Popular Science Monthly/Volume 76/June 1910/Biologic Principles of Paleogeography I
| BIOLOGIC PRINCIPLES OF PALEOGEOGRAPHY |
By Professor CHARLES SCHUCHERT
YALE UNIVERSITY
IN deciphering the ancient geography as to the position of the marine waters and the land masses, we as pioneers in this work must be controlled primarily by the known fossilized life and secondarily by the character and place of deposition of the geologic formations. This record is most extensive and best preserved in the deposits of the continental and the littoral region along the continental shelves of the oceanic areas. Back of these two principles, however, there is another that eventually will become the primary guiding factor. It is the principle of diastrophism—one seeking to explain the causes for the periodic movements of the lithosphere.
In our study of the ancient seas with their sediments and entombed life we have safe guidance in the phenomena of the present. Ludwig in 1886 estimated the species of animals then known to naturalists as upwards of 312,000, and in 1905 Stiles thought this great total had increased to about 470,000 forms. Of this sum fully 60 per cent, are insects, and of the remainder, the writer concludes that about 25 per cent., or 115,000 species, live in the sea, and 71,000 have their habitat on the land or in the waters of the land. Of the 115,000 kinds of known animals inhabiting the seas nearly 70 per cent, are Cœlenterata, Echinodermata, Molluscoidea and Mollusca, the types of organisms most often found by the stratigrapher and on which he is largely dependent in deciphering the ancient geography.
Let us now examine into the number of available fossil forms made known by the paleontologists. As early as 1868, Bigsby in his "Thesaurus Siluricus" listed 8,897 species from the strata beneath the Devonic, and in his "Thesaurus Devonico-Carboniferous" of 1878, he further enumerated about 5,600 Devonic and 8,700 Carbonic forms. In 1889 Neumayr concluded that there were then known about 10,000 Jurassic species. We may therefore estimate that the paleontologists of to-day have access to at least 100,000 species of fossils. Their numbers in the geologic scale are about as follows: Cambric 2,000, Ordovicic 8,000, Siluric 8,000, Devonic 9,000, Lower Carbonic 7,000, Upper Carbonic 8,000, Permic 4,000, Triassic 6,000, Jurassic 15,000, Cretacic 10,000 and Tertiary 25,000. The end of species-making is not at all in sight, and the day will come when paleontologists will deal with ten times as many species as are now known.
Stiles tells us that zoologists know but from 10 to 20 per cent, of the living forms, and there should therefore be from 3,760,000 to 4,700,000 different kinds of animals alive to-day, ranging from the protozoa to man. Now let us compare the abundance of living animals with those of the geologic ages, and especially with the Jurassic period, of which life we have probably a better knowledge than of any time back of the Tertiary. The European Jurassic has long been divided into 33 zones (Buckman hints at a probable 100), and if we hold that each one of these times had only one quarter as many species as in the lowest estimate of the present world, there must have lived during the entire Jurassic something like 31,000,000 kinds of animals. Yet paleontologists have described not more than 15,000 Jurassic forms. The great imperfection of the extinct life record is thus forcibly brought to our attention, and we learn from these estimates that for each kind of animal preserved in the rocks more than 2,000 other kinds are utterly blotted out of the geologic record.
Much of this more apparent than real imperfection, however, is due to the vast number of insect species now living—animals that must have been comparatively few in the Jurassic, due in the main to the absence of flowering plants. From these figures, however, we must not conclude that the geologic record is equally imperfect throughout; for the paleontologist studying marine fossils well knows that he can not, as a rule, hope to study other than those kinds of animals that have hard and calcareous or siliceous external or internal skeletons. Of such there may be in the present seas about 250,000 kinds, of which about 25,000 have been named. Therefore on this basis we can say that the student of Jurassic faunas knows 1 species in every 54 of shelled animals that lived during this period.
This admittedly great imperfection of the life record needs to be further explained so that the reader will not arrive at the erroneous conclusion that modern stratigraphy rests upon very insecure foundations. The stratigrapher in determining the age of a given deposit, and in the identification of it from place to place and from country to country, and even across the great oceans, deals in his work not with quantity of species, but with comparatively small numbers of constantly recurring hard parts of certain species that are more often of marine than of land origin. Many of these forms have but local value but others have spread thousands of miles, and some of the long enduring species range over the greater part of the earth. Some of the best guide fossils in the Paleozoic are the brachiopods because they are present in nearly all the strata of this era. The writer in 1897 listed 1,859 forms then known from these rocks of North America. Of these about 28 per cent., or 537 species, had great geographic distribution. 117 species are found in the Rocky Mountain area, the Mississippi valley and the Appalachian region, and of these 36 are also known to occur in foreign countries. The number of species common to North America and other continents, however, is 121. It is upon faunal assemblages of this quantity and nature that the stratigrapher relies most in deciphering the former extent of the continental seas.
