aus deutsch und portugiesisch Ostafrika,” Deutsch-Ostafrika, vii.
(Berlin, 1900), with bibliography. General: Potonié, Lehrbuch der
Pflanzenpalaeontologie (Berlin, 1899); Scott, Studies in Fossil Botany
(1900); Seward, Fossil Plants (Cambridge: vol. i., 1898); vol. ii. 1910,
with bibliography; Zeiller, “Revue des travaux de paléontologie
végétale,” Rev. gén. bot. (1903) et seq. Catalogue of the Mesozoic
Plants in the British Museum, (a) “Wealden Flora,” pts. i. and ii.;
(b) “Jurassic Flora,” pt. i. (1894–1901), pt. ii. (1904), with bibliography;
“On the Structure and Affinities of Matonia pectinata, with
Notes on the Geological History of the Matonineae,” Phil. Trans.
cxci. (1899); “On the Structure, &c., of Dipteris,” ibid. cxciv. (1901,
with bibliography; Seward and Ford, “The Araucarieae, recent and
extinct,” Phil. Trans. R. Soc. (London, 1906); G. R. Wieland,
“American Fossil Cycads,” Publication Carnegie Instit. (Washington,
1906); Nathorst, “Paläobotanische Mitteil.,” K. Svensk. Vetenskaps.
Akad. Hand. xlii., No. 5 (1907); The Norwegian North-Polar Expedition,
iii. (1893–1896); “Fossil Plants from Franz Josef Land;” L. F.
Ward, “Status of the Mesozoic Floras of the United States,”
Twentieth Ann. Rep. Geol. Survey (Washington, 1900); Solnis-Laubach,
“Ueber das Genus Pleuromeia,” Bol. Zeit. (1899); Newton
and Teall, “Notes on a Collection of Rocks and Fossils from Franz
Josef Land,” Q.J.G.S. liii. (1897); Hollick and Jeffrey, “Studies of
Cretaceous Coniferous remains,” Mem. New York Botanical Garden,
vol. iii. (1909); Stopes and Fujii, “Structure and Affinities of
Cretaceous Plants,” Phil. Trans. R. Soc. (1910). References to important
papers on Mesozoic botany will be found in the bibliographies
mentioned in the above list. (A. C. Se.)
III.—Tertiary
After the Wealden period, and before the deposition of the lowest strata of the Chalk, so remarkable a change takes place in the character of the vegetation that this break must be taken as, botanically, the transition point from a Secondary to a Tertiary flora. A flora consisting entirely, with a single doubtful exception, of Lower Cretaceous. Gymnosperms and Cryptogams gives place to one containing many flowering plants; and these increase so rapidly that before long they seem to have crowded out many of the earlier types, and to have themselves become the dominant forms. Not only do Angiosperms suddenly become dominant in all known plant-bearing deposits of Upper Cretaceous age, but strangely enough the earliest found seem to belong to living orders, and commonly have been referred to existing genera. From Cretaceous times onwards local distribution may change; yet the successive floras can be analysed in the same way as, and compared with, the living floras of different regions. World-wide floras, such as seem to characterize some of the older periods, have ceased to be, and plants are distributed more markedly according to geographical provinces and in climatic zones. This being the case, it will be most convenient to discuss the Tertiary floras in successive order of appearance, since the main interest no longer lies in the occurrence of strange extinct plants or of transitional forms connecting orders now completely isolated.
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| Fig. 1.—Alismacites primaevus. |
The accurate correlation in time of the various scattered plant-bearing deposits is a matter of considerable difficulty, for plant-remains are preserved principally in lacustrine strata laid down in separate basins of small extent. This it is obvious must commonly be the case, as most leaves and fruits are not calculated to drift far in the sea without injury or in abundance; nor are they likely as a rule to be associated with marine organisms. Deposits containing marine fossils can be compared even when widely separated, for the ocean is continuous and many marine species are world-wide. Plants, on the other hand, like land and fresh-water animals, occupied areas which may or may not have been continuous. Therefore, without a knowledge of the physical geography of any particular period, we cannot know whether like or unlike floras might be expected in neighbouring areas during that period. If, however, we discover plant-bearing strata interstratified with deposits containing marine fossils, we can fix the period to which the plants belong, and may be able to correlate them in distinct areas, even though the floras be unlike. This clear stratigraphical evidence is, however, so rarely found that much uncertainty still remains as to the true age of several of the floras now to be described.
