Gangamopteris.

In 1847 McCoy[1339] described a leaf-fragment from Permo-Carboniferous rocks in New South Wales as Cyclopteris angustifolia. The type-specimen of this species, which is now in the Sedgwick Museum, Cambridge, has been re-described by Mr Arber[1340]. Subsequently[1341] McCoy instituted the generic name Gangamopteris for leaves, like that previously referred by him to Cyclopteris, from the Bacchus Marsh Sandstone, of New South Wales, but he did not publish a diagnosis of the genus until several years later[1342]. Feistmantel[1343], who has described many species of Gangamopteris from the Lower Gondwana strata of India, slightly modified the original diagnosis. The genus is represented by sterile fronds only. We know nothing of the stem, and such evidence as is available in regard to the form of the fertile leaves is of a circumstantial kind. It is, however, highly probable that Gangamopteris is not a true fern but a Pteridosperm.

Leaves simple, sessile, varying in shape; obovate or spathulate, broadly lanceolate or rarely linear; the apex is usually blunt ([fig. 345]) but occasionally gradually tapered. In general appearance a Gangamopteris leaf is similar to that of Glossopteris indica, the chief distinction being the absence of a midrib. Gangamopteris leaves are on the whole larger than those of Glossopteris; many of them reach a length of 20 cm. and some of the large Indian fronds are nearly 40 cm. long. The venation of Gangamopteris shows a greater uniformity in the size and shape of the meshes than that of Glossopteris. The middle of the lamina, especially in the lower part, is occupied by a few vertical veins from which branches curve upwards and outwards towards the edge of the lamina. The secondary veins are connected by frequent anastomoses and agree very closely with those of Glossopteris. The lamina becomes narrower towards the base, which is either cuneate or in some cases slightly auriculate ([fig. 345]).

As I have elsewhere pointed out[1344], the presence or absence of a midrib is not in itself a character of real taxonomic importance. In the recent fern Scolopendrium vulgare the frond has a prominent midrib, while in S. nigripes there is no median rib. Mr Arber has expressed the opinion that “it is extremely doubtful whether the genus Gangamopteris should not be merged in Glossopteris[1345].” The retention of the two names is, however, convenient, and it would tend to confusion were we to carry to its logical conclusion the view that the recognised distinction between the two genera may not be a mark of generic difference.

Gangamopteris is confined to Palaeozoic strata, a fact which leads White[1346] to speak of the Gangamopteris rather than of the Glossopteris Flora. It occurs in South America, South Africa, Australia, and India, extending as far north as Kashmir; it has been discovered by Amalitzky in Permian rocks of Russia[1347]. The Russian rocks in which Glossopteris and Gangamopteris were found are no doubt of Permian age. In Australia, South Africa, Brazil and Argentina, and in the Indian Coal-fields, Gangamopteris is a characteristic genus of Lower Gondwana rocks. These strata are usually spoken of as Permo-Carboniferous in order to avoid the danger of attempting on insufficient data a precise correlation with European formations.

Feistmantel speaks of Gangamopteris as most abundant in the Talchir-Karharbári beds, though it is represented also in the overlying Damuda series. In Australia the genus occurs in rocks which correspond in position and in their plant fossils with the Talchir-Karharbári beds of India; similarly, in South Africa and South America the Gangamopteris beds are homotaxial with those of India and Australia. The leaf described by Carruthers[1348] from Brazil as Noeggerathia obovata (the type-specimen is in the British Museum) is no doubt specifically identical with Gangamopteris cyclopteroides Feist.[1349] In a paper by Mr Hayden on Gangamopteris beds in the Vihi Valley, Kashmir, evidence is adduced in support of the conclusion that the rocks are “not younger than Upper Carboniferous and may belong to the base of that subdivision or even to the Middle Carboniferous[1350].” It would seem that Gangamopteris was a very widely spread genus during the latter part of the Carboniferous period in the vast Southern Continent to which the name Gondwana Land is often applied, and that it flourished in the Southern Flora during at least part of the Permian period: with other members of the Glossopteris Flora it migrated to the North where it has been preserved in Permian rocks of Northern Russia. The Glossopteris Flora must have had its birth in the Southern hemisphere. The conclusion seems inevitable that the leaves of Glossopteris and Gangamopteris in the shales and sandstones of India, South Africa, South America, and Australia are relics of the vegetation of a continent of which these regions are the disjuncta membra. Darwin wrote to his friend Hooker in 1881, “I have sometimes speculated whether there did not exist somewhere during long ages an extremely isolated continent, perhaps near the South Pole[1351].” It is probable that Gangamopteris is one of the genera which flourished on this continent.

Gangamopteris cyclopteroides, Feistmantel[1352]. Fig. 345.

The specimen represented in [fig. 345] illustrates the characters of this commonest representative of the genus.

Fig. 345. Gangamopteris cyclopteroides, Feist. (Nat. size. From Arber, after Feistmantel.)

Gangamopteris kashmirensis, Seward.

This type agrees closely with G. cyclopteroides in size and in the form of the leaf, but it is distinguished by the flatter form of the arch formed by the lateral veins, by their greater inclination to the margin of the lamina, and by the more acutely pointed apex of the lamina. This species, though not very sharply distinguished from G. cyclopteroides, is important as coming from beds which have been assigned on other than palaeobotanical evidence to an Upper or possibly a Middle Carboniferous horizon[1353].

We have no definite information in regard to the nature of the reproductive organs of Gangamopteris, but such evidence as there is supports the view expressed by Dr White[1354] and shared by some other authors that Gangamopteris and Glossopteris should be assigned to the Pteridosperms. Despite the abundance of Gangamopteris leaves, no fertile specimen has been discovered. This negative evidence may prove to be as correct as that which led Stur[1355] to exclude Neuropteris, Alethopteris and Odontopteris from the ferns. The only evidence of a positive kind is that furnished by Dr David White in his recent Report on the Palaeozoic Flora of South Brazil. This author describes some small Aphlebia-like leaves under two new generic names Arberia[1356] and Derbyella[1357]. The differences between the two sets of specimens, so far as can be determined from the reproductions of imperfect impressions, are slight, and it is by no means clear that a distinction of generic rank exists. These scale-leaves are on the average about 2 cm. in length; the lamina is oval or rounded and has more or less prominent lobes. In Derbyella there are indications of anastomosing veins. The specimens referred to Arberia minasica are, as White points out, very similar to the fossil described by Feistmantel from Lower Gondwana rocks of India as probably a portion of an inflorescence of Noeggerathiopsis[1358]. Feistmantel’s specimen is represented in [fig. 346]: the curled lobes may have originally borne seeds. In the Brazilian examples the abruptly truncated lobes “bear evidence of separation from reproductive bodies.” An important point is the association of these scale-leaves with Gangamopteris fronds and with gymnospermous seeds of the Samaropsis type. On the leaves assigned to Derbyella aurita circular depressions occur at the base of the lobes which are described as probably due to sporangia.

Dr White’s discovery gives us increased confidence in expressing the view that Gangamopteris bore its reproductive organs on specialised leaves very different from the sterile fronds; it also strengthens the suspicion that the genus is a member of the class of seed-bearing fern-like plants.

Fig. 346. Arberia sp. (= Noeggerathiopsis of Feistmantel). (Nat. size. After Feistmantel.)