Popular Science Monthly/Volume 72/June 1908/The Dawn of Quadrupeds in North America
|THE DAWN OF QUADRUPEDS IN NORTH AMERICA|
THE UNIVERSITY OF CHICAGO
IN the early days of the physical sciences one who gave a commonsense interpretation to a phenomenon of nature was looked upon as dissenting from the accepted interpretation of things. It pleased the people of the early days of science to regard natural phenomena as something wonderful, as a divine creation or as something beyond the comprehension of the human intellect. It has been less than two centuries since men began to emerge from under this pall. The advance made in science constitutes one of the wonders of the age. It is the purpose of this essay to record the progress of investigation in one line of scientific research, and that is the one which leads up to a knowledge of the ancestry of one group of the vertebrates.
The early observers of nature had curious suppositions in regard to the nature of fossils. They regarded these objects, which were common, as of various origins. Some claimed that they arose spontaneously in the rocks, others were of the opinion that they arose from germs which had fallen from heaven, but the large majority believed the fossils to be the remains of "an accursed race" whose existence had been ended by the Noachian deluge. Early in the seventeenth century collectors of natural objects became familiar with certain bodies which were known as "glossopetrse." They were embedded in the rocks and the manner of their entombment was a matter of considerable dispute. In the first half of the seventeenth century, Fabio Colonna had tried to convince his contemporaries that the "glossopetræ" were nothing but shark's teeth. His arguments failed to carry conviction, however. It was not until 1669 that Steno, a Dane by birth, though a teacher in the schools of Florence, Italy, demonstrated by the dissection of a common shark's head that the "glossopetræ" and the teeth of the shark were identical. These results he published in a quaint little volume entitled "De Solido intra Solidum naturaliter contento." This was the first application of the modern method of paleontological research to the study of objects contained in the rocks. Later Cuvier continued and extended the researches of Steno and from the time of the famous investigator, Cuvier, to the present, our knowledge of the objects entombed in the rocks has progressed rapidly.
Modern researches have carried our knowledge of the distribution and relationships of animals farther and farther back into geological time. There was a time, not many decades ago, when people knew nothing of the animals of the ancient days. During the life of Cuvier the knowledge of extinct animals had not progressed much farther back into geological time than the Cretaceous or Jurassic. It was just four years before his death that Jaeger made his important contribution to the Triassic fauna of Europe by the description of the remains of Mastodonsaurus, which he had in 1824 described as Ichthyosauri. Subsequent researches by a host of observers have carried our knowledge of animals into an antiquity which had not been expected.
The animals which it is our purpose to treat here are the ancestors of the modern Amphibia. There are few groups of vertebrates whose phylogeny is more obscure than that of our common toads, frogs and salamanders. It is the popular idea that these animals are unknown back in geological time, but that they are of a rather recent origin. As a matter of fact the present-day amphibians are the descendants of the oldest group of vertebrated animals with the exception of the fishes. Our knowledge of the fishes begins near the dawn of animal life on earth, and their remains are preserved in the rocks of the Ordovician age, just west of Cañon City, Colorado, and in the Big Horn Mountains, Wyoming. Our knowledge of the amphibians begins just two ages later, and in the Devonian rocks of Pennsylvania are found the earliest traces of quadrupeds on earth. These evidences consist in footprints found by Isaac Lea in 1849 and the announcement of his discovery was given to the British Association for the Advancement of Science by Buckland in that year. These footprints represent a rather large animal which may have attained a length of several feet. The footprints were found impressed in the "Old Red Sandstone" of Pennsylvania which forms a part of the Catskill formation of that state. Marsh, forty-seven years later, announced the discovery of similar footprints from the same horizon and near the same locality but does not mention the discoveries of Lea. From these tracks in the Devonian to the deposits in the Allegheny series of the Pennsylvanian our knowledge of the Amphibia is a blank. There is not a trace recorded of any amphibians in the rocks of the Mississippian or in the Pottsville of the Pennsylvanian.
