Popular Science Monthly/Volume 77/October 1910/Paleontology and Isolation
| PALEONTOLOGY AND ISOLATION |
By Dr. JOHN M. CLARKE
STATE MUSEUM, ALBANY, N. Y.
THE notion of isolation as a factor in variation, as I am using the term, is that of geographic separation exclusively, the conception expressed most clearly by Wallace, Moritz Wagner and Jordan. I take it that while this influence has been carefully estimated in the geographical distribution of living species, it has not often been expressed in its own terms in the analysis of extinct faunas. With increasing accuracy in the record of ancient continental lines and barriers, we are coming to a point where the efficiency of this factor can be safely taken into account. The outcome of free interbreeding, as Jordan has pointed out, is to unify species and obliterate variations. Per contra, isolation checks this process and gives freer play to tendencies arising from other factors in variation. The effect is thus, as a general rule, negative, but expresses itself freely enough in geographic provinces severed by some barrier or condition which has the effect of a barrier. Among existing species the formative effects of segregation have been very largely illustrated from restricted areas such as the subdivisional valleys and forests of Hawaii with its distinctive forms of the Helicidæ and other terrestrial snails—a case that is paralleled in paleontology by the snails of Steinheim. But the effect is to be reckoned with in larger or continental areas between which there has been at one time opportunity of interchange, especially in the case of marine species, with which we chiefly deal, along the epicontinents.
I have particularly in mind phenomena which have been brought to my notice by a somewhat extended study of the Devonian faunas of the southern hemisphere and the broader application of the factor is best enforced and illustrated by this instance. I may say that this broader notion seems to be that entertained by Darwin so far as he specified the conception of geographic segregation as an element in natural selection and it was his work in South America that formed the basis of his conclusions.
With other students we recognize the existence during the Devonian of austral continental lands which have been variously designated and variously outlined. By some this land has been posited as a north and south Atlantis lying in the meridional axis of the present ocean, by others a broken land mass partly crossing the southern Atlantic from east to west. But now we begin to see its continuity and the extent of its strands, with something of its changes in outline during its early history. It was the precursor and the nucleus of Gondwánaland. With it began, so far as we now know, the long history of that continental land and the successive records of life developing under continued conditions of geographic isolation from the northern strands.
From Argentina, Bolivia and northern Brazil we have very lucid evidence, on the basis of paleontology, that in the late Silurian the shore lines were continuous with those of the north. We have no dependable knowledge of these earlier faunas at the east and indeed their entire absence is indicated by stratigraphy; but with the submergence of the Silurian at the west, there entered from the African east upon this south Atlantic field, a positive diastrophism whose axis was well nigh normal to that of the present Atlantis, and along the shores of this growing land bridge entered an invasion of marine life at the opening of the Devonian time. It seems to have come westward from a dispersion area in Africa and it evidently disseminated itself without interruption of continuity from the strands which now, as the Bokkeveld beds of Cape Colony, constitute the only evidence of marine life in the South African Paleozoic, to those of the Falkland Islands, two far distant regions which have much more of organic content in common than do the Falklands and the nearer regions of Parana, Argentina and Bolivia.
This fauna with its special and peculiar features is, however, spread through Bolivia, western Argentina, southern Brazil, including Parana and as far north as Matto Grosso, thence eastward by way of the Falklands to South Africa. From the boreal strands of the period it was separated by a barrier, often narrow and constituted only of deeper water, so that of the boreal Devonian we find no evidence much south of the equator in Brazil nor of the austral Devonian north of that line. This barrier I believe to have been overpassed at times during the early part of the Devonian by species which are of wider distribution south and north but these passages seem to have become rarer as time passed and as more complete geographic isolation was effected.
There are many evidences in this southern fauna that the land bridge was accompanied by insular strands which are evidenced by varying percentages in community of species and by bathymetrical variations. Apart from these possible island masses, there was clearly a Devonian land bridge extending from South Africa to the Falklands, westward into Argentina and northward into Bolivia, embracing also as continental or island lands parts of the states of Paranà, Matto Grosso and even of Pará.
By virtue of the evident derivation of the fauna of this time from the east along newly forming strands which were, throughout the period of the Devonian, kept asunder from the Atlantic-European lands at the north, and by its further development under conditions of isolation, the fauna presents fundamental contrasts to any development of the Devonian elsewhere in the world. It is in itself a unit and a unit also in relation to the sediments in which it is involved. There is no earlier Devonian in this southern region nor is there any later Devonian, for wherever the succession has been determined this austral fauna, bearing no evidence in itself of a later time stamp than early Devonian, is overlain by Carboniferous deposits without demonstrated unconformities between. Deposits and faunas which at the north we are accustomed to regard as of later Devonian age, are absent at the south, either because this austral land was broadly above the sea during these stages and its strands now lie buried or, as seems much more probable, this sedimentation represents the total Devonian sedimentation and this fauna the total Devonian fauna at the south.
I can not in this place analyze the peculiarities which give the austral fauna of these "Falklandia" strands their special impress but I may specially cite the trilobites which are astonishingly developed. I presume any competent student of northern faunas, being shown a series of these without knowledge of their origin, would pronounce them of early Devonian age and yet they are neither northern species nor, in any large degree, northern genera. While they bear the impress of boreal genera and resort to morphologic equivalencies thereto in fugitive epidermal structures which so richly characterize the boreal trilobites at this time, they are on the whole constructed on a series of modified types which hold their fundamental expression while developing minor details with the chronology normal to their succession at the north. The Phacopes are seldom true Phacopes, the Dalmanites seldom true Dalmanites, yet the same structural decorations and extravagances we are familiar with at the north, are distributed freely through the group. This is all equally true, in qualifying terms, of the other groups of this fauna, save for the fact that in these we can hardly venture to insist so entirely on generic distinctions south and north. The species differences declare themselves on every hand and taken as a whole the fauna presents fairly conclusive evidence of having derived its distinctiveness through its isolation from the boreal fauna from which it ancestrally took origin. Yet while it has developed this character it has also proceeded to maintain a faunal composition which declares its age, and a morphological stamp which shows that it developed all its parts in the proper time and place in the series.
In predicating geographic isolation as the prime factor in this regional development of the Devonian fauna, its efficiency should not be made to seem qualified by an illustration which is striking by virtue of its contrast with the already well known. There are evidences in plenty that geographic isolation has played a similar rôle with even more diverse effect in the development of the boreal faunas of the same geologic stage. The north Atlantic land bridge was continuous at this time, as evidenced not alone by the presence of the Coblentzian fauna in the Atlantic coast rocks but by an array of additional facts; and it seems very probable that the primary movement of these northern faunas was from the same African dispersion area as that of the south.