Succession of Floras.
Our knowledge of vegetation older than the Carboniferous is still far too scanty for any satisfactory history of the Palaeozoic Floras to be even attempted; a few, however, of the facts may be advantageously recapitulated in chronological order.
No recognizable plant-remains, if we accept one or two doubtful Algal specimens, have so far been yielded by the Cambrian. From the Ordovician and Silurian, however, a certain number of authentic remains of Algae (among many more that are questionable) have been investigated; they are for the most part either verticillate Siphonae, or the large—possibly Laminariaceous—Algae named Nematophycus, with the problematical but perhaps allied Pachytheca. The evidence for terrestrial Silurian vegetation is still dubious; apart from some obscure North American specimens, the true nature of which is not established, Potonié has described well-characterized Pteridophytes (such as the fern-like Sphenopteridium and Bothrodendron among Lycopods) from supposed Silurian strata in North Germany; the horizon, however, appears to be open to much doubt, and the specimens agree so nearly with some from the Lower Carboniferous as to render their Silurian age difficult of credence. The high development of the terrestrial flora in Devonian times renders it probable that land-plants existed far back in the Silurian ages, or still earlier. Even in the Lower Devonian, Ferns and Lepidodendreae have been recognized; the Middle and Upper Devonian beds contain a flora in which all the chief groups of Carboniferous plants are already represented. Considering the comparative meagreness of the Devonian record, we can scarcely doubt that the vegetation of that period, if adequately known, would prove to have been practically as rich as that of the succeeding age. Among Devonian plants, Equisetales, including not only Archaeocalamites, but forms referred to Asterophyllites and Annularia, occur; Sphenophyllum is known from Devonian strata in North America and Bear Island, and Pseudobornia from the latter; Lycopods are represented by Bothrodendron and Lepidodendron; a typical Lepidostrobus, with structure preserved, has lately been found in the Upper Devonian of Kentucky. Fern-like plants such as Sphenopterideae, Archaeopteris and Aneimites, with occasional arborescent Pecopterideae, are frequent; many of the genera, including Alethopteris, Neuropteris and Megalopteris, probably belonged, not to true Ferns, but to Pteridosperms; although our knowledge of internal structure is still comparatively scanty, there is evidence to prove that such plants were already present, as for example, the genus Calamopitys. The presence of Cordaitean leaves indicates that Gymnosperms of high organization already existed, a striking fact, showing the immense antiquity of this class compared with the angiospermous flowering plants.
Any detailed account of the horizons of Carboniferous plants would carry us much too far. For our present purpose we may divide the formation into Lower Carboniferous and Lower and Upper Coal Measures. In the Lower Carboniferous (Culm of Continental authors) many Devonian types survive—e.g. Archaeocalamites, Bothrodendron, Archaeopteris, Megalopteris, &c. Among fern-like fronds Diplotmema and Rhacopteris are characteristic. Some of the Lepidodendreae appear to approach Sigillariae in external characters. Sphenophylleae are still rare; it is to this horizon that the isolated type Cheirostrobus belongs. Many specimens with structure preserved are known from the Lower Carboniferous, and among them Pteridosperms (Heterangium, Calamopitys, Cladoxylon, Protopitys) are well represented, if we may judge by the anatomical characters. Of Gymnosperms we have Cordaitean leaves, and the stems known as Pitys, which probably belonged to the same family.
The Lower Coal Measures (Westphalian) have an enormously rich flora, embracing most of the types referred to in our systematic description. Calamarieae with the Arthropitys type of stem-structure abound, and Sphenophylleae are now well represented. Bothrodendron still survives, but Lepidodendron, Lepidophloios, and the ribbed Sigillariae are the characteristic Lycopods. The heterogeneous “Ferns” grouped under Sphenopterideae are especially abundant. Ferns of the genera referred to Marattiaceae are common, but arborescent stems of the Psaronius type are still comparatively rare. Numerous fronds such as Alethopteris Neuropteris, Mariopteris, &c., belonged to Pteridosperms, of which specimens showing structure are frequent in certain beds. Cordaites, Dorycordaites and many stems of the Mesoxylon type represent Gymnosperms; the seeds of Pteridosperms and Cordaiteae begin to be common. The Upper Coal Measures (Stephanian) are characterized among the Calamarieae, now more than ever abundant, by the prevalence of the Calamodendreae; new species of Sphenophyllum make their appearance; among the Lycopods, Lepidodendron and its immediate allies diminish, and smooth-barked Sigillariae are the characteristic representatives. “Ferns” and Pteridosperms are even more strongly represented than before, and this is the age in which the supposed Marattiaceous tree-ferns reached their maximum development. Among Pteridosperms it is the family Medulloseae which is especially characteristic. Cordaiteae still increase, and Gymnospermous seeds become extraordinarily abundant. In the Upper Coal Measures the first Cycadophyta and Coniferae make their appearance. The Permian, so far at least as its lower beds are concerned, shows little change from the Stephanian; Conifers of the Walchia type are especially characteristic. The remarkable Permo-Carboniferous flora of India and the southern hemisphere is described in the next section of this article. During the earlier part of the Carboniferous epoch the vegetation of the world appears to have been remarkably uniform; while the deposition of the Coal Measures, however, was in progress, a differentiation of floral regions began. The sketch given above extends, for the later periods, to the vegetation of the northern hemisphere only.
Authorities.—Potonié, Lehrbuch der Pflanzenpaläontologie (Berlin, 1899); Renault, Cours de botanique fossile, vols. i.–iv. (Paris, 1881–1885); Scott, Studies in Fossil Botany (2nd ed., London, 1908–1909); “The present Position of Palaeozoic Botany,” in Progressus rei botanicae, Band I. (Jena, 1907); Seward, Fossil Plants (in course of publication), vol. i. (Cambridge, 1898), vol. ii. (1910); Solms-Laubach, Introduction to Fossil Botany (Oxford, 1892); Zeiller, Éléments de paléobotanique (Paris, 1900). In these general works references to all important memoirs will be found. (D. H. S.)
II.—Mesozoic
The period dealt with in this section does not strictly correspond with that which it is customary to include within the limits of the Mesozoic system. The Mesozoic era, as defined in geological textbooks, includes the Triassic, Jurassic and Cretaceous epochs; but from the point of view of the evolution of plants and the succession of floras, this division is not the most natural or most convenient. Our aim is not simply to give a summary of the most striking botanical features of the several floras that have left traces in the sedimentary rocks, but rather to attempt to follow the different phases in the development of the vegetation of the world, as expressed in the contrasts exhibited by a comparison of the vegetation of the Coal period forests with that of the succeeding Mesozoic era up to the close of the Wealden period.
Towards the close of the Palaeozoic era, as represented by the Upper Carboniferous and Permian plant-bearing strata, the vegetation of the northern hemisphere and that of several regions in the southern hemisphere, consisted of numerous types of Vascular Cryptogams, with some members of the Gymnospermae, and several genera referred to the Pteridospermae and Cycadofilices (see section I. Palaeozoic). In the succeeding Permian period the vegetation retained for the most part the same general character; some of the Carboniferous genera died out, and a few new types made their appearance. The Upper Carboniferous and Permian plants may be grouped together as constituting a Permo-Carboniferous flora characterized by an abundance of arborescent Vascular Cryptogams and of an extinct class of plants to which the name Pteridosperms has recently been assigned—plants exhibiting a combination of Cycadean and filicinean characters and distinguished by the production of true gymnospermous seeds of a complex type. This flora had a wide distribution in North America, Europe and parts of