In the Allegheny series, there are several deposits in the United States, and probably one in Canada, which have produced remains of the early quadrupeds. The principal localities are in Illinois, Pennsylvania and Ohio. From the last named state great numbers of these paleontological treasures have been recovered and are preserved in the museums of the east. The most interesting place which has kept for us a record of the amphibian life of this far-off time is a deposit of coal in the eastern part of Ohio in the northern part of Jefferson county. These deposits are in the form of a thick bed of coal which has been opened up by modern industry which has cast aside as useless the blocks on which are preserved the priceless relics of the creatures of this bygone time. There have been great numbers of these blocks of coal collected by geologists, but in all probability the greater part of the animal remains preserved in the coal has gone to furnish heat for the people of the region.
The animals, which have been obtained from the old Diamond mine near the village of Linton, Ohio, are imbedded in the coal which formed from the vegetation growing on the shores of the lake in which the coal accumulated. This old lake was probably of but limited extent and may not have measured more than six miles in its greatest diameter. In this lake lived and died for ages the animals whose remains represent the first recorded appearance of quadrupeds on the earth. There are, to be sure, deposits in Illinois which are of contemporaneous age, but so far only five specimens of amphibia have been discovered in these deposits, so they are hardly to be taken into account when compared to the hundreds of specimens obtained by Dr. J. S. Newberry from the Linton locality. The animals which disported themselves in this old lake, at their death fell to the bottom and their remains, what was left of them after their former companions had feasted on their bodies, were covered with the mud and vegetation which drifted in upon them. Thus they are preserved to us.
The student of these remains finds them greatly different from the amphibians of to-day. There were some forms which were large, but the majority of them were small. Some may have reached a length of ten feet while a great many did not exceed six inches and a few were less than five in extent. There is one little form from Illinois, to be described further on, which barely attained a length of two inches in the adult state. Some of the Amphibia from the Linton mines represent snake-like forms with the proportions of the modern whip snake of the western plains, though not with its dimensions. Others more nearly resembled the modern lizards and this resemblance was carried to the extent of the possession of strong teeth and clawed digits. There was no osseous carpus and tarsus, however, so that they are distinct from the lizards structurally. Still other of these early quadrupeds must have resembled the modern crocodiles in appearance and a few may have attained nearly the dimensions of these forms. There were forms which were partially protected by hard dermal plates, at least on some parts of the body. Some, like the fishes, had rounded scales which covered the entire body, while a few appear to have been entirely naked. All the forms appear to have possessed the ventral armor of dermosseous rods or scutes which protected the abdomen much as the abdomen of the Sphenodon of New Zealand is protected to-day. When in the later ages of the existence of this old lake its waters became filled with vegetation and it had acquired the characters of a marsh there came a disturbance in the earth's crust and the lake was again submerged, and on its bottom was formed the thick stratum of good coal which is now known to geologists as the "Ohio No. 6." This coal was formed over the graves of the earliest quadrupeds. Here through the vast stretches of geological time they lay in their coaly bed. After many, many eons of time the descendants of animals which had been their contemporaries came with tools fashioned with their fore feet to dig out the coal to keep their naked bodies warm. These were men and to these miners we owe a debt of gratitude for thus bringing to light these treasures of the earliest quadrupeds.
There was a man in the days when these coal mines were being worked who appreciated the opportunity of collecting the remains of these creatures and he deserves far more credit than the miners who delved in the ground for the coal. This was Dr. J. S. Newberry, whose name is to be ever associated with the first investigators into the history of the primitive quadrupeds of this continent. Through his knowledge of the geology of the region in which the mine was located he realized, as no other did, the importance of gathering these remains as rapidly as possible. The result was worthy of the exertion. The mines have now long since been deserted, the village of Linton has gone out of existence and even the spot where the mines were located is difficult to find, so Dr. Hussakof tells me. Newberry's collection of the early quadrupeds is now in the American Museum of Natural History of New York City, and it will stand as a monument to the zeal of one of the early investigators into the "Eotetrapoda" of North America. Newberry's collections have, for the most part been described by Cope, who has done more on the morphology of the extinct Amphibia than any other investigator in North America.
