis denticulata. Another argument in favour of including C. denticulata in the Osmundaceae is supplied by the association of pinnae of this type with the petrified stem of Osmundites Dunlopi recorded by Kidston and Gwynne-Vaughan.
