Fossil plants, Vol. 2

The pinna shown in fig. 271 is the type-specimen of Sphenopteris arguta Lind. and Hutt. from the Yorkshire Inferior Oolite and is indistinguishable from the English examples on which Brongniart founded his species S. hymenophylloides. Fig. 272 shows a specimen from the York Museum illustrating the difference between the sterile and fertile pinnae. The resemblance of some fertile pinnae of Coniopteris hymenophylloides to those of Thyrsopteris elegans has led to a frequent use, without any solid justification, of the generic name of the Juan Fernandez fern for Jurassic and Wealden plants. It is not impossible that some of the fossils described by Heer from Jurassic rocks of Siberia[905] as species of Thyrsopteris are Cyatheaceous ferns, but it is impossible to say with certainty that they are generically identical with the recent species. In his monograph of the Potomac flora of Virginia[906] and Maryland, Fontaine has described as species of Thyrsopteris several specimens of fronds which afford no evidence as to the nature of the sori or sporangia. Some of the fronds referred by this author to Thyrsopteris rarinervis[907], which I examined in the Washington Museum, are in all probability examples of Onychiopsis, a genus included in the Polypodiaceae. The fragments described by Lester Ward[908] as species of Thyrsopteris from the Lower Cretaceous of the Black Hills of North America afford no satisfactory evidence of relationship to the recent type. Similarly Velenovský has described a Lower Cretaceous Onychiopsis from Bohemia[909] as a species of Thyrsopteris, although the fertile segments bear little or no resemblance to those of the Cyatheaceous genus. Some fertile portions of fronds described by Heer[910] as Asplenium Johnstrupi and afterwards as Dicksonia Johnstrupi[911] from the Cretaceous beds (Kome series) of Greenland are very similar to Coniopteris hymenophylloides.

Coniopteris quinqueloba (Phillips). Fig. 273.

This species, originally described by Phillips[912] as Sphenopteris quinqueloba, is very similar in habit to C. hymenophylloides, differing chiefly in the smaller size of the leaf and in the narrower ultimate segments. The specimen shown in fig. 273, B, illustrates the form of the sorus and sporangia.

Coniopteris arguta (Lind. and Hutt.[913]). Figs. 274, 275, A.

The sterile pinnae of this species bear pinnules of a type met with in various species of ferns from different horizons; the smaller ones are entire and slightly falcate, while on the lower part of a frond the ultimate segments are longer and have a crenulate margin. The fertile pinnae bear pinnules reduced to a midrib with a narrow border, and terminating in a cup-like indusium (fig. 275, A). In habit the sterile leaf (fig. 274) of this species is similar to the Jurassic Schizaeaceous fern Klukia exilis.

Protopteris.

Presl[914] instituted this genus for a Lower Cretaceous tree-fern from Bohemia originally figured as Lepidodendron punctatum[915] and assigned to a Palaeozoic horizon; it was afterwards named by Corda[916] Protopteris Sternbergii and referred by Brongniart[917] to Sigillaria. The genus Protopteris stands for fossil fern-stems with the habit and, in the main, the structural features of recent tree-ferns. Persistent leaf-bases and sinuous adventitious roots cover the surface of the stems: the vascular system is of the dictyostelic type characteristic of Cyathea (fig. 240, p. 313) and Alsophila. It is by the pattern formed by the vascular tissue on the exposed surface of the leaf-bases that Protopteris is most readily recognised: the leaf-trace has a horse-shoe form with the ends curled inwards and the sides more or less indented (fig. 277). The generic name Caulopteris is used by some authors in preference to Presl’s genus; but Protopteris is more conveniently restricted to Mesozoic Cyatheaceous stems and Caulopteris to Palaeozoic stems, with the internal structure of Psaronius (see Chap. XXIII.). Stenzel applies Caulopteris to Mesozoic stems in which the leaf-trace consists of several separate strands and not of a continuous band.

Lower Cretaceous casts of tree-fern stems in the Prague Museum have been described under the names Alsophilina and Oncopteris; the figures of the latter (fig. 276) given by Feistmantel[918] and by Velenovský[919] show the petiole-bases arranged in vertical rows and characterised by leaf-traces consisting of two separate strands in the form of two Vs lying on their sides.

Tree-fern stems described under various generic names are not infrequently found in European Lower Cretaceous rocks: their comparative abundance affords an example of striking changes in geographical distribution since the latter part of the Mesozoic epoch. The Cyatheaceae no longer exist in Europe and the arborescent species of the genus have retreated to more southern regions.

Protopteris punctata (Sternb.). Fig. 277.