In rocks approximately equivalent to the Lower Greensand of England, or slightly earlier, Angiosperms make their first appearance; but as the only strata of this age in Britain are of marine origin, we have to turn to other countries for the evidence. The earliest Angiosperm yet found in Europe is a single monocotyledonous leaf of doubtful affinities, named by Saporta Alismacites primaevus (fig. 1), and found in the Valenginian strata of Portugal. These deposits seem to be equivalent to British Wealden rocks, though in the latter, even in their upper part, no trace of Angiosperms has been discovered. No other undoubted Angiosperm has yet been discovered in Europe in strata of this age, but Heer records a poplar-like leaf from Urgonian strata, a stage newer than the Valenginian, in Greenland, and Saporta has described from strata of the same date in Portugal a Euphorbiaceous plant apparently closely allied to the living Phyllanthus and named by him Choffatia Francheti (fig. 2). We must turn to North America for a fuller knowledge of the earliest flowering-plants.
In S. Dakota a remarkable series has been discovered, lying unmistakably between marine Upper Jurassic rocks below and Upper Cretaceous above. There has been a certain amount of confusion as to the exact strata in which the plants occur, but this has now been cleared up by the researches of Lester F. Ward, who has shown how the American Cretaceous. Secondary flora gives place to one of Tertiary character.
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| Fig. 2.—Choffatia Francheti. |
The lower strata—i.e. those most allied to the Jurassic—contain only Gymnosperms and Cryptogams. The next division (Dakota No. 2 of Meek and Hayden) contains Gymnosperms and Ferns of Neocomian types, or even of Neocomian species; but mingled with these occur a few dicotyledonous leaves belonging to four genera. The specimens are very fragmentary, and all that can be said is that one of the forms may be allied to oak, another to fig, a third to Sapindus, and the fourth may perhaps be near to elm. The “Potomac Formation” of Virginia and Maryland is doubtless also mainly of Neocomian age, for though it rests unconformably on much older strata, the successive floras found in it are so allied to those of S. Dakota as to leave little doubt as to the general homotaxis of the series. Lester Ward records no fewer than 737 distinct forms, consisting chiefly of Ferns, Cycads, Conifers and Dicotyledons, the Ferns and Cycads being confined mainly to the Older Potomac, while the Dicotyledons are principally represented in the Newer Potomac, though occurring more rarely even down to the base of the series. Six successive stages have been defined in the Potomac formation. The Mount Vernon beds, which occur about the middle of the series, have as yet yielded only a small number of species, though these include the most interesting early Angiosperms. Among them are recorded a Casuarina, a leaf of Sagittaria (which however, as observed by Zeiller, may belong to Smilax), two species of poplar-like leaves with remarkably cordate bases, Menispermites (possibly a water-lily) and Celastrophyllum (perhaps allied to Celastrus). Proteophyllum, found in the same bed, and also in the Infra-Cretaceous of Portugal, seems to have belonged to a Proteaceous plant, though only leaves without fruits have yet been discovered in deposits of this early date. Whatever doubt may be left as to the exact botanical position of these early Lower Cretaceous Angiosperms, it is clear that both Monocotyledons and Dicotyledons are represented by several types of leaves, and that the flora extended over wide areas in North America and Greenland, and is found again at a few points in Europe. There is yet no clear evidence either of climatic zones or of the existence of geographical provinces during this period.
The next strata, the Aquila Creek series, contain a well-marked dicotyledonous flora, in which both the form and nervation of the leaves begin to approximate to those of recent times. The leading characteristic of this Middle Potomac flora is the proportion of Dicotyledons. Notwithstanding this apparent passage-bed, there is a marked difference between the Older and the Newer Potomac floras, very few species passing from the one to the other. Only 15 out of 405 plants in the older series occur in the beds above,
