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(Lower greensand and Speeton clay, Wealden
and Hastings sands, Kootanie and Queen
Thus we have a great and sudden inswarming of the higher plants of modern types at the close of the Lower Cretaceous. In relation to this, Saporta, one of the most enthusiastic of evolutionists, is struck by this phenomenon of the sudden appearance of so many forms, and some of them the most highly differentiated of dicotyledonous plants. The early stages of their evolution may, he thinks, have been obscure and as yet unobserved, or they may have taken place in some separate region, or mother country as yet undiscovered, or they may have been produced by a rapid and unusual multiplication of flower-haunting insects! Or it is even conceivable that the apparently sudden elevation of plants may have been due to causes still unknown. This last seems, indeed, the only certain inference in the case, since, as Saporta proceeds to say in conclusion : “Whatever hypothesis one may prefer, the fact of the rapid multiplication of dicotyledons, and of their simultaneous appearance in a great number of places in the northern hemisphere at the beginning of the Cenomanian epoch, cannot be disputed.” 1
The leaves described by Heer, from the Middle Cretaceous of Greenland, are those of a poplar (P. primæva). Those which I have described from a corresponding horizon in the Rocky Mountains are a Sterculites (S. vetustula), probably allied to the mallows, and an elongated leaf, Laurophyllum (L. crassinerve) (Fig. 69), which may, however, have belonged to a willow rather than a laurel. These are certainly older than the Dakota group
* Including an estimate of Fontaine's undescribed species.
"Monde des Plantes,” p. 197.
of the United States and the corresponding formations in Canada. On the eastern side of the American continent, in Virginia, the Potomac series is supposed to be
of Lower Cretaceous age, and here Fontaine, as already stated, has found an abundant flora of cycads, conifers, and ferns, with a few angiospermous leaves, which have not yet been described.
In the Canadian Rocky Mountains, a few hundreds of feet above the beds holding the before
mentioned species, are the FIG. 69.—Stercalia and Laurophyllum shales of the Mill Creek
or Salix, the oldest Angiosperms known in the Cretaceous of Canada. series, rich in many spe
cies of dicotyledonous leaves, and corresponding in age with the Dakota group, whose fossils have been so well described, first by Heer and Capellini, and afterward by Lesquereux. We may take this Dakota group and the quader-sandstone of Germany as types of the plant-bearing Cenomanian, and may notice the forms occurring in them.
In the first place, we recognise here the successors of our old friends, the ferns and the pines, the latter represented by such genera as Taxites, Sequoia, Glyptostrobus, Gingko, and even Pinus itself. We also have a few cycads, but not so dominant as in the previous ages. The fan-palms are well represented, both in America and in the corresponding series in Europe, especially by the genus Sabal, which is the characteristic American type of fan-palm, and there is one genus which Saporta regards as intermediate between the fan-palms and the pinnately leaved species. There are also many fragments of stems and leaves of carices and grasses, so that these plants, now so important to the nourishment of man and his companion animals, were already represented.
But the great feature of the time was its dicotyledonous forests, and I have only to enumerate the genera supposed to be represented in order to show the richness of the time in plants of this type. It may be necessary to explain here that the generic names used are mostly based on leaves, and consequently cannot be held as being absolutely certain, since we know that at present one genus may have considerable variety in its leaves, and, on the other hand, that plants of different genera may be very much alike in their foliage. There is, however, undoubtedly a likeness in plan or type of structure in leaves of closely allied plants, and, therefore, if judiciously studied, they can be determined with at least approximate certainty.* More especially we can attain to much certainty when the fruits as well as the leaves are found, and when we can obtain specimens of the wood, showing its structure. Such corroboration is not wanting, though unfortunately the leaves of trees are generally found drifted away from the other organs once connected with them. In my own experience, however, I have often found determinations of the leaves of trees confirmed by the discovery of their fruits or of the structure of their stems. Thus, in the rich cretaceous plant-beds of the Dunvegan series we have beech-nuts associated in the same beds with leaves referred to Fagus. In the Laramie beds I determined many years ago nuts of the Trapa or water-chestnut, and subsequently Lesquereux found, in beds in the United States, leaves which he referred to the same genus. Later, I found in collections made on the Red Deer River of Canada my fruits and Lesquereux's leaves on the same slab. The presence of trees of the genera Carya and Juglans in the same formation was inferred from their leaves, and specimens have since been obtained of silicified wood, with the microscopic structure of the modern butternut. Still we are willing to admit that determinations from leaves alone are liable to doubt.
In the matter of names of fossil leaves, I sympathise very strongly with Dr. Nathorst, of Stockholm, in his
* Great allowance has to be made for the variability of leaves of the same species. The modern hazel (C. rostrata) is a case in point. Its leaves, from different parts of the same plant, are so dissimilar in form and size that they might readily be regarded as of different species.
objection to the use of modern generic names for mere leaves, and would be quite content to adopt some noncommittal termination, as that of “phyllum” or “ites suggested by him. I feel, however, that almost as much is taken for granted if a plant is called Corylophyllum or Corylites, as if called Corylus. In either case a judgment is expressed as to its affinities, which if wrong under the one term is wrong under the other; and after so much has been done by so many eminent botanists, it seems inexpedient to change the whole nomenclature for so small and questionable an advantage. I wish it, however, to be distinctly understood that plants catalogued on the evidence of leaves alone are for the most part referred to certain genera on grounds necessarily imperfect, and their names are therefore subject to correction, as new facts may be obtained.
The more noteworthy modern genera included in the Dakota flora, as catalogued by Lesquereux, are the following: Liquidambar, the sweet-gum, is represented both in America and Europe, the leaves resembling those of the modern species, but with entire edges, which seems to be a common peculiarity of Cretaceous foliage.* Populus (poplar), as already stated, appears very early in Greenland, and continues with increasing number of species throughout the Cretaceous and Tertiary. Salix (willow) appears only a little later and continues. Of the family Cupulifere we have Fagus (beech), Quercus (oak), and Castanea (chestnut), which appear together in the Dakota group and its equivalents. Fruits of some of the species are known, and also wood showing structure. Betula
* With reference to this, something may be learned from the leaves of modern trees. In these, young shoots have leaves often less toothed and serrated than those of the adult tree. A remarkable instance is the Populus grandidentatus of America, the young shoots of which have entire leaves, quite unlike except in venation those of the parent tree, and having an aspect very similar to that of the Cretaceous poplars.