Popular Science Monthly/Volume 76/June 1910/Biologic Principles of Paleogeography II
|BIOLOGIC PRINCIPLES OF PALEOGEOGRAPHY|
C. S. GEOLOGICAL SURVEY
CONSIDERING the breadth and intricacy of the subject assigned me, and the limited time that can be given to its consideration. it has seemed best to me to restrict my remarks to two or three of the obviously more important phases of the problem.
Aside from the study of the rock-masses themselves—which are often difficult of interpretation—reliance for an interpretation of paleogeography must be placed in the former life found entombed, and of the two biologic elements, plants undoubtedly hold a very high—probably the highest—place.
In making use of plants in the study of paleogeography we may first consider distribution. If we find two fossil floras identical or similar in all essential or important details, we feel justified in regarding them for all practical geologic purposes as contemporaneous. In order that we may be certain that the two floras are identical, they must be composed of types that are readily identifiable, that is, forms so well characterized that they may be easily and certainly recognized. As examples of such floral elements mention may be made of many ferns and fern allies, most cycads, conifers and peculiar, well-marked or characteristic dicotyledons. Having settled the contemporaneity of the floras, inquiry may next be made as to the probable manner in which the separated or isolated areas were reached by these floras. Here again we must carefully consider the character of the flora and the means for its natural dispersal. The living flora, and for that matter probably the floras from at least the beginning of the Tertiary progressively to the present time, has developed in many ways means for the comparatively rapid and wide-spread dissemination of their reproductive parts (seeds, etc.). For example, a large percentage of the members of the dominant living family of seed-plants—the Compositæ—have developed seeds with an attachment of soft, fluffy hairs which serve to float them in the air, often to great distances. In many other living groups there are similar, or at least as effective, devices for dissemination, but as we go back in time adaptations calculated to be of aid in distribution grow less and less, and soon even seeds of any kind are unknown, or known but imperfectly, and reproduction is normally by means of spores, that is, reproductive bodies in which there is no embryo already formed when they leave the parent plant. It is obvious that plants that are reproduced by seeds, in which there is both an embryo and a supply of food for use during germination, must possess a decided advantage over those reproduced by means of spores. In the groups of spore-bearing plants ordinarily found fossil, the spores are not known to have developed any particular devices for their wide dissemination, such as flotation in air, attachment to animals, etc. They are produced in vast quantities, and depend upon a few reaching situations favorable for successful germination. Their vitality is also of apparently exceedingly limited duration, and it is doubtful if they could long survive immersion in salt water.
The bearing of the above digression is apparent. Given a fossil flora made up of ferns or fern allies, exclusive of what are known to belong to the cycadofilices, and when such flora is found in two or more separated areas, we are justified, in my opinion, in arguing a practically continuous land connection. They were incapable of crossing very wide reaches of open water, particularly salt water. Fresh-water streams have been to some extent avenues of distribution, but many fossil floras—and living floras as well—are too widely spread to be explained by this means. When, as is usually the case, identical floras occupying different areas are mixed floras, the bearing on the means of reaching the various areas is more complicated. An example may better serve to bring this out. Thus, the Jurassic flora is practically world-wide in its distribution, ranging from Franz Josef Land, 82° N., to Louis Philippe Land, 63° S. It is composed of ferns, fern-allies, cycads and conifers, a large percentage being true ferns. The probability of a close land connection argued on the basis of the true ferns, has already been alluded to. The cycads—the Jurassic is called the age of cycads—were abundant in individuals and numerous in forms. On the basis of our knowledge of living types, it may be stated that cycad seeds germinate immediately on falling from the cone without any necessary resting period. They are not known to retain their vitality for a longer period than three years, and usually but two years. They sink promptly in fresh water and as the stony coat is easily penetrated by water, they either germinate or rot at once. In salt water they will probably sink and decay even more quickly. Therefore, the probability of their being transported for any great distance over open water is reduced to a minimum. The conifers of the Jurassic were reproduced by seeds. They belong to types not known to enjoy any special means for transportation, nor is it probable they could better withstand freshor salt-water immersion than the cycads. All classes of vegetation present in the Jurassic, therefore, argue for a practically continuous land connection.
In considering the bearing of any flora on the paleogeographic problem the process is similar to that outlined above. That is, an analysis of the composition of the flora, a study of the means of natural dissemination which includes duration of vitality, and finally a judgment as to its probable means or avenues of transportation, involving a land connection or otherwise.
A word may be said as to the presence of land plants in marine deposits. That the trunks of trees may float for a considerable time and to great distances is undeniably possible, but unfortunately the study of fossil wood has not yet reached that degree of refinement in most cases that will permit of its general use, and reliance in identification must be placed largely in foliar and reproductive organs. The delicate fronds of ferns, leaf-clad branchlets of conifers and the leaves of seed-bearing plants are incapable of long withstanding the immersion and wave action of salt waters. In my judgment, therefore, the presence of fronds, leaves and similar organs in marine deposits argues very near-by land.
The only other point I shall consider is the bearing of plants on the interpretation of climate. Since it is generally acknowledged that plants furnish the most reliable data for this phase of the subject, an inquiry as to the kinds of plants that have been found most valuable in this connection may be of interest. Obviously our interpretation of the probable conditions under which the plants of past geological ages grew, must be on a basis of a knowledge of present conditions found to obtain for similar or closely related groups. That we may occasionally err in this is possible, especially if reliance is based on too few forms, but when all the various elements of a flora are considered, the results are thought to be within a close approximation of the truth. Thus, since Artocarpus—the bread-fruit tree—only grows at the present day within 20° of the equator, it follows that when Artocarpus is found fossil in Greenland, 72° N., the conditions at the time it flourished there must have been tropical or subtropical, and this conclusion is confirmed by the tree ferns and cycads associated with it. Palms can not nourish with a temperature below 40°; a fossil flora, rich in palms of well-defined types, could hardly have grown under very much cooler conditions. Tree-ferns are practically confined to within 30° of the equator and a temperature of approximately 60°. A fossil flora, such, for example, as the Triassic of Virginia, that contains numbers of tree-ferns, must have grown under tropical or subtropical conditions. A fossil flora rich in types, the living representatives of which can withstand a temperature of —40° to —60°, or even lower, must have been at least cool-temperate. Cycads are now found only within 30° of the tropics; a rich cycad flora argues then for a tropical or at least a subtropical climate.
Examples of this kind could be multiplied almost indefinitely. In interpreting geological climate selection is made so far as possible of the plants or groups of plants, that are confined at the present day within relatively narrow limits of temperature, be this high, medium or low.