Popular Science Monthly/Volume 77/August 1910/The Paleontologic Record III: Anatomy and Physiology in Invertebrate Extinct Organisms
|THE PALEONTOLOGIC RECORD. III|
STATE MUSEUM, ALBANY, N. Y.
THE inquiry into the position of anatomy and physiology in invertebrate paleontology seems very appropriate at present, since paleontology is steadily becoming more closely affiliated to zoology, and the sphere of zoology is at present dominated by comparative anatomy and physiology.
Since, however, invertebrate paleontology has only the hard parts, mostly outer shells, at its disposal, the view still prevailing among zoologists that little is to be expected from it in regard to the solution of the problems of anatomy and physiology of the lower animals seems natural. Nevertheless, the results already attained prove that if paleontologists do not approach their material with a geological knowledge only, as has been done in the past altogether too often, most gratifying results may be obtained, at least in some classes, for it must be conceded that the connection of the hard parts with the fleshy parts is very unlike in different classes; it is very intimate in some, as the crinoids and brachiopods, and again more indifferent, as in the gastropods.
But it is not claiming too much for invertebrate paleontology if we say that where the hard parts are of great structural importance, paleontology has earlier taken cognizance of this fact and consequently gone ahead of zoology. As an instance I may cite Zittel's investigations of the skeleton of the hexactinellid sponges which have taught the fundamental importance of the form of the spicules and the structure of the skeleton in that class and whose results have been readily adopted by zoologists. In classes which, as the brachiopods and crinoids, are to-day mere shadows of their former greatness, paleontology has its greatest chance, and it would fail in its task if it would there not become the instructor of the affiliated science; and it is gratifying to see that this fact is finding recognition, as, for instance, in Ray Lankester's "Treatise of Zoology," where the chapter on the crinoids has been entrusted to Bather, a paleontologist and one of the best authorities on crinoids.
It is apparent that in such classes as those just mentioned, of which only the last ends of the branches are still alive, the origin and nature of many structures can not be elucidated, even by the embryology and comparative anatomy of the recent forms, but only by paleontology. Such a structure is, for instance, the mystifying stem of the crinoids which, by a study of the primitive ancestors of the crinoids among the cystids, is readily recognized as a dorsal evagination of the body. Likewise, to cite another example, the siphuncle of the recent Nautilus, which is obscure as a wholly rudimentary organ, is in such primitive Paleozoic cephalopods as Nanno and Piloceras, still seen in its original form and thereby recognized in its nature.
Since that which has already been accomplished in fossil anatomy is proof that there are still larger fields to be ploughed and harvested, it is proper to inquire into the best methods of this work before us.
We first need more extensive and more intensive or more detailed purely descriptive anatomical researches of the invertebrate fossils. There are many species that, when investigated in their smallest detail, are bound to give important results. I may cite here, as examples of such accomplishments, Hudson's minute study of the strange Blastoidocrinus of our Chazy rocks with its 90,000 ossicles, or that of the Eurypterus fischeri by Holm. Of this archaic fossil marine arachnoid, a relative of the scorpion and of the king crab, it can be fairly said that, as far as its chitinous integument is concerned, it is as well known as any recent species. We know, through Holm, its gills, its complex genital appendages of both sexes, and even its fine hairs and bristles. Dr. Clarke and myself have lately continued these investigations in the American eurypterids, and there observed the structure of the compound eyes, the pore system of the segments, the genital apertures, the mode of moulting, the arrangement of some of the principal muscles and other anatomical facts of interest.
It can be said that this field of detailed descriptive anatomy has been merely touched thus far, as far as our fossil invertebrates are concerned, and altogether too much neglected. This is not only true as to the gross anatomy, but still more so as to the microscopic structure. It must be conceded that owing to the secondary changes in the rocks, this latter line of investigation meets with great obstacles not fully appreciated by the zoologist, and that it is only in its infantile stage in some classes. But the results obtained by the microscopic research of the Paleozoic bryozoans in this country may be considered as a striking example of what persistency and enthusiasm may still accomplish. In microscopic anatomy of the fossils the training of a geologist is as much required as that of a zoologist and the history of these investigations shows that a zoologist without geologic training may be badly misled by the deceptive states of preservation of the fossils.
