Popular Science Monthly/Volume 77/July 1910/The Migration and Shifting of Devonian Faunas
| THE MIGRATION AND SHIFTING OF DEVONIAN FAUNAS |
By Professor HENRY S. WILLIAMS
CORNELL UNIVERSITY
IN the year 1881 I presented before the American Association for the Advancement of Science the first definite announcement of the theory of recurrent faunas, applying it to the fauna of the Marcellus, Genesee and Ithaca black shales of New York, which I then conceived to be represented by the continuous fauna of the black shales of Ohio, Indiana, Kentucky and Tennessee; and also in the same paper the theory of shifting of faunas was applied to the Hamilton and Chemung faunas of central New York.[1] Since that time a large amount of evidence has been accumulated confirming these hypotheses.
The two hypotheses are correlated. Recurrence, or the departure of a fauna, its replacement by another and its final reappearance in the same section at a higher level, become the facts upon which the hypothesis of shifting of the faunas is based; and only on the assumption of the continuance and shifting of a fauna without losing its characteristics can we satisfactorily explain its recurrence.
The following facts are among the more important which have come to light in the course of my studies:
§ 1. The Catskill sedimentation was shown to be thicker and to start lower down in the geological column in eastern New York than in middle and western New York. In eastern New York it began while the Hamilton marine fauna was still present and cut it off, bringing in estuarian conditions with a brackish water and land fauna and flora. In middle New York no Catskill sedimentation is present until after the arrival of the Chemung fauna; and in western New York no trace of the Catskill type of sediments appears till after the close of the Devonian.
These facts are direct evidence of shifting of the environmental conditions of the edge of the continent westward as the deposits of the middle and upper Devonian were being laid down. With this shifting westward of the off-shore conditions of the sea, there went on a corresponding shifting of several faunas that were adjusted to each phase of those conditions.
These facts were stated in a paper on the classification of the upper Devonian published in 1885.[2]
§ 2. The Appearance of Dominant Species of a General Fauna in Reversed Order of Succession at the Close of a Fossiliferous Zone.—The case of Spirifer lævis in the Ithaca Zone and of the frequent appearance of Leiorhynchus at the opening and close of a fossiliferous zone were among the earliest observed facts suggesting the actual shifting of the body of the fauna entering the area in one order of succession and its departure in the reverse order. In the Ithaca section there occurs at the base of the fossiliferous zone of the Ithaca member a bed containing abundance of Spirifer (Reticularia) lævis. The discovery of the same species at the top of the fossiliferous zone as the normal Ithaca fauna become sparse gave the first suggestion that the faunas were moving or shifting. The Reticularia zone marked the first trace of the fauna to enter and the last to leave the area. Confirmatory evidence was also found in the order of succession of the dominant species of the Ithaca fauna. These facts were reported in 1883.[3]
§ 3. The study of the mode of occurrence of Leiorhynchus still further drew attention to the definite order in which series of species came in and went out of any given area. The species of the genus were generally found abundantly at the base or at the top of the fossiliferous zones rich in the brachiopods in the midst of which Leiorhynchus was rare.[4]
§ 4. The reappearance in a single or few strata of several representatives of an earlier fauna long after the formation to which they were normal had ceased.
Slight traces of this fact were observed in the first survey of the Devonian section passing through Ithaca, reported in 1883, Bull. 3, U. S. G-. S., and the fauna ~No. 14 N (p. 15) was called a recurrent Hamilton fauna because of the appearance there of such species as Spirifer fimbriatus, S. augustus, Pleurotomaria capillaria and others; and higher up in the midst of the Chemung section at Chemung narrows Tropidoleptus carinatus and Cypricardella bellistriata, Phacops bufo and Dalmanites calliteles were found.
The discovery of such traces of an earlier fauna led to further search; and as the evidence accumulated an elaboration and definite formulation of the theory of recurrence of faunas was made which has been set forth in several papers, and is illustrated in detail in the folio of the Watkins Glen-Catatonk quadrangles, which is now in press, for the U. S. Geological Survey (December, 1909).
