One of the most complete specimens so far discovered has recently been described by Fritel[1219] from Eocene beds of the Paris Basin as Salvinia Zeilleri. This species, founded on portions of stems bearing floating leaves, submerged root-like leaves, and sporocarps, is compared with a recent tropical American species S. auriculata.
It is noteworthy that no authentic records of Hydropterideae have been discovered in Palaeozoic rocks[1220]. Comparisons have been made in the case of the genera Traquairia Carr. and Sporocarpon Will. with the reproductive organs of Azolla[1221], but these rest on a wholly insufficient basis.
Dawson[1222] proposed the generic name Protosalvinia for some spores of Devonian age, which he regarded on inadequate grounds as evidence of Palaeozoic Hydropterideae.
Zeiller[1223], in discussing the possible relationships of the problematical type Chorionopteris gleichenioides Cord., suggests a possible alliance with the Hydropterideae. Corda founded the genus Chorionopteris[1224] on some small fragments of pinnules, 6–7 mm. long, found in the Carboniferous rocks of Radnitz in Bohemia.
The lobes of the pinnules are incurved distally to form a capsule, containing four sporangia, which apparently opened on dehiscence into four valves; the spores are of one size. The material is however insufficient for accurate determination.
There is no evidence contributed by fossil records which indicates a high antiquity for the Hydropterideae. It is unsafe to base any conclusion on the absence of undoubted Palaeozoic representatives of this group; but the almost complete absence of records in pre-Tertiary strata is a fact which may be allowed some weight in regard to the possible evolution of the heterosporous filicales at a comparatively late period in the earth’s history.
A description of the Mesozoic genus Sagenopteris may be conveniently included in this chapter, though as in many other instances the inclusion of a genus under the heading of a recent family name does not by any means imply that the position of the extinct type is regarded as settled.
Sagenopteris.
This generic name was applied by Presl[1225] to small fronds composed of four or rarely two palmately disposed leaflets with a more or less distinct midrib and anastomosing secondary veins. Schimper[1226] compared Sagenopteris with Marsilia, but did not regard the resemblance as evidence of relationship. Nathorst[1227] expressed the opinion that certain fruit-like bodies obtained from the Rhaetic beds of Scania are of the nature of sporocarps and were borne by Sagenopteris, with the leaves of which they were associated. He published a drawing of part of a fruit showing on its partially flattened surface some raised oval bodies which are considered to be spores. Dr Nathorst kindly placed at my disposal the drawings reproduced in [fig. 325] made from some of his specimens found at Bjuf in Scania.
In contour and superficial features, e.g. the veining on the wall, these bodies bear a fairly close resemblance to the sporocarps of recent species of Marsilia. They were found in association with the leaves of Sagenopteris undulata Nath., an abundant Scania type similar in form to the English Jurassic species S. Phillipsi (figs. [327], [328]). Heer was independently led by an examination of some examples of the Swedish “fruits” to compare them with the sporocarps of Marsilia. A small spherical body is figured by Zigno[1228] close to a leaf of his species S. angustifolia, which may be a sporocarp. In a recent paper, Salfeld[1229] says that he found fructification on the lower face of the leaflets of S. Nilssoniana Brongn. from German Jurassic rocks, but he brings forward no evidence in support of this statement. The systematic position of Sagenopteris is by no means settled. In a previous account of the genus I expressed the view that it is probably a member of the true ferns[1230], but the resemblance of Dr Nathorst’s drawings to the Marsilian sporocarps influences me in favour of his opinion that Sagenopteris may belong to the Hydropterideae. The evidence, as Solms-Laubach[1231] states, is not wholly satisfactory: Schenk points out that the frequent occurrence of detached Sagenopteris leaflets suggests that they easily fell off the petiole, whereas in Marsilia the leaflets do not fall off independently. The discovery of a new type of Marsiliaceae in Brazil, which Lindman has described as Regnellidium diphyllum[1232] ([fig. 326], A), affords an additional piece of evidence bearing on the comparison of Sagenopteris with members of this family. In Regnellidium the leaves differ from those of Marsilia in bearing two instead of four leaflets, and in the former the veins are repeatedly forked, and do not anastomose as in Marsilia. In the possession of only two leaflets Regnellidium agrees with some forms of Sagenopteris ([fig. 328]).