Popular Science Monthly/Volume 77/July 1910/The Paleontologic Record II: Paleontologic Evidences of Climate
|THE PALEONTOLOGIC RECORD|
By T. W. STANTON
U. S. GEOLOGICAL SURVEY
TO every one climate is an interesting theme. The climates of the past, especially when they can be shown to differ in character or distribution from those of the present, attract the attention of the general public, and they are of importance to the special student of geologic history whether his researches deal with the purely physical aspects of the subject or include some branch of paleontologic study.
The evidence as to former climates comes from many sources. The records of deposition and denudation in themselves sometimes give more or less definite indications concerning variations in temperature or moisture or both; the land floras when compared with those now living by their general characters and by the details of their structure, show more or less clearly the climatic conditions under which they lived; the land animals, especially the higher vertebrates, afford a good basis for inferring their habits and hence indirectly their environment, including climate; marine invertebrates give trustworthy evidence of differences in temperature of oceanic littoral waters at least in the later periods. It is obvious, however that the data furnished by any one of these lines of evidence will make only unconnected fragments of the history of past climates and that the evidence on the climate of any particular epoch, if derived from a single source, is seldom so complete or so convincing that corroborative testimony from other sources is not desirable. The subject is one in which general cooperation is essential.
It should be stated at the outset that the most abundant and most definite evidence comes from paleobotany, and will be outlined in Mr. White's paper. The discussion of the data derived from fossil vertebrates must also be left for some one who is qualified to present it, and the whole Paleozoic era may be passed over with the statement that so far as indications from the animal life are concerned the climate of the whole earth was mild and equable. The proof of local exceptions to this statement comes from other sources.
All inferences from paleontologic evidence as to former climatic conditions rest in the final analysis on a comparison with the present distribution of animals and plants with reference to climate. Such comparisons may be general or specific, direct or indirect, and the convolutions that may be drawn from them vary greatly in positiveness. To take a familiar example, the reef-building corals are now restricted to shallow waters in which the mean temperature during the coldest month in the year is not less than 68° F., and such conditions are not found in the northern hemisphere north of latitude 32°. Since late Tertiary corals differ but little from those of the present time it is justifiable to assume that coral reefs in late Tertiary rocks indicate waters of about the temperature stated. But when Jurassic coral reefs are found as far north as latitude 53° it is by no means so certain that they indicate a minimum monthly mean temperature of 68° F., and concerning Devonian and Silurian coral reefs in high latitudes the doubt must be still greater. At the present time large reptiles are mainly confined to hot moist climates, but that fact alone can not be considered proof that the Mesozoic dinosaurs required the same kind of a climate.
The impress of climate on the present fauna is shown in various ways. A tropical fauna contains the greatest number of species and exhibits its luxuriance in other ways. Thus, taking shell-bearing marine mollusks to illustrate the general law, Dall has shown in Bulletin 84, U. S. Geological Survey, that the average tropical fauna in shallow waters consists of over 600 species, while the temperate fauna has less than 500 species, and the boreal fauna only 250. Again, there are certain genera that are characteristic of particular zones, and assemblages of forms that are recognized as belonging only to frigid, or temperate, or tropical waters, and in genera that have a wide range many of the species are restricted to certain limits of temperature.
In the late Tertiary faunas which contain a large proportion of living genera and many living species justifiable inferences as to climate may be made from direct comparison with living faunas. By one or another of the tests just indicated, or by a combination of them, Dall has produced convincing evidence that the Oligocene fauna of the Atlantic states was subtropical and that the Oligocene maintains its' subtropical character even as far north as Arctic Siberia. He has also shown that the Miocene fauna of Maryland indicates a temperate climate and that a similar cool-water fauna extended at that time as far south as Florida. The fossils of the raised Pliocene beaches at Nome, Alaska, according to the same investigator, furnish evidence of warmer climate during Pliocene time even at that high latitude. By similar methods, in a paper published in the Journal of Geology, Vol. XVII., Arnold has recently argued for a series of climatic changes in the late Tertiary and Pleistocene of California.
When the investigation is carried back to the Mesozoic and earlier faunas in which few of the genera and none of the species are identical with those now living the problem becomes more difficult and the conclusions are much less definite, as the comparisons must be more general. Proofs of actual temperatures as measured in degrees should not be expected unless the botanists can furnish data. There is, however, great local differentiation of faunas and it is fair to ask the question to what extent this is due to differences in climate. One of the earliest discussions of this question was by Ferdinand Roemer, who more than fifty years ago in "Die Kreidebildungen von Texas" noted the fact that the Cretaceous of the highlands in Texas is lithologically and faunally much like the Cretaceous of southern Europe and the Mediterranean region, that it differs from the Cretaceous of New Jersey in about the same way that the southern European Cretaceous differs from that of England and northwestern Germany, and that in each case the European deposit is approximately 10° farther north than its American analogue. He concluded that the differences between the northern and southern facies were due to climate and that the climatic relations between the two sides of the Atlantic were about the same in Cretaceous time as they are now. Roemer's conclusion that there were climatic zones in the Cretaceous may be true, but his reasoning was based on false premises so far as the American deposits are concerned, for the New Jersey type of marine Cretaceous extends with little change all the way from New Jersey to the Rio Grande, and the "Cretaceous of the highlands" with which he contrasted it, now known as the Comanche series, is not represented by marine beds on the Atlantic coast. This shows the necessity for careful stratigraphic and areal work as well as for good paleontology before such broad conclusions can be safely made.
The more general work of Neumayr recognized in the Jurassic and Cretaceous of Europe three faunal provinces designated as boreal, central European, and alpine or equatorial, which on account of their zonal distribution he regarded as indicating climatic differences. He believed that these zones are recognizable throughout the northern hemisphere and cited evidence to show that similar zones exist south of the equator. In recent years Neumayr's conclusions have been questioned by many because in so many instances genera supposed to be characteristic of one zone have been found mingled with those of another. For example, the alpine ammonite genera Lytoceras and Phylloceras occur in Alaska (lat. 60°) associated with the boreal Aucella, and Aucella itself ranges from the Arctic Ocean to the torrid zone. Still, in spite of such exceptions and anomalies in distribution, there is much evidence for a real distinction between boreal and southern faunas in the Jurassic and in the Cretaceous which may indicate a zonal distribution of temperature in Mesozoic time. It should be remembered, however, that a boreal climate probably did not then mean a frigid climate, and that the differences in temperature were probably not so great as at the present time.
The conclusions justified by the evidence from fossil invertebrates are:
1. In the Paleozoic there is practically no faunal evidence of climatic zones comparable with those that now exist.
2. In the Mesozoic there is a more or less definite zonal distribution of faunas which may be in part due to differences in climate but this conclusion in each case should be checked by the study of the floras and all other available lines of evidence.
3. From the middle of the Tertiary on through the Pleistocene trustworthy conclusions as to climatic conditions and changes can be made by direct comparisons with the distribution of living faunas.