rocks of Cracow, Marchantites erectus from the Inferior Oolite rocks
of Yorkshire, and M. Zeilleri from the Wealden beds of Sussex.
These fossil Hepaticae are unfortunately founded only on sterile
fragments, and placed in the Liverworts on the strength of their
resemblance to the thallus of Marchantia and other recent genera.
The Palaeozoic Calamites were succeeded in the Triass'ic period by large Equisetites, differing, so far as we know, in no essential respect from existing Equisetums. The large stems represented by casts of Triassic age, Equisetites arenaceus other species, probably possessed the power of secondary growth in thickness; the cones were of the modern type, Equisetaceae. and the rhizomes occasionally formed large underground tubers like those frequently met with in Equisetum arvense, E. sylvaticum and other species. Equisetites Muensteri is a characteristic and fairly widely spread Rhaetic and Liassic species, having a comparatively slender stem, with leaf-sheaths consisting of a few broad and short leaf-segments. Equisetites columnaris, a common fossil in the Jurassic plant-beds of the Yorkshire coast, represents another type with relatively stout and occasionally branched vegetative shoots, bearing leaf-sheaths very like those of Equisetum maximum and other Horsetails. In the Wealden strata more slender forms have been found—e.g. Equisetites Burchardti and E. Lyelli—in England, Germany, Portugal, Japan and elsewhere, differing still less in dimensions from modern species. Of other Equisetales there are Schizoneura and Phyllotheca; the former first appears in Lower Gondwana rocks as a member of the Glossopteris flora, migrating at a later epoch into Europe, where it is represented by a Triassic species. The latter genus ranges from Upper Carboniferous to Jurassic rocks; it occurs in India, Australia, and elsewhere in the “Gondwana Land” vegetation, as well as in Palaeozoic rocks of Asia Minor, in Permian rocks of Siberia, and in Jurassic plant-beds of Italy. This genus, like the allied Calamites, appears to have possessed cones of more than one type; but we know little of the structure of these Mesozoic Equisetaceous genera as compared with our much more complete knowledge of Calamites and Archaeocalamites. (See section I., Palaeozoic.)
Reference has already been made to Sigillaria oculina and to the genus Pleuromeia. Palaeobotanical literature contains several records of species of Lycopodites and Selaginellites; nearly all of them are sterile fragments, bearing a more or less close resemblance to living Club-Mosses and Selaginellas, but lacking the more important reproductive organs. Lycopodiates. Nathorst has recently described a new type of lycopodiaceous cone, Lycostrobus Scotti, from Rhaetic rocks of Scania, from which he obtained both megaspores and microspores. An investigation by Miss Sollas of a plant long known from Rhaetic rocks in the Severn valley as Naiadita acuminata has shown that this genus is in all probability a small lycopodiaceous plant, and neither a Moss nor a Monocotyledon, as some writers have supposed. One of the best-known European species is Lycopodites falcatus, originally described by Lindley and Hutton from the Inferior Oolite of Yorkshire.
|
Fig. 5. |
|
A, Otozamites Beani. B, O. Bunburyanus. Inferior Oolite, England. |
Among the large number of Mesozoic Ferns there are several species founded on sterile fronds which possess but little interest from a botanical standpoint. Some plants, again, have been referred by certain authors to Ferns, while others have relegated them to the Cycads. As examples of these doubtful forms may be mentioned Thinnfeldia, characteristic of Rhaetic and Lower Filicales. Jurassic rocks; Dichopteris, represented by some exceptionally fine Jurassic specimens, described by Zigno, from Italy; and Ctenis, a genus chiefly from Jurassic beds, founded on pinnate fronds like those of Zamia and other Cycads, with linear pinnae characterized by anastomosing veins. Plants referred to Schimper’s genus Lomatopteris and to Cycadopteris of Zigno afford instances of the difficulty of distinguishing between the foliage of Ferns and Cycads. The close resemblance between specimens from Jurassic rocks placed in one or other of the genera Thinnfeldia, Dichopteris, Cycadopteris, &c., illustrates the unsatisfactory custom of founding new names on imperfect fronds. It is of interest to note that some leaf-fragments recently found in Permian rocks of Kansas, and placed in a new genus Glenopteris, are hardly distinguishable from specimens of Jurassic and Rhaetic age referred to Thinnfeldia and other Mesozoic genera. The difficulty of distinguishing between Ferns and Cycads is a necessary consequence of the common origin of these two classes; in Palaeozoic times the Cycadofilicies and Pteridospermae (see section I., Palaeozoic) played a prominent