The main object of anatomical research is to result in comparative anatomy and to determine what parts are fundamental or primary and what have undergone modifications due to functional changes. It is obvious that here invertebrate paleontology is in a position to answer a host of questions that could not be successfully approached by comparative anatomy of recent forms, by the direct observation of successive changes. Its methods of investigation have already been applied with wonderful success to large parts of our Paleozoic crinoids, brachiopods, bryozoans and cephalopods. And I do not doubt that the time has come when the preliminary stage of mere description of fossils is passed, and a monographic treatment of each class that would fully enter into the comparative anatomy of all structures preserved, could be profitably undertaken.
It is only by this work that paleontology can hope to make those contributions to philosophical anatomy in revealing the causes of the different structures which it is especially fitted and called upon to furnish by its ability to study the gradual development of the structures. Wherever a class of fossils has been thus thoroughly treated, it has given a fruitful crop of new hypotheses and principles, as is instanced by Hyatt's investigation of the fossil cephalopods. Most classes, and especially the corals, echinoids and trilobites, await such treatment by competent investigators.
Since physiology is that branch of biology that treats of the laws of phenomena of living organisms, it might seem hopeless to expect any information from the fossil world. This is apparently the more true in regard to the invertebrates, since a special physiology exists thus far only for men and the higher invertebrates and the recent invertebrates are largely a virgin field. For this reason also, only the most general foundations of comparative physiology have been laid, and an invertebrate fossil physiology would get as yet but little support from that side. Moreover, the main source of exact information in recent physiology is the experimental method, and this is wholly inapplicable to the fossil world.
And yet it seems to us that the empiric method upon which physiology has so long flourished promises also rich fruit in paleontology. I can do no more now than briefly mention the problems that most readily suggest themselves here. Invertebrate paleontology will be especially competent to furnish contributions to the mechanics of physiology by throwing light on the development of the means and modes of locomotion. In connection with this problem invertebrate paleontology also shows most clearly the deep-reaching influence of secondary fixation on the structure of the organism, as in the case of the strange Richthofenia among the brachiopods and the Rudistæ among the lamellibranchs. It can not fail that the progress in recent invertebrate physiology will stimulate inquiry into the physiology of the fossils; and further that, as invertebrate fossil anatomy progresses, the data for such inquiry will also come forth.
Another problem closely connected with that of the mode of locomotion is that of the origin of the organs of sense, and also upon this, as far as the organs of seeing at least are concerned, the fossil invertebrates are able to throw some light, as in the trilobites and eurypterids.
Another line of inquiry is that of the mode of nutrition as recognizable by the appendages, and its influence upon the general structure. Under this heading such interesting minor problems as that of the origin of parasitism arise and may be solved, as indicated by a recent publication as to the time of beginning, causes and gradual changes of parasitism, to its very complex present conditions.
Probably also the physiology of respiration will in time receive important additions as far as the echinoderms, crustaceans, scorpions and eurypterids are concerned.
The widest scope, however, will have those problems that are connected with the reactions of the organisms to their physical and chemical surroundings. The invertebrate paleontologist meets forever, in sight of the ever-changing faunules, the question, what exterior influences caused these changes? Often they can be directly recognized, as in the dwarfed faunules of the Devonic pyritiferous Tully limestone or of the bituminous Marcellus and Genesee shales or the eurypterid faunas of the Salina lagoons. The systematic investigation of these reactions through the series of formations is an inviting task.
A special problem of singular interest connected with the reaction of the organisms to the chemical surroundings is that of the composition of the shell of the invertebrates. There is good evidence for the view that the shells were at first chitinous and that but gradually they became calcareous or siliceous. This important question again is intimately connected with that of the original composition of the ocean, and this line of inquiry again leads us to the highly fascinating paleophysiological problem, lately so happily dealt with by Professor Lane, as to the geological evidence on the original composition and origin of the vital liquid, the original body temperature and the physiological origin of the hard parts of the invertebrates in general.