The fossils of the Dakota group consist of a few marine mollusks, some few remains of fish and Saurians, but it is more particularly noted for its dicotyledonous plants. The Molluscoe are rare, having been found in a few localities only. Two of these are in the western part of Saline county, in the vincinity of Bavaria, and another is in the western portion of Clay county. On one of these spots, covering not over two acres, we procured twelve species, new to science, and described by Prof. F. B. Meek in U. S. Geological Survey, Hayden, 1870, pp. 297-313.
A few fish and one Saurian (Hyposaurus vebbii*) have been found in this group.
* Vide Cope's "Cretaceous Vertebrata," p. 17, where this specimen is incorrectly stated to be from the Niobrara. Brookville, the locality there named, is clearly on the Dakota.
Of the fish, the most interesting is the Pelocarapis varius, Cope, an ally of the flying fish, found near the dividing line between the Dakota and Ft. Hays.
But it is in its fossil flora, represented largely by dicotyledonous leaves, that the Dakota claims the attention of the student of nature. Prof. Lesquereaux, our greatest American fossil botanist, has devoted to this flora most careful and valuable study. In his Cretaceous Flora, and other publications connected with Prof. Hayden's geological survey, he has given us the results of many years' study, to which we refer the reader. Prof. L. says: "The plants of the Dakota group, as known mostly by detached leaves, are striking, by the beauty, the elegance, the variety of their forms and of their size. In all this they are fully comparable to those of any geological epoch, as well as those of our time."*
The fossil flora is almost entirely represented by leaves, though a few specimens of fruit, imperfectly preserved, have been collected; also some poor specimens of wood and bark. The leaves, however, are usually in excellent preservation, the veins and veinlets as they lie imprinted on the stone being frequently as clearly visible, in all their outlines, as those just taken from the living tree. Among these fossils are thirteen specimens of conifers and one palm. The latter is the oldest known, being found lower down geologically than any other. I discovered it in the northwest corner of Marion county. It is a Sabal, or the same genus as the Palmetto of South Carolina.
Among the conifers are a pine and four species of the celebrated gigantic redwoods (Sequoia) of California, and one is closely related to those trees. In examining fossil leaves we have frequently inspected every visible outcrop for fifteen or twenty miles without finding a specimen ; then perhaps a single square mile would present several good localities. In this irrgeular manner we have collected specimens from Washington county to Ft. Larned, a distance of one hundred and fifty miles.
The fossil plants are usually obtained from thin layers and strata, extending in a horizontal position along a ravine or around a hill. They may occur at several places in the same vicinity, but usually without any connection. They are found at all depths in the Dakota, from within thirty-five feet of the Permian to within forty feet of the Ft. Benton shales. The numerous indications show that the trees must have grown on islands near the shore line, and that the leaves were imbedded in the marine sediment immediately after dropping. Worm borings are also found in the same strata with the leaves.
The contrast between this fossil flora and the plants of the older formation is very strong, while its resemblance to those now living is equally remarkable. The interest attached to this numerous variety of modern plants is enhanced by the fact that in the earlier formation no dicotyledons are found, the conifers which come down from the Devonian age being the highest type. But in our Dakota, and in the corresponding age in Europe, we have a sudden influx of a new type covering nearly all the forms now living. These are "the first known of the great modern group of Angiosperms and the ordinary fruit trees of the temperate zones, distributable not in a single one but in all the essential groups of vegetables living at one time."*
*Lesquereaux
This sudden appearance of the full type of modern vegetation will be more apparent on examination in detail. Prof. Lesquereaux, in his Cretaceous, describes one hundred and thirty-eight species, distributed among seventy-two genera and twenty-three orders, of which one hundred and seven species of nineteen orders and fifty-two genera are dicotyledonous plants. Of these, more than one-half have been collected in Kansas, and about thirty of the new species described by Prof. Lesquereaux were discovered by the writer. To these are to be added twenty-six new species described by the same author in a recent bulletin (VII. of No. 5, second series) of Hayden's Reports. Additions to these are constantly being made. There are nine species of conifers, five of poplar, six of willow, eight of oak, six of Platanus or buttonwood, seven of sassafras, five of magnolia, two of fig, one of palm, and two of cinnamon. The last two were probably hardy species of their kind. Still they indicate a warmer climate than now exists. When we recollect that at the period of their growth this part of the country was nearly on a level with the ocean, and the dry land was composed principally of a few islands, the variance of the climate is easily explained. Taking Prof. Lesquereaux's list of dicotyledons, we find fifty-six per cent. of his genera are identical with those now living east of the Rocky Mountains, in the temperate zone of the United States. To this must be added twenty-four per cent., which are apparently identical, represented by the Populites, Betulites, Acerites, Negundoides, Lauraphillum, etc. Of the remaining twenty per cent., some, like the fig and cinnamon, are now living in the tropics, while a few are probably extinct genera. It is also a marked feature that the proportions of the different kinds of plants, or those represented by the classes now living in the temperate zones, are the same.
Those organized in a low as well as perfect condition are represented by about the same percentage as we find them in our present flora. I draw attention to this fact, as it has been stated that the lower forms (apetalous, etc.) of dicotyledonous plants appear in geology long before the higher (polypetalous, etc.) are discovered. But in the Dakota we have the first and oldest of all kinds mingled together in about the same proportions as they now exist. Intermingled with these are the still lower and older forms (cryptograms) represented by ferns. This feature of resemblance to living vegetation is increased by the examination of specific forms. At first Lesquereaux was disposed (like all paleontologists who find familiar forms in an unexpected geological age) to say that all the species were extinct; but in his later writings, after exchanging opinions with the best floral paleontologists of Europe, he has been led to change his opinion on at least one species. In naming a new sassafras, he honored me by calling it S. Mudgei.
By a comparison of numerous specimens from Greenland and Europe with our Dakota and the living Sassafras officinale, we obtain the following conclusions by the highest authorities. Prof. W. P. Schimper says "that these leaves, very variable in size, present such a remarkable likeness to those of S. officinale that one would be obliged to consider them as belonging to an homologous species." And Lesquereaux adds: "Comparing leaves of S. officinale with those represented by Count Saporta in the flora of Sezane, and the specimens of S. Mudgei from Kansas, it is impossible for me to recognize any character, even any specific difference, by which these leaves could be separated."*
* See Hayden's Geological Report, 1874, p. 328.
This extreme persistence, (by which I lose my namesake,) it must be recollected, covers a period of one-eighth of the earth's geological history. On more careful study of these fossil leaves, it is most probable that others may be found specifically like those now living.
The fig, in its nervation, and especially in its areolation, is of the same character as many species now living in Cuba and Florida.** Had these leaves been found in Post-Pliocene, very many of them would be assigned to living species.
** Ibid, P. 327.
The persistence of vegetable forms has been more strong through all geological ages than any other organic life.
Prof. Asa Gray has recently, in the American Journal of Science, given us a very interesting essay on the origin, by migration, of our present North American forests, from those fossilized in Greenland. He describes the close relationship of the living forests and those found in the rocks of that cold region, and from a series of well digested facts, he comes to the conclusion that the ancestry of our trees lived in Greenland, in the Miocene epoch, and migrated by the pushing southward of new generations. He stops there, and goes no farther back. He does not inquire from whence came the Greenland stock.
By a similar arrangement of the facts, and train of argument, it becomes apparent that our Dakota forests were the parent stock of the fossil forests now seen in Alaska, in the valley of the Mackenzie river, Greenland, and probably at intermediate points. The close resemblance, which we have stated elsewhere, of the Dakota leaves to our living trees, also shows the resemblance to the Greenland species, half-way between in geological time.