In the making of paleogeographic maps or in the determination of geologic time, using fossils as the essential basis, we have guidance in those of marine faunas, and the floras and faunas of the land and its fresh waters. Of these widely differing realms or habitats we now know that the fossils of the marine faunas are the more reliable not only because there are so many more of them than of the land dwellers, but more especially because their geologic succession is far more complete. The conditions of preservation, that is, appropriate burial in sediments, are always at hand in marine waters, but on the land entombment occurs only exceptionally, whereas the life of fresh waters is very meager and almost unchanging during geologic time. Then, too, marine life is "less affected by meteorologic factors, and more dependent upon conditions which affect the whole hydrosphere rather than small areas of it. The struggle for life is less intense, the food supply generally more adequate, enemies less vigorous, and dangerous fluctuations of temperature far less frequent, in the sea than on land. The same features make the land fauna more clearly indicative of minor divisions of the scale, and of the progress of organic evolution in the general region concerned; while less conclusive as to the contemporaneity of widely separated though analogous faunas."[1]
In regard to the probable geographic position of the shore lines we rarely have safe guidance in the fossils, and for this depend on the nature of the deposits. Greatest dependence is placed upon the geographic position of sandstones and especially on conglomerates to indicate the probable former shores. Limestones of uniform character and wide distribution are indicative of greater distance from land. Shallowness of the continental seas is proved by a rapid change in the character of the sediments both laterally and vertically, and by the oolite and dolomite deposits. Intraformational conglomerates, coral reefs, ripple marks, and shrinkage cracking furnish further evidence to the same conclusion. Storm waves are known to plough the present sea-bottom to depths of 160 feet. Calcareous muds are now forming in tropical and subtropical waters at sea-level around coral reefs, and elsewhere in these latitudes at depths from 200 to 600 meters. It is probable that all of the ancient great limestone deposits are of warm waters, and, if so, are an additional aid in discerning the geologic times and regions of milder climates.
Phosphatic concretions form in the littoral region where the temperature changes are rapid, as off the coast of the New England states, and periodically cause much destruction of the individual life. The carcasses decompose at the bottom of the sea, making nuclei for the accretion of phosphate of lime, and because of the irregular periodicity of accumulation come to be arranged in definite stratigraphic zones. Old Red sandstone fishes are also usually found in clay nodules but abundantly only in limited zones (Scaumenac, Canada and Wildungen, Germany). Have these also been killed by rapid changes in the temperature of these waters? In any event the fish-bearing beds are always found near the shore lines of Devonic seas.
Scour of sea bottom is met with in the present seas where great streams of water are forced through narrow passages, as the Gulf Stream in the Floridian area; or where such streams impinge against the continental shelf, as north of Cape Hatteras, or flow across submerged barriers "a few miles broad," as the Wyville-Thomson ridge connecting the British and Faeroese plateaus (Johnstone, 1908, 31). Strong currents preventing sedimentation also occur in long and narrow bays, as that of Fundy, where the undertow caused by the very high tides of this region sweeps the bottom clean. These exceptional and, after all is said, rather local occurrences can not be the explanation for the many known breaks in the geological sedimentary record, the disconformities of stratigraphers. These breaks are at times as extensive as the North American continent (post Utica break), and are usually of very wide extent. Scour of the bottom by the currents of the ancient continental seas will not explain away the presence of these truly land times, but it is to be sought in the oscillatory nature of the seas of all time which is probably caused by the periodic unrest of the earth's crust due to earth shrinkage. We agree with Suess that "Every grain of sand which sinks to the bottom of the sea expels, to however trifling a degree, the ocean from its bed," and every movement of the sea-bottoms and the periodic down fracturing of the horsts causes the strand lines to tremble in and out, be they of a positive or transgressive or of a negative or land-making character.
