Popular Science Monthly/Volume 80/April 1912/Ancient Portals of the Earth
|ANCIENT PORTALS OF THE EARTH|
STANFORD UNIVERSITY, CALIFORNIA
Portals Defined.—A study of the distribution and relationships of ancient marine faunas shows that there have been certain critical areas through which these faunas were connected from time to time. These critical areas are depressions on or between continental masses, and are invariably regions of permanent instability of the earth's crust, where mountain-making and the accompanying volcanic and earthquake disturbances have been prevalent.
When these areas were depressed below sea-level, they formed straits, or channels, connecting sea-basins, and affording avenues for intermigration of marine faunas. When they were elevated, they formed barriers impassable to the dwellers in the sea. Thus neither the name strait nor barrier is applicable to them as a general term. Therefore the name portal is selected, as indicating a gateway that may be either open or closed, and still retain its identity.
Of the portals that were important in the ancient world only three are now open: the North Pacific portal, of which Bering Strait is a shrunken remnant; the Iberian, still recognizable in the Strait of Gibraltar; the Malaysian, seen in the inter-island passages in the East Indian Archipelago. And one has shifted its position, growing from an arm of the sea into the noble expanse of the Indian Ocean.
One still shows its nature as a portal in the narrow strip of land joining the two Americas. The others are now concealed as parts of continental masses, namely the Crimean, the Asia Minor portal, and the Bokharan, revealing their nature as former arms of the sea only in the extinct marine fossils now buried in their sediments.
Still other bodies of water that loom up large in our present-day geography, as the North Atlantic and the South Atlantic, too wide to be called merely passages, or to be differentiated from the main ocean by a special name, did not even exist in the ancient days.
These great changes have been wrought chiefly by the world-wide Tertiary mountain building, and the accompanying, or causing, disturbances of the continental areas. Also there was much readjustment in the late Paleozoic topographic revolution, and in the late Jurassic Cordilleran revolution.
Paleogeography.—In recent years the reconstruction of ancient physical geography has been a favorite field of research, not to say of speculation, of geologists; paleontologists, too, have taken their part in this, since ancient continents can be outlined only by the distribution of the faunas and floras of the land, and ancient seas can be traced only in the remains of petrified inhabitants of the waters.
In the Book of Genesis we read: "And God said, Let the waters under the heaven be gathered together into one place, and let the dry land appear; and it was so. And God called the dry land Earth; and the gathering together of the waters called He seas: and God saw that it was good." This is certainly the shortest account of the origin of continents and seas, and probably as good as any. But we do not any longer think that it all happened at one time.
Since we can know definitely the geology of only that part of the earth which is now land, and since we know the underlying strata of only a small part of that, we can only conjecture concerning the history of regions now buried under the oceans. No one man, nor group of men, is qualified to make a dogmatic statement as to the origin of continents and seas, such as that quoted above from Holy Writ. There is ample room for differences of opinion, starting from the same facts. Consequently, geologists and physical geographers are divided into two camps. One holds that the major divisions of land and sea were always as they now are; this is the doctrine of permanence of continental plateaus and oceanic basins. The other group advocates the idea of constant change in the position of land masses and oceanic troughs. To them the grand features of the earth do not bear the marks of hoary antiquity, but are youthful characters, due to rather modern diastrophism of the crust.
The truth probably lies somewhere between the two extremes, and the differences between the two camps consist rather in statement than in fundamental doctrine. Even the most conservative upholders of the theory of permanence admit that some of the continental areas have been covered, in the past, by seas of almost oceanic size and depth. And the most radical advocates of the shifting of lands and seas believe that some of the continental masses have always been continents, and that some of the great depressions have always been oceans.
Further, it becomes plainer, as paleogeographic studies go deeper into the history of the earth, that the dominant ancient features are not obliterated by later changes, but are merely obscured. Continents that were dismembered have been united again; seas that existed in the early days have recurred. Which is to say that whether continental plateaux and oceanic troughs have been permanent or not, the regions of diastrophism have been permanent, that when crumpling and dislocation of the earth's crust have once started, they have kept up with recurrent activity all through the succeeding ages.
The ancient portals all lie in regions where the Tertiary mountain folds touch the great lines of crumpling begun in the late Paleozoic topographic revolution. The present distribution of continental teaus and oceanic troughs dates from the Permo-Carboniferous revolution, although the North Atlantic and the South Atlantic were not occupied by the sea until early Tertiary time.
During Cambrian time the Bering, Central American, Iberian, and Asia Minor portals were open, probably corresponding to regions of Pre-Cambrian folding. The others had not yet come into existence, or at any rate information is lacking concerning them. In the Lower Silurian we know of the existence of the Bering portal. During the Devonian the Bering and the Asia Minor portals were open all the time, the Crimean part of the time, while the Bokharan portal was merely part of the open Asiatic sea. During the Carboniferous era all the major portals were open at times except that leading down to Madagascar. Somewhere near the border between Coal Measures and Permian the Paleozoic topographic revolution inaugurated centers of distribution and portals connecting them that held sway during nearly the whole of Mesozoic time.
