Fossil plants, Vol. 2

The unsatisfactory and meagre records in regard to the past history of these heterosporous Filicales render superfluous more than a brief reference to the recent species.

Marsiliaceae.

This family is usually spoken of as including the two genera Marsilia and Pilularia. Lindman[1202] has however founded a third genus, Regnellidium, on a Brazilian plant which is distinguished by some well-defined characters from all species of Marsilia. The members of the Marsiliaceae live for the most part in swampy situations. Marsilia is represented in Europe by M. quadrifoliata L. which occurs in Portugal, France, Germany and other parts of the Continent, extending also to Kashmir, Northern China, and Japan. Of the other 53 species, 17 are recorded from different regions in Africa, while others occur in South America, Asia[1203], Australia, and elsewhere.

Pilularia globulifera L. is the only British representative of the Hydropterideae. The remaining four species of the genus occur in South America, California, New Zealand, Australia, and P. minuta Dur. is met with in the South of France, Algeria, and Asia Minor in subtropical or warm temperate regions.

The Marsiliaceae are regarded as more nearly related to the Schizaeaceae than to any other family of homosporous ferns[1204]. Their heterospory, the production of sporangia in closed fruit-like sporocarps, and the anatomical features associated with existence in marshy habitats, tend to obscure the resemblances to the true ferns.

The genus Marsilidium proposed by Schenk[1205] for a piece of an axis, bearing apparently a whorl of six leaflets, from the Wealden of Osterwald, cannot be regarded as satisfactory evidence of the existence of the Marsiliaceae in the Wealden flora of North Germany.

The six leaflets of Marsilidium speciosum, having a length of 5 cm., are similar in shape to the four leaflets of recent species of Marsilia, but they differ in the repeated dichotomy of the veins from the reticulate venation of the recent forms. It is worthy of note, however, that in Lindman’s Brazilian type Regnellidium diphyllum (fig. 326, A), the leaflets are characterised by dichotomous and not by anastomosing veins.

Hollick[1206] has described some impressions of imperfect orbicular leaves with a “finely flabellate obscurely reticulated(?) venation” from Cretaceous rocks of Long Island as Marsilia Andersoni, but these are too fragmentary to be accorded this generic designation. My friend Dr Krasser informs me that he is describing some well-preserved leaves from Cretaceous beds of Grünbach in Lower Austria as Marsilia Nathorsti[1207]. He compares these with the recent form Marsilia elata, a variety of M. Drummondi.

Another Lower Cretaceous species Marsilia perucensis has been figured by Frič and Bayer[1208] as a stalked fruit-like body from Bohemia. This was originally described by Velenovský as M. cretacea, but under this name Heer[1209] had previously recorded a supposed sporocarp from Greenland. These fossils have little claim to recognition as examples of Marsiliaceous plants.

The fragment figured by Heer[1210] from Tertiary rocks of Oeningen as Pilularia pedunculata is too small to determine with reasonable accuracy. Other supposed representatives of the family mentioned in palaeobotanical literature are not of sufficient importance to describe.

Salviniaceae.

The two genera of Salviniaceae, Salvinia and Azolla, are water plants, and are usually described as annuals which survive the less favourable season in the form of detached sporocarps. Goebel[1211] states that all the tropical species of Salvinia known to him have an unlimited existence.

Salvinia natans, Hoffm., the only European species, extends from the South of France to Northern China and the plains of India: the other twelve species are mostly tropical. Azolla, represented by four species, occurs in Western and Southern North America, South America, Madagascar, Australia, New Zealand, and is widely spread in tropical Asia and Africa.

Species of Azolla frequently form a considerable proportion of the floating carpet of vegetation on inland waters[1212] growing under conditions which might be supposed favourable for preservation in a fossil state.

The Salviniaceae, though probably rather farther removed than the Marsiliaceae from the homosporous Filicineae, are considered by Bower[1213] to be related to the Gradatae, but modified in consequence of their aquatic habit and the assumption of heterospory.

No undoubted examples of fossil species of Azolla have been described. Salvinia, on the other hand, is represented by several Tertiary species, for the most part founded on leaves only, and Hollick[1214], who published a list of fossil Salvinias, has described detached leaves as Salvinia elliptica Newb. from what may be Upper Cretaceous rocks from Carbonado, Washington. Some of the leaves figured as Tertiary Salvinias are of no value as evidence of the former distribution of the genus[1215].

