Popular Science Monthly/Volume 76/June 1910/Adequacy of the Paleontologic Record I
|ADEQUACY OF THE PALEONTOLOGIC RECORD|
UNIVERSITY OF IOWA
WHEN or how life began on our planet no one may be able to tell us; but that life has been present and has been an important factor in the world's geological development since before the beginning of the Cambrian is known to the most callow of embryo geologists taking his first course at the village high school. So far as relates to the skeleton-bearing, marine invertebrates which have lived on floors of epicontinental seas, there are remarkably complete records of this long history of living things, the order of their succession, their migrations, their geographic distribution during any given portion of geologic time, as well as of the progressive and orderly modifications which resulted in the extermination of decadent or unfit types, on the one hand, or resulted, on the other hand, in the advancement of certain types and their adaptation to the conditions prevailing in the living world to-day.
The zoologist, confining attention to living forms, gets a view of the animal creation as it exists, after ages of development and modification, during a fraction of a single faunal stage. The paleontologist, while unable to see the beginnings of life, gets the broader view which comes from a study of the organic world as it has appeared during numberless successive stages. He may trace the origin of forms and note the trend and tendency of variations in ways denied the zoologist. Neither the depth of the water nor the distance from the shore at the points where the objects of his study lived interferes with the thoroughness of his explorations. He is not limited to what he may learn by taking samples of the old sea bottom, here and there, with a dredge; he traces his life zones with practical continuity over areas of continental extent.
The faithfulness with which the paleontological record has been kept since the beginning of the Cambrian is a matter of constant surprise. No organism was too small for preservation, if only its soft parts were supported or protected by a stony skeleton of some kind; no parts of the skeletal structure were too minute to be kept practically unaltered to the smallest microscopic detail; no period of time has been so long that the records of the large or the small things of life were necessarily obliterated. The shells of such minute and delicate things as radiolaria and foraminifera, on the one hand, and that king of invertebrates, the giant Camaroceras, on the other, have all been kept through the ages with equal fidelity. The hinge characters of the brachiopods, their internal arm supports, their spires and loops, the distribution of the ramifying blood channels in the mantle, the surface markings of every rank and grade down to the smallest which can be observed only with the lens, and the microscopic structure of the shell itself, are other examples of the faithfulness with which details, however insignificant in point of magnitude, have been guarded, protected, preserved. Strangely enough, in respect to a very large proportion of the animal remains buried in the ancient sediments it looks as if time had been standing still; it has neither marred nor destroyed. The organic remains from the Ordovician formations are quite as perfectly preserved as those from the Tertiary.
The profusion of the life of the ancient seas is as much a source of surprise as the detailed perfection of the record. In the Mississippi Valley limestones constitute a very large proportion of the sedimentary rocks, and it is unnecessary to say that these limestones record the life and death of countless myriads of organisms. In some cases the waters of the old seas were comparatively quiet, and the shells or other hard parts, undisturbed and unbroken, remain in the positions they occupied when the individuals they represent were alive. There is a bed of marly shale carrying many thin lenses of limestone, lying between the Platteville and the Galena, from 60 to 70 feet above the base of the Mohawkian, and these lenses are made up in large part of unbroken brachiopod shells. On the surface of one of these slabs, in an area measuring 35 square inches, one may count more than 60 perfect specimens of Dalmanella subæquaia and Orthis tricenaria. The rate is about 290 individuals to the square foot. The number on a square mile of such sea bottom runs up into the billions. The number of individuals of the species Pentamerus oblongus that swarmed on the bottom of the Niagaran sea is strikingly demonstrated in every paleontological museum. The wide geographic range of this species is well known; its range in time was such as to make possible the accumulation of beds of limestone, 70 feet in thickness, from the detritus of its broken shells. Like other persistent or widely ranging species, it gave rise to a very large number of varietal forms, some of which have been described as specifically distinct.