If diastrophism should be the final arbiter of the division of geological time, then on the basis of physiography the Permian, including the Artinsk stage, should be included in the Mesozoic. For while Permian life is distinctly more closely related to that of the Paleozoic, Permian physiography is like that of the Triassic.
Interregional Faunal Zones.—It is customary to speak of cosmopolitan faunas in the past ages, but this term is properly applicable only to the Cambrian and Silurian, when local differentiation had not yet caused the extremes of later times. And even in the Cambrian, as the scanty faunas become better known, provincial differences appear. In Devonian time provincial distinctions were already well developed, and the invasion of a region by an exotic fauna is easily recognized. The first interregional migration that is definitely known occurred early in the Upper Devonian, when the American waters were invaded by a fauna that could not have sprung from its predecessors in that region, but was endemic in Eurasia. This is the zone of the Cuboides fauna, which was followed by still further immigration from the same center of dispersion, in the zone of Manticoceras intumescens of the Upper Devonian. The connection with Europe was through the back door, through northern Siberia, across North America, for the continental mass of North Atlantis and Appalachia prevented direct communication.
With the opening of the Carboniferous age the subsidence of the southern part of Appalachia allowed direct intermigration between the waters of western Europe and the Mississippian Sea through the Poseidon basin. Here we find the faunal zone of Aganides rotatorius common to the two regions, but unknown anywhere else. This is the first direct invasion of the American seas by a population from the western Tethys, or ancient Mediterranean basin. In the latter part of the Lower Carboniferous epoch another migration from western Europe to the Mississippi basin took place; this was in the zone of Goniatites striatus, which, as was the case with the preceding zone, did not extend beyond the Mississippian Sea. At this time in the western part of North America, the Great Basin Sea was connected with northern Asia, and through that region with Europe. The Pacific region lacks the fauna of Goniatites striatus, but has, instead, that of Productus giganteus. Here, for the first time, we see a sharp differentiation into Mediterranean and Pacific types of faunas, separated by the land barrier of the Rocky Mountain area.
In the age of the Coal Measures we find further evidence of continued invasion of the Mississippian Sea by immigrants from the western European Tethys, in the faunal zone of Gastrioceras Listen. This gi'oup was common in the shallow epicontinental expansion of the old Mediterranean Sea, and reached America along the shores of the Poseidon Ocean, the Paleozoic ancestor of the modern Atlantic, and into the Mississippian basin through the Gulf of Mexico, up into Arkansas, Texas, Oklahoma, Missouri, Kansas, Illinois, etc. But the western sea of the Great Basin was dominated by a fauna from the boreal waters, which came down through the Bering portal, along the old shore line of northwestern America. The Mediterranean fauna extended eastward through the Tethys to Sumatra, and up into the Ural Mountains, showing that the Iberian, the Bokharan, and the Asia Minor portals were all open. The Malaysian portal was apparently closed, and the Madagascar geosyncline as yet shows no evidence of its existence.
The Artinsk, or Permo-Carboniferous transition, fauna has approximately the same regional distribution as that of the zone of Gastrioceras Listeri, except that it reached further northward, to Nova Zembla. But while the Carboniferous cephalopod fauna may be considered as Mediterranean in origin, the Artinsk fauna probably came from further east; its real home seems to have been on the border between Asia and Europe, for there it is most abundantly developed, and bears the closest resemblance to its predecessors.
At this time the northern Asiatic-Pacific fauna had a different character, but is as yet little known. In America it is known in California, and in the southern embayment that stretched from the Pacific to western Texas. It is called the Guadalupian fauna, from Guadalupe Mountains in Texas, where it was first described. In Texas these two faunas, the Artinsk and the Guadalupian, come within a few hundred miles of each other. The east and the west, the Atlantic and the Pacific, were separated then, as they were during most of the Carboniferous.
On the west, the Guadalupian or Pacific fauna, penetrated the Great Basin Sea, but did not extend far inland on the continent, the high lands of Utah and Idaho still separating it from the Cordilleran extension of the Mississippi sea.
During at least a portion of the Permian the Bering portal was open, the Central American closed, the Iberian, Asia Minor, and Bokharan portals open, and the Malaysian portal closed, for the characteristic cephalopod fauna, with Medlicottia and its associates, occurs in Texas, Sicily, the Ural Mountains, Nova Zembla, the Himalayas, and Timor in the Indian Archipelago. The Pacific, or Guadalupian type, on the other hand, is distributed from Japan around the old Pacific shore line to California, and in another gulf across the more southerly part of the American continent to western Texas.