The ancient marine life had similar zoogeographic arrangement to that of the present. It can be grouped into local faunas and these combined into subprovinces, provinces and realms. Their distribution is governed primarily by the presence or absence of land barriers, and secondarily by temperature and latitude. In the present seas temperature is one of the main factors controlling the distribution of the species, but during the geologic ages the climate was, as a rule, far more uniform than now, as we are living under the influence of polar ice caps and a passing glacial period, or possibly even an Interglacial period.
The faunas with which the stratigraphic paleontologist works appear in many instances as suddenly introduced biotas. Our collaborators of half a century ago explained them as Special Creations, but since their time we have learned that the suddenly appearing faunas are not such in reality but only seem to appear rather quickly due to the slowness of sedimentary accumulation. Ulrich estimates that the American Paleozoic has less than 100 mapable units or formations, each with a duration of probably not less than 175,000 years. Accordingly, each foot of average sedimentary rock has taken not less than 833 years to accumulate. Our knowledge regarding the average rate of sedimentary marine accumulation is, however, as yet very insecure, and to make this clear some of the remarks made by Sollas, President of the Geological Society of London (1910), will be quoted. He was led to make these remarks after the reading of a paper by Buckman correlating the Jurassic sections of South Dorset. He said, "The correlation of thin seams with thick deposits was a matter of great importance. . . . It might afford some hints as to the order of magnitude of the scale of time. If we assumed that one foot of sediment might accumulate in a century, in an area of maximum deposition, then in the case of the seam two inches thick, which was represented by 250 feet in the Cotteswolds, the rate of formation would be less probably than 1 foot in 150,000 years." What Ulrich's estimate of time necessary for the accumulation of one foot of average sediment means to migratory faunas may be illustrated by the spreading of Littorina littorea. In the last century this edible European gastropod was introduced at Halifax, Nova Scotia, and in 50 years attained the Delaware Bay and north to Labrador. Taking this dispersion as the basis for calculating faunal mi- grations, we learn that they may spread 500 miles, while one sixteenth of an inch of average sediment is depositing, or 8,000 miles during the time of one foot of sedimentary accumulation. If, therefore, Paleo- zoic faunas migrated "only one fiftieth as fast as this living shell, then we may reasonably assert essential contemporaneity for stratigraphic correlations extending entirely across the continent." We have here an explanation for the apparently sudden distribution of the Ordovicic brachipod Rhynchotrema capax, that everywhere holds an identical geologic horizon from Anticosti to the Big Horns and from El Paso, Texas, to Arctic Alaska. Spirifer hungerfordi spreads during the first half of Upper Devonic time from the Urals to Iowa, and another brachiopod, Stringocephalus burtoni, migrates during the last third of Middle Devonic time from western Europe to Manitoba.
The life of the present seas extends from the strand-line to the deepest abyss, but by far the greatest quantity and variety lives in the upper sunlight, photic or diaphanous region. Photographically the light of the sun is detectable in exceptionally clear-water tropical seas to a depth of about 2,000 feet, but Johnstone places the average depth for all waters at 650 feet, beyond which there is more or less of total darkness, the aphotic realm.
Sunlight is the first essential for the existence of life. Where it penetrates, there plant life is possible, and this life is the substratum on which all animal life is ultimately dependent for food. Near the surface of the sea lives the plankton, sometimes referred to as the "pastures of the sea" and compared with the "grass of the fields." Most of this plankton consists of diatoms that at present are by far more prolific in the cooler polar waters. At times of greatest abun- dance in Kiel Bay as many as 200 of these "jewels of the plant world" are contained in a drop of water, and in the Antarctic seas there is an area of ten and one half million square miles where diatom ooze is accumulating. They are the principal food supply for most of the ses- sile benthos, or bottom life, among which the mollusca and brachiopods are of the greatest importance in paleogeography.
Geologic deposits rich in diatoms are sometimes regarded as those of the deep sea, at least as of deeper waters than those of continental seas. The English Carbonic deposits, rich in diatoms, have a fauna whose species are all of the shallow water kinds. The vast Miocene diatom deposits of California, described by Arnold, have living bottom types of foraminifera that, according to Bagg, do not indicate a depth of over 500 fathoms.
From the present distribution of marine life we learn that t greatest bulk of invertebrates are restricted to the bottom of the shallow seas within the depth to which sunlight readily penetrates, that is, a depth on the average not over 600 feet. The value of this observation to the paleogeographer and the student of fossil marine life lies in the confirmation of paleontologists that continental seas are shallow seas, to the bottom of which in most places sunlight permeates. These seas are to be compared with the littoral regions of the present oceans, and they are the areas that are most exposed to climatic and physical changes, due to their proximity to the atmosphere and the lands. The life of these waters is, therefore, subject to an environment that is more or less changeable, and one of the basic causes underlying organic change. It is the invertebrates of the littoral and shallow seas that the paleontologist studies.