The Devonian formations furnish similar evidence of the wonderful profusion of the ancient life and help us to appreciate the wealth of material that the paleontologist has at his command. In the quarries at Independence, Iowa, there are beds crowded with beautifully preserved forms, mostly brachiopods, as perfect to-day in every detail of shell structure and ornamentation as when the currents of life pulsated within. A coral reef, no species lost, has been cut into by a small intermittent stream near Littleton, Iowa; and perfect coralla, wagon loads of them, are strewn along the sandy channel a quarter of a mile or more. A successor to the reef just noted, composed of different species, the corals still in place, may be seen and studied on the west side of the river opposite the village of Littleton. The state quarry beds near North Liberty are simply cemented masses of brachiopods; they illustrate the remarkable prodigality of the Devonian life, but the individuals are not in good condition for study. It is a different case that is presented by the fossils in the marly beds of the Lime Creek shales at the exposures between Mason City and Eockford. A very large proportion of the specimens here are as perfect as when the animals lived; and there is a beauty and delicacy and exquisite refinement about most of them that is scarcely matched, certainly not surpassed, anywhere among fossils of any age or time. More than 65 species occur in the Lime Creek fauna, and thousands of individuals of some of the species, illustrating wide ranges of variation, enrich the museums of the world.
Along the Aux Sables River at many points near Thedford and Arkona, Ontario, there are calcareous shales containing a marine fauna, or rather a succession of faunas which once flourished in wonderful profusion and is still preserved in equally wonderful perfection. Statistics and computations would fail to give an adequate conception of the abundance and character of the material here offered for study. No detail of the skeletal parts has been lost; and as for the number of individuals, they are simply uncountable. There lies before me a small fragment of this old sediment having a surface of less than 15 square inches and it shows 51 identifiable individual specimens, not counting stem segments of crinoids. The 51 individuals are distributed among eleven species, and these represent eleven genera—namely, Phacops, Platyceras, Tentaculites, Spirifer, Chonetes, Hederella, Ortliopora, Chætetes, Arthracantha, Striatopora and Aulopora. Can any bit of modern sea bottom of similar size make a better showing? Above and below the Rocky Glen, near Arkona, from which this specimen came, there are opportunities to study continuous sections approximately 100 feet in thickness, the successive beds crowded with organic remains and revealing the historic sequence of varying organic types as the life responded to slight changes of environment. Here, as at countless other localities, the paleontologist gets a view of changes, of movements, of trend and tendency among living things ranging over a period of time equal to many millenniums.
Another life record of especial interest, typical of many scattered up and down the land areas of the globe, is furnished by the Osage division of the Mississippian at Burlington and Keokuk. Of one group of crinoids, the Camerata, these Mississippian limestones have yielded about 250 species, and of other groups a number about equally as great. The beauty and perfection of the individual specimens can be appreciated only by those who have had the good fortune to see the superb collections of Wachsmuth and Springer. Crinoids flourished here in such numbers that beds of limestone 150 feet in thickness are built practically of crinoidal remains and nothing else. The time represented was long enough to allow of a series of modifications of such extent that the crinoid fauna of the Upper Burlington is very distinct from that of the lower beds of the same formation, while the fauna of the Keokuk differs from both. Here again the paleontologist is favored, not only with a wealth of material, but with an opportunity to note the trend and tendency of things. This was the time of greatest development, of highest prosperity, among camerate crinoids. But in the midst of this prosperity the trained paleontologist may discover signs of degeneration, the prophecy of speedy extinction. The law enunciated by Beecher and quoted by Professor Woodward in his address before the geological section of the British Association at its meeting in Winnipeg last summer, is well exemplified in the Mississippian history of this particular group of crinoids. The tendency among any division of skeletal-bearing animals to run to extravagant ornamentation in the way of ribs, nodes, spines or other excesses of dead, useless, skeletal matter, is something that precedes and presages the decline and death of the race. Even in the Upper Burlington the skeleton of the crinoids is heavier than in the Lower; stronger nodes on the plates are produced; more arm plates are incorporated in the dorsal cup; the animals are weighted down with useless matter. This tendency is carried to extremes in the Keokuk limestone, a fact well exemplified by the species figured on plate 15 of Hall's "Geology of Iowa," Volume I., part II. In these species the development of massive spines and heavy nodose plates reaches its maximum. The race has come to the end of its career. When the Keokuk closes, only a few of the simpler forms of the Camerata survive, and even these shortly disappear. The paleontologist sees the operation of the same law, the same trend and tendency, among the Cretaceous Ammonoids; in many other groups of animals it is as clearly manifest; but it would not be profitable, before such a body as this, to carry the discussion farther, even if the limits of the paper permitted. Let me close by quoting from the address of Professor Woodward, to which reference has already been made:
Geology and paleontology in the past have furnished some of the grandest contributions to our knowledge of the world of life; they have revealed hidden meanings which no study of the existing world could even suggest; and they have started lines of inquiry which the student of living plants and animals alone would scarcely have suspected to be profitable.