Fossil plants, Vol. 2

Schizaeaceae.

Evidence bearing on the existence of this family in Carboniferous floras is by no means decisive. The generic name Aneimites proposed by Dawson[822] for some Devonian Canadian plants resembling species of the recent genus Aneimia, and adopted by White[823] for a species from the Pottsville beds of Virginia, is misleading. The Canadian plants give no indication of the nature of the reproductive organs, and the fronds described by White are, as he shows, those of a Pteridosperm and bore seeds.

An examination of the suspiciously diagrammatic drawings published by Corda[824] of the small fertile pinnules of a Carboniferous fern from Bohemia, which he named Senftenbergia elegans, leads us to conclude that the sporangia are almost certainly those of a Schizaeaceous species. The small linear pinnules bear two rows of sessile sporangia, singly as in recent Schizaeaceae and not in sori, characterised by 4–5 rows of regular annular cells (fig. 270, A) surrounding the apex. It has already been pointed out that the apical annulus of recent Schizaeaceae, though normally one row deep, may consist in part at least of two rows. Zeiller[825] examined specimens of Corda’s species and decided in favour of a Schizaeaceous affinity; he describes the sporangia as 0·85–0·95 mm. in length, with 3 to 5 and occasionally only two rows of cells in the apical annulus. Zeiller’s figures (fig. 270, A) confirm the impression that Corda’s drawings are more beautiful than accurate. Stur[826], on the other hand, who first pointed out that the type-specimens of Senftenbergia came from the Radnitz beds of Bohemia and not from the Coal-Measures, convinced himself that the sporangia have no true annulus (fig. 270, E). He describes them as characterised by a comparatively strong wall and by the presence of a band of narrow vertical cells marking the line of dehiscence, features which lead him to assign the plant to the Marattiales, a group which seems to have exercised a dominating influence over his judgment. In a later publication Zeiller[827] replies to Stur’s criticism but adheres to his original opinion. Solms-Laubach[828], while expressing himself in favour of Marattiaceous affinity, recognises that Zeiller’s arguments cannot be set aside.

The question must remain open until further evidence is forthcoming; but it would seem that this Carboniferous type, not as yet recognised in Britain, possessed sporangia having a distinct resemblance to those of the Schizaeaceae, though this similarity does not amount to proof of the existence of the family in the Palaeozoic era.

Palaeozoic floras may be described as rich in generalised types, types foreshadowing lines of evolution, which in the course of ages led to a sorting and a redistribution of characters. It may be that Senftenbergia is one of these generalised types.

It is not until we ascend the geological series as far as the older Jurassic rocks that we meet with a type which can with confidence be classed with the Schizaeaceae, as least so far as sporangial characters are concerned. The species Klukia exilis is selected as the best known and most widely-spread representative of Jurassic Schizaeaceae.

Klukia exilis (Phillips)[829]. Fig. 259.

The generic name Klukia was proposed by Raciborski[830] for a species originally described by Phillips[831] from the Inferior Oolite of the Yorkshire coast as Pecopteris exilis. Bunbury’s[832] discovery (supplemented by additional evidence obtained by Raciborski) of well-preserved sporangia justified the substitution of a distinctive designation for the provisional term Pecopteris.

The species may be defined as follows:—

Frond tripinnate, of the Cladophlebis type; pinnae linear, lanceolate, attached to the rachis at a wide angle. Ultimate segments short and linear, entire or, in the lower part of a frond, crenulate, 5 mm. long or occasionally longer. Sporangia 0·5 mm. in length, borne singly on the lower surface of the lamina in a row on each side of the midrib.

A re-examination[833] of the specimen described by Bunbury confirmed his account of the structure of the sporangia. The pinna shown in fig. 259 is characterised by unusually small fertile pinnules some of which bear 10 sporangia in two rows; the annulus includes about 14 cells. Fertile specimens of this and similar forms are figured by Raciborski[834] from Jurassic rocks of Poland, and good examples of the English species may be seen in the Leckenby collection, Cambridge, in the British Museum, the museums of Manchester, Scarborough, and other places.

It is possible that specimens referred to K. exilis by Yokoyama[835] from Wealden strata in Japan may afford evidence of the persistence of the species beyond the Jurassic era, but in view of the close resemblance of the sterile fronds described from Wealden strata as Cladophlebis Brownii[2] and C. Dunkeri[836] to those of Klukia exilis, identity can be established only by an examination of fertile specimens. A Jurassic fern recently described by Yabe[837] from Korea as Cladophlebis koraiensis may be identical with K. exilis and there is little doubt as to the existence of the species in Jurassic Caucasian strata[838].