part, and even among recent Cycads and Ferns we still see a few indications of their close relationship. There is reason to believe that compound or generalized types—partly Ferns and partly Cycads—persisted into the Mesozoic era; but without more anatomical knowledge than we at present possess, it is impossible to do more than to point to a few indications afforded by external, and to a slight extent by internal structure, of the survival of Cycadofilicinean types. The genus Otozamites, which it is customary and probably correct to include in the Cycadales, is represented by certain species, such as Otozamites Beani (fig. 5, A), a characteristic Yorkshire fossil of Jurassic age, which in the form of the frond, bearing broad and relatively short pinnae, exhibits a striking agreement with the sterile portions of the fronds of Aneimia rotundifolia, a member of the fern family Schizaeaceae. Again, another species of the same genus, O. Bunburyanus (fig. 5, B), suggests a comparison with fern fronds like that of the recent species Nephrolepis Duffi. The scaly ramenta which occur in abundance on the leaf-stalk bases of fossil Cycads constitute another fern-character surviving in Mesozoic Cycadales. Without a fuller knowledge of internal structure and of the reproductive organs, we are compelled to speak of some of the Mesozoic plants as possibly Ferns or possibly Cycads, and not referable with certainty to one or other class. It has been found useful in some cases to examine microscopically the thin film of coal that often covers the pinnae of fossil fronds, in order to determine the form of the epidermal cells which may be preserved in the carbonized cuticle; rectilinear epidermal cell-walls are usually considered characteristic of Cycads, while cells with undulating walls are more likely to belong to Ferns. This distinction does not, however, afford a safe guide; the epidermal cells of some ferns, e.g. Angiopteris, have straight walls, and occasionally the surface cells of a Cycadean leaf-segment exhibit a fern-like character. Leaving out of account the numerous sterile fronds which cannot be certainly referred to particular families of Ferns, there are several genera which bear evidence in their sori, and to some extent in the form of the leaf, of their relationship to existing types.
The abundance of Palaeozoic plants with sporangia and sori of the Marattiaceous type is in striking contrast to the scarcity of Mesozoic ferns which can be reasonably included in the Marattiaceae. One of the few forms so far recorded is that known as Marattia Muensteri from Rhaetic localities in Europe and Asia. Some species included in the genus Marattiaceae. Danaeites or Danaeopsis from Jurassic rocks of Poland, Austria and Switzerland may possibly be closely allied to the recent tropical genus Danaea. Of the Ophioglossaceae there are no satisfactory examples; one of the few fossils compared with a recent species, Ophioglossum palmatum, was described several years ago from Triassic rocks under the name Cheiropteris, but the resemblance is one of external form only, and practically valueless as a taxonomic criterion. It would appear that the eusporangiate Ferns suddenly sank to very subordinate position after the Palaeozoic era.
|
|
Fig. 6.—Cladophlebis denticulata. Inferior Oolite, England. |
The Osmundaceae, represented by a few forms of Palaeozoic age, played a more prominent part in the Mesozoic floras. A species described by Schenk from Rhaetic rocks of Franconia as Acrostichites princeps is hardly distinguishable from Todites Williamsoni, a widely distributed species in Inferior Oolite strata. This Jurassic species bore bipinnate fronds Osmundaceae. not unlike those of the South African, Australian, and New Zealand Fern Todea barbara, which were characterized by a stout rachis and short broad pinnules bearing numerous large sporangia covering the under surface of the lamina. Specimens of Todites have been obtained from England, Poland, and elsewhere, sufficiently well preserved to afford good evidence of a correspondence in the structure of their sporangia with those of recent Osmundaceae. This Jurassic and Rhaetic type occurs in England, Germany, Poland, Italy, East Greenland, North America, Japan, China and Persia (Map A, X.). Bipinnate sterile fronds of Todites have in some instances been described under the designation Pecopteris whitbiensis. This and other names, such as Asplenium whitbiense, A. nebbense, Asplenites Roesserti, &c., have been given to bipinnate fronds of a type frequently met with in different genera and families of recent Ferns, e.g. Onoclea Struthiopteris, species of Cyathea, Asplenium, Gymnogramme, &c. In most cases the Rhaetic, Jurassic and Wealden Ferns included under one or other of these names are sterile, and cannot be assigned to a particular family, but some are undoubtedly