From the Coal-beds of Yen-Bäi (Tonkin), probably of Miocene age, Zeiller[1216] has figured some well-preserved impressions of oval or orbicular leaves, 15 mm. long and 10–20 mm. broad, characterised by reticulate venation and by cordate bases, which he refers to Heer’s Swiss species Salvinia formosa[1217].

Dr Zeiller[1218] in the most recently published part of his series of valuable résumés of palaeobotanical literature refers to a description by Brabenec of specimens of this species from Bohemian Tertiary beds showing both microspores and megaspores.

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).

Sagenopteris Phillipsi (Brongniart)[1233]. Figs. 327, 328.

The fronds of this common Jurassic species, which is recorded from many European localities, from North America, Australia, the Antarctic regions[1234], and elsewhere, are very variable as regards the form, size, and number of the leaflets.

Frond petiolate, in some forms the petiole bears four linear or oval-lanceolate leaflets having a distinct midrib and oblique anastomosing veins. In others a shorter winged petiole bears one or two shorter and broader, somewhat obcuneate, leaflets without a midrib.

It is probable that Bunbury[1235] was correct in his opinion that the specimen figured by Lindley and Hutton[1236] as Otopteris cuneata, characterised by two leaflets (fig. 328), is not specifically distinct from the normal form with four leaflets (fig. 327).

Similarly, such specimens as that represented in Pl. XVIII., fig. 3 of the first part of my Jurassic Flora, in which a short stalk bears only one leaflet may, provisionally at least, be included in Brongniart’s species. Yabe[1237] describes a form with two leaflets from Jurassic rocks of Korea as Sagenopteris bilobata which resembles S. Phillipsi; and Moeller[1238] records a specimen similar to that represented in fig. 328 from Bornholm as S. cuneata (Lind. and Hutt.).

The leaf shown in fig. 327, A, in which the longest segments are 4·5 cm. in length, represents the most abundant form and illustrates the very close agreement between S. Phillipsi and the Rhaetic species S. rhoifolia. Fig. 327, B, which is drawn from a specimen figured by Lindley and Hutton[1239], shows a leaf with longer (6·5 cm.) and much narrower segments. Broader leaflets are occasionally met with in which the lamina reaches a length of 11 cm.[1240]

Leaves with leaflets narrower (3 mm. broad) than those represented in fig. 327, B, are described by Zigno[1241] from Jurassic beds of Italy as S. angustifolia and by Moeller[1242] from the Jurassic of Bornholm as S. Phillipsi f. pusilla. A coarser type of venation than that of S. Phillipsi is occasionally found in Jurassic examples, as in S. grandifolia Font.[1243] from Oregon and S. Nathorsti Barth. from Bornholm[1244].

Sagenopteris is recorded also from several Rhaetic floras. The best known species, S. rhoifolia Presl[1245], is hardly distinguishable from some forms of S. Phillipsi or from the Italian Jurassic species described by Zigno as S. Goeppertiana[1246], though the leaflets are usually rather larger. This species was first described by Brongniart as Filicites Nilssoniana[1247], and a few authors[1248] have adopted this specific name because of its priority over Presl’s designation. As Nathorst remarks, to give up the well-known name S. rhoifolia for S. Nilssoniana is “mere pedantry.” The epidermis of S. rhoifolia as figured by Schenk[1249] consists of cells with straight and not undulating walls: stomata occur on the lower surface (fig. 326, B).

Rhaetic leaves of the type represented by S. rhoifolia have a wide geographical distribution.

The specimens described by Feistmantel from the Damuda series of India as Sagenopteris longifolia are no doubt fronds of Glossopteris longifolia[1250].

The Wealden species Sagenopteris Mantelli (Dunk.)[1251] agrees closely in habit and in the form of the leaflets with S. Phillipsi and S. rhoifolia. It is probable that some of the leaves described by Velenovský[1252] from Lower Cretaceous rocks in Bohemia as Thinnfeldia variabilis are portions of Sagenopteris fronds. S. Mantelli is recorded from several European localities, from California[1253], and elsewhere.

Sagenopteris appears to have been widely distributed during the Rhaetic, Jurassic and Lower Cretaceous floras. The very great similarity between the specimens recorded from these three formations renders the genus an uncertain guide in regard to geological age. Decisive evidence as to its position in the plant kingdom is at present lacking: the inclusion of the genus as a possible member of the Hydropterideae has still to be justified.