Ruffordia Goepperti (Dunk.). Fig. 260.

This Wealden fern[839] has been doubtfully assigned to the Schizaeaceae on the ground of the resemblance of the sterile fronds to those of some species of Aneimia, and because of the difference between the sterile and fertile pinnae (Fig. 260). Ruffordia cannot be regarded as a well authenticated member of the Schizaeaceae.

Lygodium Kaulfussi, Heer. Fig. 261, B, B′.

Fragments of forked pinnules, agreeing very closely in venation and general appearance with recent species of Lygodium, have been identified by Gardner and Ettingshausen[840] from English Eocene beds and by Knowlton from the Miocene beds of the Yellowstone Park[841] as Lygodium Kaulfussi Heer (fig. 261, B). Despite the absence of sporangia it is probable that these fragments are correctly referred to the Schizaeaceae. The sterile and fertile specimens figured by Heer[842] from Tertiary beds of Switzerland agree very closely with recent examples of Lygodium. Similar though perhaps less convincing evidence of the existence of this family in Europe is furnished by Saporta[843], who described two Eocene species from France.

Gleicheniaceae.

The application by Goeppert[844] and other earlier writers of the generic name Gleichenites to examples of Palaeozoic ferns was not justified by any satisfactory evidence. One of Goeppert’s species, Gleichenites neuropteroides, is identical with Neuropteris heterophylla[845], a plant now included in the Pteridosperms.

The resemblance of sporangia and sori, whether preserved as carbonised impressions or as petrified material, from Carboniferous rocks, to those of recent species of Gleicheniaceae is in many cases at least the result of misinterpretation of deceptive appearances. Williamson[846] drew attention to the Gleichenia-like structure of some sections of sporangia from the English Coal-Measures, but he did not realise the ease with which sections of Marattiaceous sporangia in different planes may be mistaken for those of annulate (leptosporangiate) sporangia. In the regular dichotomous habit of Carboniferous fronds described as species of Diplothmema (Stur) and Mariopteris (Zeiller)[847] we have a close correspondence with the leaves of Gleichenia, but the common occurrence of dichotomous branching among ferns is sufficient reason for regarding this feature as an untrustworthy criterion of relationship. It is, however, interesting to find that in addition to the existence of some Upper Carboniferous ferns with sori like those of recent Gleichenias, the type of stelar anatomy illustrated by Gleichenia dicarpa (fig. 237, C, p. 310) and other species is characteristic of the primary structure of the stem of the Pteridosperm Heterangium. We find in Carboniferous types undoubted indications of anatomical and other features which in succeeding ages became the marks of Gleicheniaceae.

Some Carboniferous fronds with short and small pinnules of the Pecopteris type, bearing sori composed of a small number of sporangia, have been assigned by Grand’Eury and other authors to the Gleicheniaceae; the same form of sorus is met with also on fronds with Sphenopteroid segments. The former is illustrated by Oligocarpia Gutbieri[848] and the latter by O. Brongniarti described by Stur and by Zeiller[849]. Zeiller has described the circular sori of Oligocarpia (fig. 270, B) as consisting of three to ten pyriform sporangia borne at the ends of lateral veins and possessing a complete transverse annulus, but Stur[850] believes that the annulus-like appearance is due to the manner of preservation of exannulate sporangia. In this opinion Stur is supported by Solms-Laubach[851] and by Schenk[852]. Despite an agreement between Oligocarpia and Gleichenia, as regards the form of the sori and the number of sporangia, it is not certain that the existence of a typical Gleicheniaceous annulus has been proved to occur in any Palaeozoic sporangia[853].

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).

Gleichenites Zippei (Corda). Fig. 262, D.

This species, originally described by Corda as Pecopteris Zippei[864] and afterwards figured by Heer[865] as Gleichenia Zippei (fig. 262, D) from Urgonian rocks of Greenland, affords a striking example of a Mesozoic member of the Gleicheniaceae. It is characterised by the dichotomous branching of the frond and by the occurrence of arrested buds in the forks. The long and slender pinnae, reaching a length of 9 cm. and a breadth of 6–8 mm., bear small crowded pinnules occasionally with circular sori which are described by Heer as consisting of a small number of sporangia (cf. fig. 262, C). Several other Lower Cretaceous species are recorded by Heer from Greenland, some of which are probably unnecessarily separated from Gleichenites Zippei. Examples of these are represented in fig. 262, A, B, C.