The earliest information in regard to the anatomy of this widely spread Lower Cretaceous fern we owe to Corda, who showed that the species agrees in essentials with existing tree-ferns. The English example described by Carruthers[920] from Upper Greensand beds in Dorsetshire (now in the British Museum) shows only the external features. The sandstone cast (14 cm. in diameter), of which a portion is seen in fig. 277, was described by Heer from Disco Island (Greenland) as a Carboniferous species[921], but afterwards correctly assigned to the Cenomanian series[922] This species is recorded also from the Lower Cretaceous of Bohemia by Frič and Bayer[923] Among examples of petrified stems exhibiting a general agreement with Protopteris punctata are those described by Stenzel[924] from Turonian rocks in Germany. In one of these, Rhizodendron oppoliense Göpp., attention is drawn to branches given off from the stem stele which have a solenostelic structure in contrast to the dictyostele of the stem; also to the minute structure of the tracheae which appear to have their ends perforated, a feature shown by Gwynne-Vaughan[925] to be characteristic of the xylem elements of many ferns.

Protopteris Witteana Schenk[926] (fig. 278, G, H), a Wealden species recorded from Germany and England, represents a closely allied or possibly an identical type. The section of the stem (fig. H) shows the narrow vascular bands, x, of a dictyostele similar to that of recent Cyatheaceous tree-ferns and a form of meristele (fig. G, x) resembling that of P. punctata. Adventitious roots are seen in section at R (figs. G and H).

Polypodiaceae.

Sections of petrified sporangia from the English Coal-Measures (Pteridotheca sp.) occasionally exhibit a striking resemblance to those of recent Polypodiaceae[927], but in the absence of material in which it is possible to recognise the true orientation of the sporangia, the exact position of the annulus is almost impossible to determine. We have as yet no satisfactory evidence of the existence of true Polypodiaceae in the Palaeozoic era. It is noteworthy that apart from the absence of ferns which can reasonably be included in this family, the anatomical features of the Botryopterideae (Coenopterideae) and of the Cycadofilices or Pteridosperms do not foreshadow those of Polypodiaceous ferns. On the other hand, as we have already noticed, anatomical characters of such families as the Gleicheniaceae, Hymenophyllaceae, and Schizaeaceae are met with in certain generalised Palaeozoic types. These facts are perhaps of some importance as supplying collateral evidence in favour of the relatively more recent origin of the dominant family of ferns in modern floras.

The use of the generic name Adiantites for fern-like fronds of Lower Carboniferous age characterised by cuneate pinnules like those of species of Adiantum, suggests an affinity which is in all probability non-existent. It has been pointed out that this generic name was applied in the first instance to the leaves of the Jurassic plant Ginkgo digitata[928] and should, therefore, be discarded. Schimper[929] used the designation Adiantides, and Ettingshausen[930], more rashly than wisely, preferred Adiantum. The specimens described by Kidston[931] as Adiantides antiquus (Ett.) (fig. 279, A) from the Carboniferous limestone of Flintshire are portions of tripinnate fronds bearing cuneate segments with numerous forked veins radiating from the contracted base of the lamina. It is not improbable, in view of Dr White’s[932] discovery of seeds on a very similar plant from the Pottsville beds of North America, that this characteristic Lower Carboniferous genus is a Pteridosperm.

From Jurassic rocks in various parts of the world numerous fossils have been described under the generic names Aspidium, Asplenium, Davallia, Polypodium, and Pteris. In the great majority of cases such records leave much to be desired from the point of view of students who appreciate the dangers of relying on external similarity between vegetative organs, and on resemblances founded on obscure impressions of sori. The generic term Woodwardites[933], which suggests affinity with the recent genus Woodwardia, has been used for Rhaetic plants belonging to the Dipteridinae.

A plant described as Adiantides Lindsayoides from Jurassic rocks of Victoria[934], characterised by marginal sori which appear to be protected by the folded-over edge of the leaflets, and by the resemblance of the pinnules to those of recent species of Lindsaya, may be a true Polypodiaceous fern; but in this case, as in many similar instances, nothing is known of the structure of the sporangia. Some sterile pinnae described by Yabe from Jurassic rocks of Korea as Adiantites Sewardi[935] may perhaps be identical with the Australian species.

In such a species as Polypodium oregonense Font., from Jurassic rocks of Oregon, the generic name is chosen because the “fructification seems near enough to that of Polypodium to justify the placing of the plant in that genus[936].” But the fact that no sporangia have been found is a fatal objection to this identification.

Onychiopsis.

