From Upper Triassic beds of Virginia, Fontaine has figured several fronds for which he instituted the genus Mertensides[854]. The habit, as he points out, is not dichotomous, but the sori are circular and are said to be composed in some species of four to six sporangia. No satisfactory evidence is brought forward in support of the use of a designation implying a close relationship with recent Gleichenias (sect. Mertensia). One of the species described by Fontaine was originally named by Bunbury Pecopteris bullatus[855], the imperfect type-specimen of which is now in the Museum of the Cambridge Botany School. In the form of the frond, the thick rachis, and in the pinnules this Triassic species resembles Todites Williamsoni, but the resemblance does not extend to the sori. Two of Fontaine’s species are recorded by Stur from Austria[856], but he places them in the genus Oligocarpia and includes them in the Marattiaceae.
Leuthardt[857] figures what appears to be a Gleicheniaceous fern from the Upper Triassic beds of Basel as Gleichenites gracilis (Heer) showing sori composed of five sporangia ([fig. 265], C) with a horizontal annulus. A Rhaetic species Gleichenites microphyllus Schenk[858] from Franconia agrees in the form of its small rounded pinnules with Gleichenia, but no sporangia have so far been found.
An impression of a frond from Jurassic rocks of northern Italy figured by Zigno as Gleichenites elegans[859] closely resembles in habit recent species of Gleichenia; though no sporangia have been found, the habit of the frond gives probability to Zigno’s determination.
A Jurassic species from Poland, Gleichenites Rostafinskii, referred by Raciborski[860] to Gleichenia, exhibits a close agreement in habit and in the form of the soral impressions to some recent species of Gleichenia.
As we pass upwards to Wealden and more recent rocks it becomes clear that the Gleicheniaceae were prominent members of late Mesozoic floras in north Europe and reached as far north as Disco Island. In English Wealden beds portions of sterile fronds have been found which were assigned to a new genus Leckenbya[861], but it is probable that these specimens would be more correctly referred to Gleichenites. Similarly fragments of Gleichenia-like pinnae with very small rounded pinnules occur in the Wealden rocks of Bernissart, Belgium[862], in north Germany[863], and elsewhere. Conclusive evidence has been obtained by Prof. Bommer of the existence of Gleichenites in Wealden beds near Brussels, where many plant remains have been found in a wonderful state of preservation. The specimens, which I had an opportunity of seeing some years ago, might easily be mistaken for rather old and brown pieces of recent plants. Some of the Belgian fragments, of which Prof. Bommer has kindly sent me drawings and photographs, are characterised by an arrangement of vascular tissue identical with that in the petioles and rhizomes of some protostelic Gleichenias. The stele of one of the Belgian rhizomes appears to be identical with that of Gleichenia dicarpa ([fig. 237], C. p. 310).
Fig. 262.
- Gleichenites longipennis Heer.
- G. delicatula Heer.
- G. Nordenskioldi Heer.
- G. Zippei. (Corda.)
(After Heer; A, B, D, very slightly reduced.)