A Gleicheniaceous species described by Debey and Ettingshausen from Lower Cretaceous rocks of Aix-la-Chapelle as Didymosorus comptonifolius[866] is very similar in habit to some of Heer’s Greenland species: this should probably be referred to the genus Gleichenites.

Gleichenites hantonensis, Wank. Fig. 263.

From the Eocene beds of Bournemouth, Gardner and Ettingshausen[867] have described under the name Gleichenia hantonensis what is in all probability a true Gleichenia (fig. 263). This species, originally recorded by Wanklyn[868], is characterised by a slender forked rachis showing what may be traces of arrested buds between the arms of the branches, by circular sori of six or eight sporangia and by the presence of peculiar tendril-like appendages on the pinnae. If the description of the tendrils is correct, this British species affords one of the few instances of ferns adapted for climbing and may be compared with the recent species Davallia aculeata (fig. 232, p. 299).

Matonineae.

The genera Laccopteris and Matonidium may be described as examples of Mesozoic ferns exhibiting a very close agreement with Matonia.

Laccopteris. This genus, founded by Presl[869], may be described as follows:—

Frond pedate, in habit resembling Matonia pectinata, with pinnate or pinnatifid pinnae; ultimate segments linear, provided with a well-marked midrib giving off numerous dichotomously branched secondary veins which are in places connected by lateral anastomoses. Sori circular, forming a single row on each side of the midrib (fig. 278, B); sporangia 5–15 in each sorus, with an oblique annulus and tetrahedral spores. The presence of an indusium is not certainly established.

Schenk[870], who described several specimens of Laccopteris from Rhaetic rocks of Germany, compared the genus with Gleichenia but he also recognised the close resemblance to Matonia pectinata. Zeiller[871] first established the practical identity of the sori and sporangia of Laccopteris and Matonia. The Rhaetic species, such as L. Muensteri, L. elegans, and L. Goepperti, agree very closely with L. polypodioides and need not be described in detail.

The Rhaetic species Laccopteris elegans, represented in fig. 264, illustrates the characteristic habit of the genus and shows a feature usually overlooked[872], namely the occurrence of anastomoses between the lateral veins. The form of the sorus of another Rhaetic species is shown in fig. 265, E. Schenk figures an interesting series of fronds of L. Goepperti in different stages of growth[873]; one of the younger leaves is seen in fig. 265, D. An examination of Rhaetic specimens of Laccopteris in the Bergakademie of Berlin convinced me of the correctness of the published descriptions of the sori.

Laccopteris polypodioides (Brongniart). Figs. 266–268; 278, A.

In habit this species closely resembles Matonia and Matonidium, the long petiole divides distally into several spreading pinnatifid pinnae with linear ultimate segments (fig. 278, A). Circular sori (indusiate?) occur in a single row on each side of the midrib containing 12–14 large sporangia (fig. 266) characterised by an obliquely vertical annulus. The midrib of the pinnules gives off secondary veins at a wide angle and these form a series of elongated meshes parallel to the median rib, as in the recent genus Woodwardia; forked and anastomosing branches are given off from these to the edge of the lamina (fig. 267).

The specimen shown in fig. 268 is probably a young frond of this species.

A very similar, possibly a specifically identical plant, was described by Leckenby from English Jurassic rocks as Phlebopteris Woodwardi[875], the distinguishing features of which are the greater number of lateral veins and the smaller sori (fig. 267, A).

The name Microdictyon was proposed by Saporta[876] for pinnules differing slightly from those of Laccopteris in venation characters: he included Laccopteris Woodwardi in this genus, but such differences as are recognisable in the venation hardly justify the use of a distinct generic title. Similarly, specimens described by Debey and Ettingshausen[877] from Lower Cretaceous rocks of Aix-la-Chapelle as species of Carolopteris may also be included in Laccopteris.

Laccopteris Dunkeri (Schenk)[878].

This species is represented in several Wealden localities by fragments of fertile pinnae similar to those of L. polypodioides. It is almost impossible to distinguish small specimens of the Wealden fern from Heer’s genus Nathorstia (Marattiaceae) unless the sori are well preserved. This species occurs in Wealden beds in England, Germany, Belgium, and elsewhere and has been discovered by Dr Marcus Gunn in Upper Jurassic plant-beds of Sutherlandshire (N.E. Scotland).

Laccopteris is widely spread in Rhaetic, Jurassic and Lower Cretaceous floras. It affords evidence of the former abundance in northern latitudes of a family now represented by the two species of Matonia confined to a restricted area in the southern hemisphere.

Matonidium.