The facts there brought out are substantially as follows: There are exhibited in the sections mapped for the quadrangles two series of fossiliferous zones; the separate zones of the two series alternate in succession; the zones of one series dominate the western sections of the area and thus thin out or disappear on tracing them eastward; the zones of the second series dominate the eastern sections and particularly the whole eastern New York sections, but thin out westward and in some cases are entirely wanting in sections west of the Watkins Glen quadrangle. The first set of faunal zones includes the faunas of the Genesee shale, the Portage formation and the several divisions of the Chemung formation.
The second set of zones includes the Hamilton fauna proper and recurrent representatives of that fauna which I have named the Paracyclas lirata zone, the Spirifer mesistrialis zone, the Leiorhynchus globuliformis or Kattel Hill zone. These zones are represented by the typical Ithaca group of Hall in its typical sections at Ithaca; and above them appear the first, second and third recurrent Tropidoleptus faunas (which I originally named the Van Etten, the Owego and the Swartwood Tropidoleptus zones, respectively). All of these several fossiliferous zones of the second set become decidedly thin on passing westward across the region. The Ithaca fauna is, occasionally, detected west of the Watkins Glen quadrangle, but is confined to less than 100 feet thickness at Watkins, is recognized for three hundred feet at Ithaca and ranges through at least 600 feet along Tioughnioga River.
Only a slight trace of the Paracyclas zone is seen as far west as Ithaca, but it is well expressed in the section on the east side of the area. The Van Etten, Owego and Swartwood Tropidoleptus zones appear in thin tongues of strata as far west as the Waverly quadrangle and are seen in occasional traces as far west as the Elmira quadrangle. When followed eastward they appear to blend together as a modified Hamilton fauna sparsely appearing in the strata up to the income of the Catskill type of sedimentation.
Where the Hamilton recurrent zones are seen in sharpest expression the recurrent species range through only a foot or a few feet of strata, hold in abundance four or five characteristic Hamilton species such as Tropidoleptus carinatus, Cypricardella bellistriata, Rhipidomella vanuxemi, Spirifer marcyi and Delthyris mesacostalis (= D. consobrinus) and others; and the Owego and Swartwood zones appear in the midst of a characteristic Chemung fauna both above and below them. In the Owego recurrent zone both Phacops rana and Dalmanites calliteles occur.
The Van Etten recurrent zone lies entirely below the range of Spirifer disjunctus and associated species of the Chemung formation. On following the sections eastward from the Waverly quadrangle the species of the Chemung fauna become scarce, and east of the Chenango River very few species of the typical Chemung fauna have been detected—although they are still abundant in the Chemung rocks to the southeast and southward across Pennsylvania, Maryland and Virginia.
§ 5. These facts have been interpreted as evidence not only of a general shifting of faunas coincident with a rising of the land along the eastern edge of the present continent, but of oscillation of conditions and alternate occupation of the area by two sets of faunas coming from opposite directions and temporarily living in abundance in the area of central New York.
§ 6. The lithologic changes in the sediments containing the different faunas are not sufficient to account for the change in fauna. In quite a number of sections there is no appreciable difference in lithologic constitution between the strata which for a hundred feet thickness have been filled with characteristic Chemung species and the immediately following thin zone (of a foot or two) with scarcely a trace of the Chemung species, but holding, in great number, species which if found by themselves would be undisputed evidence of the Hamilton formation.
§ 7. It becomes necessary therefore to suppose that the controlling cause determining the presence of one or other fauna is not the character of the bottom on which the sediments which preserved the fauna were laid. We are thus led to conclude that the qualities of the ocean water have determined the shifting or migration of the faunas. The conditions to which the faunas were adjusted were evidently those of depth, salinity or temperature of the waters in which the species lived; and their change of habitation was occasioned by change in the direction, path or extent of flow of oceanic currents.
