On the other hand, fern-like seed-plants, known as Pteridosperms, and Gymnosperms belonging almost entirely to families now extinct, were abundant, while the Pteridophyta attained a development exceeding anything that they can now show.
We must bear in mind that throughout the Palaeozoic period, and indeed far beyond it, vascular plants, so far as the existing evidence shows, were represented only by the Pteridophyta, Pteridosperms and Gymnosperms. Although the history of the Angiosperms may probably go much further back than present records show, there is no reason to suppose that they were present, as such, amongst the Palaeozoic vegetation.
It is, however, probable that a considerable group of true Ferns, allied to Marattiaceae, existed in Palaeozoic times, side by side with simpler forms. In one respect the fronds of many Palaeozoic Ferns and Pteridosperms were peculiar, namely, in the presence on their rachis, and at the base of their pinnae, of anomalous leaflets, often totally different in form and venation from the ordinary pinnules.
In all these cases there is reason to suspect that the plants may have been Pteridosperms, rather than Ferns.
Although doubts have lately been cast on the authenticity of Palaeozoic Marattiaceae owing to the difficulty in distinguishing between their fructifications and the pollenbearing organs of Pteridosperms, the anatomical evidence (stem of Psaronius) strongly confirms the opinion that a considerable group of these Ferns existed.
The genus Aneimites, resembling the Maidenhair Ferns in habit, has now been transferred to the Pteridosperms, the seeds having been discovered in 1904 by David White.
The Pteridosperms, of which only a few examples have been considered, evidently constituted a group of vast extent in Palaeozoic times.
There appears, in fact, so far as stem-structure is concerned, to have been no sharp break between the typical Palaeozoic Gymnosperms and pronounced Pteridosperms such as Lyginodendron.
In the isolated seeds of Cordaitales and Pteridosperms, pollen-grains are often found within the pollen-chamber, and the pluricellular structure of these pollen-grains has been repeatedly demonstrated.
Anatomically the connexion of the family with the Pteridosperms (and through them, presumably, with some primitive group of Ferns) seems clear, but we have as yet no indications of the stages in the evolution of their reproductive organs.
Many other forms of seed, and especially those which show radial symmetry, as for example Trigonocarpus, Stephanospermum and Lagenostoma belonged, as we have seen, to some of the plants grouped under Pteridospermeae, though other Pteridosperms had flattened seeds not as yet distinguishable from those of Cordaitales.
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.
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.
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.
" 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.
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.