Schenk[879] instituted this convenient term for fossil fern fronds agreeing in habit and in their sori with Matonia pectinata (figs. 227, 228, p. 292). Zeiller[880] has drawn attention to the fact that the Mesozoic species differ from the surviving types in the greater number of sporangia in each sorus, and, it may be added, in Matonidium the fertile pinnules are more richly supplied with sori than are those of Matonia. Unfortunately our knowledge of the structure of the sporangia of Matonidium is less complete than in the case of Laccopteris, but such evidence as is available justifies the conclusion that Matonia is a direct descendant of ferns which formed a prominent feature in European Jurassic and Wealden floras. It is interesting to find that in a Cretaceous species, described by Krasser (fig. 265, A) since the publication of Zeiller’s paper, the sori appear to be identical in distribution and in appearance with those of the recent species.

I am indebted to Prof. Bommer for permission to reproduce the unpublished drawing represented in fig. 237 D (p. 310) of a section of the rhizome of Matonidium from the Belgian Wealden beds of Hainaut (“Flore Bernissartienne”). The section shows an arrangement of vascular tissue identical with that in the recent species: there may be two solenosteles and in addition a solid axial strand. The form of the leaf-trace in the fossil appears to be identical with that in Matonia pectinata (fig. 237, A, p. 310).

Matonidium Goepperti (Ettingshausen)[881]. Fig. 269.

Under this name are included specimens from Inferior Oolite and Wealden strata in Britain and elsewhere. It is, however, not impossible that if more information were available, we should find adequate reasons for recognising two specific types. Fontaine[882], adhering rigidly to the rules of priority, speaks of this species as Matonidium Althausii (Dunker), but Ettingshausen’s specific term is better known.

Fronds pedate and apparently identical in habit with those of Matonia pectinata; ultimate segments linear, slightly falcate and bluntly pointed. Sori circular or oval, numerous, containing 15 to 20 sporangia with an oblique annulus, in two rows on the lower surface of the pinnules; indusium as in Matonia.

The English examples have so far afforded no information in regard to sporangial structure, but Schenk[883] has recognised a distinct annulus in German material. In his description of fossil plants from Lower Cretaceous rocks in California, Fontaine[884] doubtfully identifies two very small fragments as Matonidium Althausii; the evidence is, however, wholly inadequate.

Matonidium Wiesneri, Krasser[885]. Fig. 265, A.

This Cenomanian (Cretaceous) species from Moravia appears to be identical in habit with the older type. The pinnules are larger and bear fewer sori. Krasser’s figures of the sterile pinnules show no lateral anastomosing between the secondary veins, but the small vascular network below each sorus (fig. 265, A) is identical with that in Matonia pectinata. The indusiate sori contain about six sporangia with an oblique annulus.

The very wide geographical distribution of the Matonineae during the Mesozoic era affords a striking contrast to the limited range of the Malayan survivals.

Hymenophyllaceae.

The frequent use of the generic name Hymenophyllites as a designation of Palaeozoic ferns, more particularly in the older literature, is another instance of the undue importance which palaeobotanists have always been prone to attach to external resemblances of vegetative organs. The fragment of lamina described by Stur for the Culm Measures of Austria as Hymenophyllum waldenburgense[886] has no claim to consideration as evidence of Palaeozoic Hymenophyllaceae. On the other hand, there are a few records of fertile fronds which, though not to be accepted without reserve, are worthy of more careful examination. Some petrified sporangia described by Renault[887] from the Culm of Esnost are referred to Hymenophyllites on account of the position of the annulus, which appears to encircle about two-thirds of the circumference; it is, however, not certain that the annulus is horizontal as in the recent genus.

The Culm species Rhodea patentissima described by Ettingshausen[888] as Hymenophyllites patentissima and subsequently referred by Stur[889] to Rhodea, is regarded by these authors as closely allied to Hymenophyllum simply on the ground of the finely divided and delicate sterile fronds; another species, Rhodea moravica (Ett.), which Ettingshausen referred to Trichomanes, is compared with recent species of that genus. In neither case do we know anything of sporangial characters.