This generic name was instituted by Yokoyama[937] for a Japanese Wealden species, previously described by Geyler[938] as Thyrsopteris elongata, on the ground that, in addition to a similarity in habit of the sterile fronds, the fertile pinnae present a close agreement to those of the recent genus Onychium.

Onychiopsis Mantelli[939] (Brongn.). Figs. 278, D; 280, A and B.

The Japanese species Onychiopsis elongata may perhaps be identical with this common Wealden fern which, as Fontaine points out, should be called O. psilotoides if the rule of priority is to be observed irrespective of long usage.

Onychiopsis Mantelli may be defined as follows:—

Frond bipinnate, ovate lanceolate, rachis winged; pinnae approximate, given off at an acute angle; pinnules narrow, acuminate, with a single vein; the larger segments serrate and gradually passing into pinnae with narrow ultimate segments. Fertile segments sessile or shortly stalked, linear ovate, sometimes terminating in a short awn-like prolongation.

The fertile segments (fig. 278, D) bear so close a resemblance to those of species of Onychium that it would seem justifiable to regard the plant as a member of the Polypodiaceae. This fern is one of the most characteristic members of the Wealden floras; it occurs in abundance in the English Wealden, in Portugal, Germany, Belgium, Japan, Bohemia, South Africa, and elsewhere. A piece of rhizome figured from the English Wealden[940] is very similar to the creeping rhizomes of recent species of Polypodiaceae. The English Wealden specimens shown in fig. 280, A and B, illustrate the difference in form presented by leaves of this species; the smaller pinnae reproduced in fig. A are more characteristic of the species than are those of the slightly enlarged example represented in fig. 280, B.

Among British Tertiary species referred to Polypodiaceae, it is interesting to find what may well be an authentic record of a fern closely allied to the recent tropical species Acrostichum (Chrysodium) aureum. This Eocene species from Bournemouth is described as Chrysodium lanzaeanum[941]. The frond is simply pinnate and apparently coriaceous in texture, with lanceolate or oblong lanceolate pinnules (fig. 261, A, A′, p. 350), differing from those of Acrostichum aureum in being sessile. A prominent midrib gives off numerous anastomosing veins. No fertile pinnules have been found.

Specimens described by Forbes from the Eocene beds of the Island of Mull as Onoclea hebraidica[942] bear a strong likeness to the North American and Japanese recent species Onoclea sensibilis. Fertile specimens referred to the latter species are recorded by Knowlton[943] from Tertiary beds of Montana.

A species described by Saporta[944] from the Eocene of Sézanne as Adiantum apalophyllum is recorded by Gardner and Ettingshausen from Bournemouth; an identification which is based on somewhat meagre evidence.

The following remarks by Gardner and Ettingshausen are worthy of repetition as calling attention to circumstances often overlooked in analyses of fossil floras. They speak of ferns as relatively rare in British Eocene rocks and add,—“the floras consist principally of deciduous dicotyledonous leaves, which ... fell into the water and were tranquilly silted over. Ferns, on the other hand, would require some violence to remove them from the place of their growth, and their preservation would consequently be exceptional, and they would be mutilated and fragmentary. This may account for their rarity. Few as the British ferns are in the number of species, they nevertheless form the largest and most important series of Eocene ferns, even of Tertiary ferns, yet described from one group of beds[945].”

Dipteridinae.

Dictyophyllum.

This genus was founded by Lindley and Hutton for a pinnatifid leaf from the Jurassic rocks of Yorkshire which they regarded as probably dicotyledonous and named D. rugosum[946]. Several ferns of this genus have since been found with well-preserved sori which demonstrate a close similarity to the recent fern Dipteris. Dictyophyllum may be defined as follows:—

Fronds large and palmate, characterised by the equal dichotomy of the main rachis into two arms which curve outwards and then bend inwards (fig. 281); from the surface of each arm are given off numerous spreading pinnae with a lamina more or less deeply dissected into lobes varying in breadth and in the form of the apex. Each lobe has a median vein, from which branches are given off approximately at right angles and then subdivide into a reticulum, in the meshes of which the veinlets end blindly (fig. 282, A and E). Sori composed of annulate sporangia are crowded on the lower surface of the lamina. In habit and in sporangial characters the genus closely resembles Dipteris, and in the branching of the frond suggests comparison with Matonia. The rhizome (Rhizomopteris) is creeping and dichotomously branched, bearing leaf-scars with a horse-shoe form of vascular strand.

Dictyophyllum is represented by several types to which various specific names have been assigned, the distinguishing features being the form of the pinna lobes, the degree of concrescence between the basal portions of the pinnae, and similar features which in some cases can only be safely used as criteria when large specimens are available for comparison.