In the tropical and subtropical shallow seas one meets with the greatest variety of life and with the brighter colored and more ornamental shelled animals, but we are much surprised when told that the greatest number of individuals occur in the colder shallow waters of the temperate and polar regions. Johnstone states, "There is little doubt that the distribution of life in the sea is exactly opposite to that on the land. The greatest fisheries are those of the temperate and arctic seas. . . . Nowhere are sea birds so numerous as in polar waters. The benthic fauna and flora are also most luxuriant." The Bay of Naples has a "richly varied, but (in mass) a scanty fauna and flora," and "at the very least the amount of life in polar seas is not less than in the tropics."[2]
Marine life is also more prolific near river mouths of the temperate zones, probably because of the great quantities of dissolved "salts of nitrous and nitric acid and ammonia, and other substances which are the ultimate food-stuffs of the plankton." Just outside of the estuary of the Mersey in Lancashire there were "not less than twenty, and not more than two hundred animals varying in size from an amphipod (one fourth inch long) to a plaice (eight to ten inches long) on every square meter of bottom" (Johnstone, 1909: 149, 176, 195-6). Finally the quantity of life in the shallow waters of the sea is not directly governed by favorable habitat, such as shallow sunlight waters in constant circulation and of equable temperature, but seems to be primarily controlled by the amount of the minimal food elements. Sea-water may be regarded as a dilute food-solution having the essential materials on which life is dependent. Of these nitrogen and the compounds of silica and phosphoric acid are present in the smallest amount. Johnstone tells us that "The density of the marine plants will therefore fluctuate according to the proportions of these indispensable food-stuffs" (234). "It is only the protophyta among the plankton which can utilize the CO2 and the nitric acid compounds, and so we see that upon these rest the greater part of the task of elaborating the dissolved food-stuff of the sea" (239).
Undoubtedly much of the land-derived nitrogen, estimated at 38 million tons per annum, is used up in the shallow areas by the plants. TVe therefore arrive at the conclusion that shallow seas bordering naked, cold, or arid lands should have the smallest amount of life, and that those of temperate regions adjacent to low lands under pluvial climates should have the greatest number of individuals. This conclusion, however, may be decidedly altered by the oceanic currents in that they distribute far and wide the salts of the sea.
These factors also suggest that during "critical periods" the faunas should be least abundant and varied, and that at the times of extreme base levels and sea transgressions they ought to be at their maximum development. These suggestions are borne out by the small Cambric, Permic and earliest Eocene faunas and the large cosmopolitan biotas of the Siluric, Jurassic and Oligocene times.
Sessile algæ are not common on muddy or sandy grounds, and these areas in the present seas have been compared with the desert areas of the lands. That muddy grounds are now nearly devoid of algous growth has particular significance in stratigraphy, because in the geologic column at many levels and in nearly all regions occur black shale formations that are not only devoid of plant fragments but are also usually very poor in fossils of the sessile benthos. When the latter are present it is seen that they are usually thin-shelled and small forms, or are types of organisms that live in the upper sunlight realm and are either of the swimming plankton or the floating nekton. As examples of such deposits may be cited the widely distributed Utica formation of the Ordovicic extending from southern Ohio to Lake Huron and east to Montreal, and the Genesee (Devonic) of New York. In these cases what appears to be of the sessile benthos is thought to belong to the nekton attached to floating seaweeds or other floating objects, and eventually all of the life of the nekton and the plankton sinks to the bottom of the sea. Therefore the carbonaceous matter of the black shales may be of algous origin like that of the New York Genesee, but it is far more probable that it is largely of animal origin, as the crude petroleum of such deposits usually has the optical properties of animal oil and especially those of fish oil.[3] Plants may be torn from rocky bottoms of the shallow areas by the action of the storms and then carried by the currents into eddying areas like the present Sargossa Sea, which has among its algæ a very characteristic assemblage of animals. It is probable, however, that black shales having wide distribution were more often the deposits in closed arms of the sea (cul de sacs), or when of small areal extent, as the result of fillings of holes in the sea bottom. In all such places there is defective circulation and lack of ox}-gen resulting in foul asphixiating bottoms.