This leads us to consider the principles of migration as affecting marine organisms.
§ 8. Migration of Species and Shifting of Faunas.—Migration as commonly applied in natural history means the movement of large numbers of the same species from one place to another in a general definite direction at more or less regular periodic times. So birds migrate northward with the advance of warm weather; some fish migrate from sea up rivers in breeding seasons; pigeons fly eastward or westward in great flocks, or grasshoppers invade a rich country devouring the vegetation in their path, or lemmings migrate across country in great quantities.
The term in these cases has to do with movements of one kind of animal in relation to the comparatively stable range of feeding-ground for the remainder of the fauna inhabiting the areas concerned. The term is rarely if ever applied to the slower movement of the whole body of animals of a fauna, coincident with great changes of climate, such as the advance of the glacial cover over the northern parts of Europe or America produced during the glacial age, or the advance of an Asiatic fauna across the Bering Straits and down the west coast of North America at some Pleistocene time when an ice bridge furnished means of communication by land from one continent to the other. Perhaps there is no impropriety in extending the application of the term migration to these latter cases in which the whole fauna and flora of a region is affected instead of single or a few species; and in which the change of position of habitat is slow and spread over a great period of time instead of being coincident with annual change of seasons. The term may equally well be applied to movements in the seas and movements on the lands.
There is, however, one reason for choosing a separate name for the movements of the latter kind to distinguish them from typical migrations.
In the first class of cases the migration is voluntary and is performed by those organisms which have the power of more or less rapid locomotion. They may be said to do the migrating themselves. In the second case the movements are involuntary and the movement is forced upon all the living organisms of the region and the change in position may be supposed to take place by the contracting on one side of the area of the conditions of possible existence for the species and the extension on the other side of favorable conditions of environment. The movements extend over many generations of life so that relatively sedentary species may gradually adjust their locus habitans to a given direction of migration. To this latter process of migration I have been accustomed to apply the term "shifting of faunas."
Migration of species is an expression of the ability of some organisms to appreciate slight changes of favorable conditions of environment and to take advantage of the better conditions during the lifetime of an individual. Shifting of faunas is an expression of the necessity for the perpetuation of the race of certain conditions of environment and the dying out of the whole fauna in the areas from which the favorable conditions are removed with corresponding spread of the fauna into new areas into which the favorable conditions have been shifted.
Shifting of faunas is an expression of the inability of the species of the fauna to survive under the changed conditions of environment which have overwhelmed them in the original habitat; but of an ability on the part of all those which migrate to follow the favorable conditions as they shift from one area to another.
In both typical migration of species and shifting of faunas change in the environmental conditions of life constitute the stimulus to change of habitat on the part of the organisms; and the movement of the organisms is a direct response to the stimulus—those organisms in the first case which migrate showing their greater vitality compared with their neighbors who stay at home; while those who stay at home show a greater power of endurance and organic adjustment to wider range of environmental conditions.
In the case of the shifting faunas those which endure without change of characters exhibit an acquired closeness of adjustment to some particular combination of environmental conditions which they are forced to follow or die and suffer annihilation. The evidence of their endurance is indicated by return and reoccupation of the same area at a later geological stage when by their reappearance, the original condition of environment may be assumed to have recurred.
In the case of living organisms evidence of migration is found in the actual presence of the species at one time in a region at a considerable distance from its ordinary locus habitans; and in some cases by seeing the species in the process of migration, as for instance the temporary alighting in fatigued condition of flocks of northern land birds on Bermuda Island on their migration southward.
In the case of fossil species the shifting of a fauna is expressed by the presence of a number of species representing an earlier fauna in a stratum in the midst of rocks containing a different and dominantly later set of species.
The fauna is then said to recur and it is the recurrence of the fauna which forms the basis for the inference that the fauna has shifted its locus habitansduring the period of time represented by the sedimentary deposits separating the formation in which the fauna is dominant from the zone in the higher formation in which the recurrent species are found.