A fertile sphenopteroid frond figured by Schimper as Hymenophyllum Weissi[890] from the Coal-Measures of Saarbrücken bears some resemblance to recent Hymenophyllaceae, but the figures are by no means convincing: an examination of the type-specimens in the Strassburg Museum led Solms-Laubach[891] to express dissent from Schimper’s determination. A more satisfactory example is that afforded by the fertile pieces of a frond described by Zeiller[892] from French Coal-Measures as Hymenophyllites quadridactylites (Gutbier). Some of the ultimate segments with a truncated tip are preserved in close association with a group of oval sporangia with a complete transverse annulus (fig. 270, F, G). The position of the sporangia is such as to suggest their separation from a terminal columnar receptacle like that in Trichomanes and Hymenophyllum. In his account of this species from the Coal-Measures of the Forest of Wyre, Kidston[893] states that Zeiller informed him that he had noticed traces of what appeared to be a columnar receptacle in the French specimens.

The records of Hymenophyllaceae from the Mesozoic and Tertiary formations are not such as need detain us. The facts bearing on the geological history of this family are singularly meagre. There is no evidence which can be adduced in favour of regarding the Hymenophyllaceae as ferns of great antiquity, which played a prominent part in the floras of the past.

It is interesting to find that the genus Ankyropteris[894], one of the Botryopterideae (a group of Palaeozoic Ferns for which I propose the name Coenopterideae), has a morphological character in common with Trichomanes, namely the production of axillary buds: there are also features in the stelar anatomy shared by the Botryopterideae and Hymenophyllaceae[895]. These resemblances, though by no means amounting to proof of near relationship, point to a remote ancestry for certain features retained by existing members of the Hymenophyllaceae.

Cyatheaceae.

The specimens from the Culm rocks of Moravia on which Stur founded the species Thyrsopteris schistorum[896] are too imperfectly preserved to warrant the use of this generic name. Goeppert[897] in 1836 instituted the genera Cyatheites, Hemitelites, and Balantites for species of Carboniferous ferns believed to be closely allied to recent Cyatheaceae, but a fuller knowledge of these types has clearly demonstrated that in all cases the reference to this family had no justification.

The Upper Carboniferous species Dicksonites Pluckeneti, of which Sterzel[898] described fertile specimens in 1886 as possessing circular sori, has since been shown by Grand’Eury[899] to be a Pteridosperm bearing small seeds. In Sphenopteris (Discopteris) cristata (Brongn.) Zeiller[900] has described sori very like those of Cyathea and Alsophila, but differing in the exannulate sporangia: this species, like so many of the Palaeozoic ferns, is probably more akin to the Marattiaceae than to the Cyatheaceae.

We have as yet no satisfactory evidence of the existence of the Cyatheaceae in Palaeozoic floras. It is not until we reach the Jurassic period that trustworthy data are obtained. Raciborski[901] has identified as Cyatheaceous fertile Jurassic fronds from Poland, but his figures are inconclusive. In Alsophila polonica it is not clear whether the annulus is vertical or oblique, and in another supposed member of the family, Gonatosorus Nathorsti, in which the indusium is described as bivalvate, there is no proof of affinity to Cyatheaceae.

In attempting to decipher the past history of the Cyatheaceae it is important to remember the close resemblance between the fertile segments of some species of Davallia (Polypodiaceae) and those of Dicksonia (fig. 229, C, D, p. 294). Unless the sporangia are well enough preserved to show the position of the annulus, it is frequently impossible to feel much confidence in the value of the grosser features, such as the reduced lamina of the fertile segments and the form of the sori. It is, however, probable that the widely-spread Jurassic species Coniopteris hymenophylloides is correctly referred to the Cyatheaceae, but even in the case of this species the evidence of external form needs confirmation by an examination of individual sporangia.

Coniopteris.

This genus was instituted by Brongniart[902] for fossil fronds characterised by pinnules more or less intermediate between the Pecopteris and Sphenopteris type and agreeing in the form of the sori with the leaves of recent species of Dicksonia. It should be noted that Stur included in this genus a species, Coniopteris lunzensis[903] from the Upper Trias of Lunz, which he regarded as a Marattiaceous fern.

Coniopteris hymenophylloides, Brongn. Figs. 271, 272, 275, B.

The above list represents a small selection of the names applied to Jurassic ferns from different localities which there are good grounds for regarding as referable to a single type[904].

Frond tripinnate; pinnae linear acuminate, attached to the rachis at a wide angle; the pinnules vary considerably in size and shape; in some the lamina is divided into a few broad and rounded lobes (fig. 275, B) while in others the leaflets are dissected into narrow linear segments. The sori are borne at the ends of veins; the fertile pinnules have a much reduced lamina and, in extreme cases, bear a close resemblance to those of Thyrsopteris elegans (fig. 229, A, p. 294). The sori are partially enclosed in a cup-like indusium and the sporangia appear to have an oblique annulus.

Venation and habit of frond of the Sphenopteris type.