These are the "halistas" of Walther and the "dead grounds" of Johnstone. To-day such are the Black Sea and the Bay of Kiel, where sulphur bacteria abound in greatest profusion. These decompose the dead organisms that rain from the photic region into such suffocating areas, or the carcasses which are drawn there by the slow undertow from the higher ground. These bacteria in the transforming process deposit in their cells sulphur that ultimately combines with the iron that is present and replaces the calcareous skeletons of invertebrates by iron pyrite or marcasite. In this way are formed the wonderfully interesting pseudomorphs of Triarthrus becki, the Utica trilobite preserving the entire ventral limbs, and of the other well preserved but small invertebrates from the Coal Measures black shale of Danville, Illinois.
Brackish-water and especially deep-sea shelled animals tend to have thin shells, while increase of salinity tends towards the thickening and roughening of the calcareous shells. It is a well known fact that in the dolomite-depositing continental seas like that of the Guelph (Siluric). all of the molluscs have ponderous thick shells. These have been interpreted as reef-living species but actual reefs in the Guelph are unknown. The molluscs are often common but corals are represented by but a few species. Similar conditions are known to occur in other dolomite faunas. Further, the Guelph was of a time of decided progressive emergence and restrictional seas under an arid climate, and therefore the waters must have been abnormally salty.
Rivers constantly discharge into the sea great quantities of plant material, but as a rule little of it other than the wood is swept far out to sea. At present the rivers of northern Siberia float into the sea vast numbers of logs that drift with the currents to Spitzbergen, East and West Greenland and Arctic America. This wide dispersal of wood by the sea is met with only in the cold regions, whereas in tropical waters the wood is rapidly decomposed. Single leaves are rarely transported far from their place of origin, and when of good preservation in geologic deposits, give decisive evidence of the nearness of the shore. On the other hand, tough palm leaves have been seen in the sea 70 miles from land and rafts of leaves are often met with 200 or more miles beyond the mouths of the Kongo and the Amazon. Proximity to shore is also indicated by the presence in marine faunas of land molluscs, insects and bones of land vertebrates.
With tillites now known in the Lower Huronian of Canada, in the Lower Cambric of northern Norway, China, South Africa and Australia, and in the Permic of India, South Africa, Australia and Brazil, we observe the recurrence of glacial climates. The Siluric and Devonic coral reefs occurring in Arctic regions, the sponge, coral and bryozoa reefs in the Jurassic of northern Europe, the rudistid and other cemented pelecypods in reefs of wide distribution in the Cretaceous, and the almost world-wide distribution of the Nummulitidæ (north of Siberia) in the late Eocene and Oligocene point as clearly to warm waters and mild polar climates. Further the widely distributed Carbonic foraminifers of the family Fusulinidæ that swarmed in temperate and tropical regions are unknown to Arctic and Antarctic regions. In other words, long before we have a fossil record the earth had climatic zones, and for long periods the climate was mild to warm, punctuated by shorter intervals of cold to mild climates.
The volume of sea water to-day is very great, but we must ask ourselves: Has this quantity always been such or was it even greater, as some geologists still hold? We no longer agree with Laplace and Dana that the earth passed through an astral stage, but rather agree with Chamberlin that it always has had a more or less cold exterior. Through volcanic activity much juvenile water from the interior of the earth was extruded in geologic time and was added to the vadose waters of the surface. Suess states that "the body of the earth has given forth its oceans and is in the middle phase of its gas liberations." Accordingly, the Paleozoic oceans must have been quantitatively smaller than those of the present, and the gradual increase in the volume of vadose waters has been accommodated by the periodic increase of oceanic depth.
We also agree with Walther that the oceans of Paleozoic and earlier time did not have the great abyssal depths they now have. The accentuated deepening of the permanent oceanic basins did not begin until the Triassic, for in none of the great depths of the present oceans are found traces of Paleozoic organisms, and all here are of Mesozoic or Tertiary origin. In the shallow regions, however, are still found a few Paleozoic testaceous-bearing genera of brachiopods, tubicular annelids, pelecypods, gastropods, Nautilus, and Limulus. The deepening of the Pacific, the Indian, and especially the Atlantic oceans has been at the expense of the lands or horsts, for the ancient continents, Gondwana and Laurentia, have each towards the close of the Mesozoic been broken into several masses. We may therefore speak of permanent oceans, and transgressed, fractured, and partially down faulted, continents or horsts.
These are some of the factors that control the making of some of the modern paleogeographic maps.