Dictyophyllum exile (Brauns). Figs. 281, 282, D, E.

The restoration, after Nathorst[947], shown in fig. 281 illustrates the habit of this striking fern, examples of which or of closely allied species are recorded from Rhaetic rocks of Germany, Scania, Persia, Bornholm, Tonkin, China, and elsewhere[948]. The petiole, reaching a length of 60 cm., forks at the apex into two equal arms leaving between them an oval space and occasionally crossing one another. The axes of these branches are twisted so that the pinnae, which may be as many as 24 on each arm, and arise from the inner side, by torsion of the axes assume an external position. An interesting analogy as regards the twisted rachis of Dictyophyllum exile and Camptopteris is afforded by the leaves of the Cycads, Macrozamia Fawcettiae and M. corallipes, which are also characterised by the torsion of the rachis. The habit, justly compared by Nathorst with that of Matonia pectinata, affords another illustration of the common occurrence in older ferns of a dichotomous system of branching. The pinnae, characterised by circinate vernation, reach a length of 60 cm. and are divided into linear lobes inclined obliquely or at right angles to the pinna axis. The whole of the under surface of the lamina may be covered with sporangia, 4–7 sporangia in each sorus; the annulus is incomplete and approximately vertical (fig. 282, D). The rhizome is probably represented by the dichotomously branched axis described by Nathorst from Scania as Rhizomopteris major; the leaf-scars show a horse-shoe leaf-trace.

Dictyophyllum Nathorsti Zeiller[949].

This type, represented by a splendid series of specimens from the Rhaetic beds of Tonkin, agrees very closely with D. exile. It differs, however, in the basal parts of the pinnae which are concrescent for a length of 5 to 8 cm. instead of free as in D. exile; and, to a slight degree, in the form of the ultimate segments. In habit and in soral characters the two species are practically identical. Each sorus contains 5 to 8 sporangia, which are rather larger than those of Dipteris.

Dictyophyllum rugosum, Lind. and Hutt. Fig. 283.

This species, which is characteristic of Jurassic rocks, is less completely known than the two types described above, but in the form and venation of the pinnae there is little difference between the Rhaetic and Jurassic plants. The leaves of the Jurassic species appear to have been smaller and more like those of Dipteris conjugata (fig. 231); there are no indications of the existence of the two curved arms at the summit of the petiole which form so striking a feature in D. exile and D. Nathorsti. No sporangia have been found on English specimens, but it is safe to assume their agreement with those of other species. A more complete list of records of D. rugosum is given in the first volume of the British Museum Catalogue of Jurassic plants[950].

Nathorst[951] has recently drawn attention to certain differences between Dictyophyllum and Dipteris. The pinnate division of the pinnae is not represented in the fronds of the recent species, but this method of lobing, which is a marked characteristic of Dictyophyllum, is less prominent in Clathropteris; and in Camptopteris lunzensis Stur[952], an Austrian Upper Triassic species, the pinnae are entire. In Dictyophyllum the sori cover the whole lower surface of the leaf; in Dipteris they are more widely separated and the sporangia have a diameter of 0·02 mm., but in Dictyophyllum the diameter is 0·4–0·6 mm. Moreover in Dictyophyllum the sori contain 5 to 8 sporangia, whereas in Dipteris they are much more numerous. Despite these differences it is clear, as Nathorst says, that Dictyophyllum, Clathropteris, and Camptopteris are existing types very closely allied to Dipteris. It is a matter of secondary importance whether we include all in the Dipteridinae or follow Nathorst’s suggestion and refer the fossil genera to the separate family Camptopteridinae.

Thaumatopteris.

This genus, founded by Goeppert[953] for a Rhaetic plant from Bayreuth, is by some authors[954] regarded as identical with Dictyophyllum, but it has recently been resuscitated by Nathorst[955] for specimens which he names T. Schenki, formerly included by Schenk in his species T. Brauniana[956]. It bears a close resemblance, in the long linear pinnules with an entire or crenulate margin, to Dictyophyllum Fuchsi described by Zeiller[957] from Tonkin, and it would seem hardly necessary to adopt a distinctive generic designation. The sporangia have a vertical or slightly oblique annulus and the rhizome is similar to that of Dictyophyllum exile. The habit of the genus is shown in fig. 284, which represents one of the German Rhaetic species.

Clathropteris.

Clathropteris meniscoides, Brongn. Fig. 285.