At the end of the Paleozoic era there was much mountain-making, and readjustment of physiography. These disturbances separated some regions that had been united, and joined others that had been divided. In the earliest epoch of the Lower Triassic, the zone of the Meekoceras fauna, the Asia Minor portal, through which the Artinsk fauna had migrated between the western Mediterranean and the Oriental Tethys, was closed, while the Malaysian portal into the Pacific was open. The barrier between the Atlantic and the Pacific still existed in Central America. The Meekoceras fauna is distributed from Spitzbergen, down to India, eastward to Timor in the Indian Archipelago, with a southward arm of the sea extending down to Madagascar, connecting with the Pacific, stretching along the Siberian coast at Wladiwostok, and then across to Idaho and California in the Great Basin Sea.
The next fauna in the Lower Triassic, that of the Tirolites zone, is known only in the Mediterranean Region and in Idaho, the connection being from the Mediterranean-Poseidon Ocean to the Pacific, through the Central America portal, which was temporarily opened by subsidence in that region. All the other portals were closed, so far as we have any information concerning them, this conclusion being based on the provincial character of the faunas of the seas.
In the epoch immediately following, the Columbites zone, is seen the same provincial, or restricted distribution of inhabitants of the seas. The Columbites fauna is found in Idaho, in northern Siberia, and in Albania, but not in India, nor the Oriental Tethys. At this time the Arctic Sea was the center of dispersion, and immigrants went southwestward to Albania, and southeastward to Idaho, but did not reach the Indian waters, in which a different group of inhabitants lived.
In the Middle Triassic the Asia Minor portal and that of Central America were reopened, and the Ceratites trinodosus fauna, with the Mediterranean as its center of dispersion, was distributed westward through the Poseidon-Atlantic to Idaho, and eastward to India. There was also some connection northward to Spitzbergen, and southward to New Zealand, for the binular genus, Daonella, is represented by nearly identical species in all these regions.
In the Karnic horizon zone of Tropites subbullatus, in the Upper Triassic, the same connections still existed, with even closer relationships of widely separated faunas. A large proportion of the Karnic species of India is identical with the Mediterranean fauna, and in California about one third are common forms in the Alps and Sicily.
Immediatelly above the beds with Tropites suhbullatus in India, the Alps, and in California occur massive limestones full of coral reefs, with some few identical species in all three regions. This same group of ancient reef-builders has also been found in Alaska, in the same stratigraphic position. This shows that the wide distribution of the Upper Triassic species was made possible by the nearly uniform distribution of warm water over a great part of the globe.
In the latter part of the Upper Triassic widespread physiographic disturbances had again divided the great zoologic regions. The Atlantic-Pacific connection had disappeared, but there was still free communication between the Alpine province and the Orient, for Mediterranean species reached as far as India and Timor. Around the North Pacific, with the Arctic sea as its center of dispersion, a different fauna, that of the bivalve, Pseudomonotis ochotica, was distributed. This extended from northern Siberia southwestward to the Crimea, southeastward to the Great Basin Sea, and southward along the shores of Asia through Japan to New Zealand.
In the Lower Jurassic we find a recurrence of the same conditions that prevailed in the Karnic epoch, free communication through the Central American and Asia Minor portals, and between the Pacific and the Oriental Tethys, with nearly identical species ranging from the far north to the south temperate region in Argentina, and from the Mediterranean eastward to Timor and westward to California.
In the Middle Jurassic similar geographic relations continued, with clear evidence of a warm climate from Franz Joseph Land to Madagascar and Argentina, and no division into climatic zones. Cycads flourished on the land, and corals built reefs in the seas. This was too good a state of affairs to last long in this world, and there soon came a change. The Central American portal was closed, as was that between the Pacific and the Tethys. The Arctic Sea became the center of dispersion of a boreal fauna, which made its way down to Russia on one side, and to California and Mexico on the other. This arrangement of the geographic provinces reminds us strongly of that during the Triassic epoch of Pseudomonotis subcircularise and it continued into the Cretaceous period.
The Upper Cretaceous is signalized by the closing of the Bering portal, and the wide distribution of a tropical fauna from the Indian waters westward to the Mediterranean waters, and eastward through Japan, around the North Pacific shore-line to California. This same fauna was also distributed from the Mediterranean westward through the Poseidon Sea to Mexico, and southward into South America. And a southern arm of the Oriental Tethys carried the same assemblage of forms down to South Africa.
The early Tertiary Eocene epoch witnessed the reopening of the Central American portal, and the immigration of the Mediterranean Venericardia planicosta fauna into the Pacific waters of the Californian province. This is the last true interregional zone in the history of the earth, and it is the last time when widespread warmth of the waters made such distribution possible. Since that time the Central American and the Asia Minor portals have been closed, and the continually increasing demarcation of climatic zones have prevented migration between the greater sea basins through northern waters. The later Tertiary epoch witnessed the inauguration of modern conditions, and the geographic regions were restricted as they are now.