This theory of the shifting of place and the recurrence in time of the same fauna involves certain conceptions as to the nature of species and the laws of evolution which it is important to consider.
§ 9. Evidence of Continuity.—To establish evidence of motion in migration as in any other kind of motion it is all important to know that the body or bodies to which the motion is ascribed is continuously the same.
In the Devonian case I have been studying the moving body is a fauna; not only have I found it necessary to establish identity of the species in the recurrent zones with those of the initial zones, but it is essential to show that the faunas as a whole are the same.
To put this in another form of statement we must establish the fact that not only the individual species have retained their specific characters, but the further fact that the equilibrium of adjustment to each other in the faunal community has not been changed, in order to prove that the recurrent fauna is the direct successor of a fauna represented in the rocks at a lower horizon.
This has led to such distinction as rare and dominant species of the fauna, and only as some such comparative frequency of the species in the faunal combination is apparent can we be sure that we are not considering an accidentally accumulated sample of a general fauna.
The presence of occasional associated species belonging to the normal fauna of the formation in which the recurrent zone appears is not antagonistic to the theory, because the theory proposes an invading of the territory occupied by the normal fauna, and whatever were the causes which brought about the shifting of the fauna they were not so completely different as to annihilate all evidence of the fauna previously occupying the ground. Hence it is only necessary to find an abrupt change of the grand majority of species to make the induction that the faunas have shifted their habitat.
The theory involves the further conception of grand general faunas which have their center of habitat and distribution in permanent oceanic basins, as distinguished from the special and (in geological strata) temporarily expressed faunas such as we are accustomed to associate with individual geologic formations.
In the case before us two such general faunas are in evidence, one of which in its dominant characteristics is traced westward into Iowa, Idaho and Arizona and up the Mackenzie River valley to the north and across the polar regions to Russia and northern Europe. The other is traced eastward and southward into central and southern Europe and also dominantly into South America.
Although, with our present knowledge, it is not possible to determine in any temporary expression of marine faunas those particular species which were derived from one from those derived from the other grand source, it is possible to recognize numerous species which belong to one center of distribution and others that belong normally to the other.
§10. Interpretation of the Facts.—It is also important to keep our heads clear in interpreting the facts.
It is only by close examination and comparison of the fossils themselves that identity of species or identity of faunas can be established.
The fixed characters of species are not only the characters by which one species is distinguished from another, hilt they are of generic, ordinal and even class value, and they may be of immense age in the race and mark no special, narrow stage of its history.
It is a question of interpretation whether each particular phase of expression of fluctuating characters is a matter of time or of environment.
I have reached the conclusion that it is those species which have the greater degree of normal and persistent fluctuation of character which migrate and follow the shifting conditions of environment, and their life period is correspondingly longer.
On the other hand species whose plasticity of characters is narrow, are more closely adjusted to their environment, are local in their range of habitat, and temporary in their geological life-period.
Interpreting the facts on this basis it is the phases of continuously fluctuating characters in species of wide geographic distribution and long geologic range which furnish the most satisfactory evidence of temporary stages in the life history of faunas.
Another question of interpretation arises when we attempt to reconstruct the physical condition of the environment at successive stages of time.
In a single vertical section we have positive evidence of succession in time. If we were sure that no recurrence of the same fauna could take place we could correlate two vertical sections strictly upon the fauna contained in the strata, on the basis of the supposition that the single fauna appeared but once in the section and that when it ceased in a given section its whole life period was expressed. But the facts show us that this is not the case in nature. In geological times as in the present, we know that many distinct faunas are living on the face of the earth at the same time, even for very similar conditions of environment. It becomes therefore a very complex matter to correlate two sections in which the order of faunas and the character of the sediments differ; which is generally the case for any two sections separated by fifty miles from each other, although on stratigraphic evidence they may be properly interpreted as covering the same interval of time.