Clathropteris, founded by Brongniart[958] for Rhaetic specimens from Scania, agrees very closely with some species of Dictyophyllum, but in view of the more rectangular form of the venation-meshes it is convenient to retain both names. The type-species was originally named Filicites meniscoides[959] and afterwards transferred to Clathropteris. An examination of Brongniart’s specimens has convinced Nathorst of the specific identity of C. meniscoides and C. platyphylla. The Tonkin leaves described by Zeiller[960] under the latter name should, therefore, be included in C. meniscoides, which may be thus defined:

The petiolate frond is characterised by an equal dichotomy of the rachis, as in Dictyophyllum; each branch bore 5–15 pinnae, disposed en éventail, reaching a length of 20–30 cm. and fused basally as in D. Nathorsti Zeill. Pinnae linear lanceolate, slightly contracted at the lower end and gradually tapered distally. The lamina, 3–14 cm. broad, is characterised by obtusely pointed marginal lobes. From the midrib of each pinna lateral veins are given off at a wide angle, and adjacent veins are connected by a series of branches which divide the lamina into a regular reticulum of rectangular and polygonal meshes (fig. 285). The sori are abundant and contain 5–12 sporangia like those of Dictyophyllum.

What is probably the rhizome of this species has been described by Nathorst (Rhizomopteris cruciata); it is similar to that of Dictyophyllum, but the leaf-scars are more widely separated. This species occurs in Upper Triassic, Rhaetic or Lower Jurassic rocks of Scania, France, Germany, Switzerland, Bornholm, North America, China, Tonkin, and Persia and is represented by fragments in the Rhaetic beds of Bristol[961].

Clathropteris egyptiaca Sew.[962] Fig. 286.

The specimen on which this species was founded was discovered in the Nubian Sandstone east of Edfu; the age of the beds is uncertain, but the presence of Clathropteris suggests a Lower Jurassic or Rhaetic horizon[963]. Seven strong ribs radiate through the lamina from the summit of the petiole; at a and b small pieces of the projecting ribs are shown in the grooves. From the main veins slender branches are given off at right angles and, as seen in the enlarged drawing, these again subdivide into a delicate reticulum with free-ending veinlets.

Camptopteris.

Camptopteris spiralis, Nath. Figs. 282, C; 287.

Nathorst proposed this generic name for Rhaetic fronds[964] resembling those of Clathropteris and Dictyophyllum, but differing in the form of the pinnae and in habit. The habit of the type-species, C. spiralis, is shown in fig. 287. An examination of the specimens in the Stockholm Museum convinced me of the correctness of Nathorst’s restoration[965]. Each of the forked arms of the rachis bore as many as 150–160 long and narrow pinnae characterised by an anastomosing venation (fig. 282, C) and by a spiral disposition due to the torsion of the axes. The sporangia agree in essentials with those of Dictyophyllum.

Hausmannia.

A critical and exhaustive account of this genus has been given by Prof. Von Richter[966] based on an examination of specimens found in the Lower Cretaceous rocks of Quedlinburg in Germany. The name was proposed by Dunker[967] for leaves from the Wealden of Germany characterised by a deeply dissected dichotomously branched lamina. Andrae subsequently instituted the genus Protorhipis[968] for suborbicular leaves with dichotomously branched ribs from the Lias of Steierdorf. A similar but smaller type of leaf was afterwards described by Zigno[969] from Jurassic beds of Italy as P. asarifolius, and Nathorst[970] figured a closely allied form from Rhaetic rocks of Sweden. While some authors regarded Hausmannia and Protorhipis as ferns, others compared them with the leaves of Baiera (Ginkgoales); Saporta suggested a dicotyledonous affinity for leaves of the Protorhipis type. The true nature of the fossils was recognised by Zeiller[971], who called attention to the very close resemblance in habit and in soral characters to the recent genus Dipteris. A comparison of the different species of Dipteris, including young leaves (fig. 231, p. 297), with those of the fossil species reveals a very striking agreement[972]. There can be no doubt, as Richter points out, that the names Hausmannia and Protorhipis stand for one generic type.

Hausmannia may be defined as follows:

Rhizome creeping, slender, dichotomously branched; leaf-stalks slender (2–25 cm. long), bearing a leathery lamina (1–12 cm. long and broad), wedge-shaped below, occasionally cordate or reniform, entire or more or less deeply lobed into broad linear segments. The leaf is characterised by dichotomously branched main ribs which arise from the summit of the rachis as two divergent arms and radiate in a palmate manner, with repeated forking, through the lamina. Lateral veins are given off at a wide angle, and, by subdivision, form a fairly regular network similar to that in Dictyophyllum, Clathropteris, and Dipteris.