Dr. Newberry found the first recognized amphibian in the Linton deposits in 1856. The next year Dr. Wyman read a note on the specimen before the meeting of the American Association and the next year he published a description of the form under the name Raniceps lyelli. It was necessary to change the name Raniceps, so ten years later Dr. Wyman proposed in its stead the term Pelion and the form is still known as the Pelion lyelli Wyman. This is perhaps one of the mostextraordinary of all of the Amphibia which have came from these deposits (Fig. 1). It was thought by Wyman, and later by Cope, that the form had the characters of the modern frogs and in its general appearance it certainly shows great resemblances to the modern frogs, especially in the shape of its head and the length of the hind leg, which Cope seems not to have observed. Among the other forms collected by Newberry is the form shown in Fig. 2. One half of the slab containing
Fig. 1. The Type of Pelion lyelli Wyman from the Carboniferous of Ohio. In the collection of the American Museum. Natural size
the lesser part of the skull went to Dr. Newberry, and in some way the other half of the slab containing the major portion of the cranial elements was obtained by Mr. W. F. E. Gurley and it is now in the collection of the University of Chicago. The illustration is made from the latter specimen. This form is peculiar in the possession of horns which projected backward over the neck. Jaekel suggests that these horns were tor the protection of the external gills, but the Microsauria, so far as we know, had no gills, at least in the adult state.
Geologically the record of the amphibians, as it has been given, is the correct one, but chronologically it is not. Long before a single specimen had been taken from the Linton beds Sir William Logan, in 1842, found evidences of amphibians in the Carboniferous of Nova Scotia in some footprints later named by Dawson Hylopus logani. These footprints Logan took with him to London and submitted them to the famous paleontologist, Sir Richard Owen, who unhesitatingly pronounced them to be "reptilian." Logan's discovery constitutes the earliest recognition of amphibians in the Carboniferous. A few years Fig. 2. The type of Stegops divaricata Cope from the Carboniferous of Ohio In the collection of the University of Chicago. Natural size later Sir J. W. Dawson and Sir Charles Lyell in breaking apart one of the stumps of the Large Sigillariæ came across some interesting vertebrate remains. When the announcement of this discovery was given to the Geological Society of London, the president or secretary remembered a skull which Dawson had sent in three years previously and which had lain in the collection of the society all of this time. Dawson had been delayed one day at Albion, Nova Scotia, and in order to while away the time between trains looked over a pile of rubbish from a coal mine near by. In so doing he split open a large slab of shale in which he found a nearly perfect skull of some unknown animal which he thought might be a fish. This he sent to the Geological Society with other specimens, and it was described in 1853 by Sir Richard Owen as Raphetes planiceps, Fig. 3. The Specimen of Amphibamusgrandiceps Cope from the Mazon Creek Beds of Illinois. In the collection of Mr L. E. Daniels of La Porte, Indiana. Natural size. and its relationships were shown to be with the Amphibia. There have been but few remains discovered since at this locality, although it was frequently examined by Dawson.
For many years after 1853, Dawson continued his researches into the Amphibia of the Joggins section in Nova Scotia, and he has left us a great amount of knowledge which he collected into his "Acadian Geology" and into his "Air-Breathers of the Coal Period." The forms described by Dawson differed in an essential respect from those discovered in the United States. The animals whose remains occur in the hollow sigillarian stumps in the Joggins deposits, seem to have all been of terrestrial habits, and Dawson frequently speaks of them as "lizard-like" and his restorations of some of the forms, though based on very scanty remains, indicate forms which would be taken for modern lizards. While his restorations are in part fanciful, he has given, in the main, an accurate idea of the animals as indicated by the remains preserved in the old stumps of the swamp in which the animals lived.
Cope's researches into the structure of the early quadrupeds constitute an interesting chapter in the history of the early vertebrates. His researches on the extinct Amphibia began in 1865 by the publication of the description of the form known as Amphibamus grandiceps, from the Mazon creek beds of Illinois. This specimen was loaned to the Illinois Geological Survey for Professor Cope to study by Mr. Joseph Even of Morris, Illinois. After Cope's description of the form, the specimen was returned to Mr. Even and, as Mr. L. E. Daniels tells me, it was later destroyed by fire. Mr. Daniels has recently been kind enough to allow me to study a specimen of this form in his collection, the only one, so far as I am aware, now in existence (Fig. 3). The peculiar characters of this form are the possession of sclerotic plates in the eyes and the possession of long curved ribs which have been recently described by Dr. Hay from this same specimen. A specimen of another species from his same deposit has been in the Gurley collection for nearly thirty years. Dr. Newberry saw it when he studied Mr. Gurley's fishes and said in a note that Professor Cope should see it. It was never sent to Cope, however, and it is described elsewhere by the writer as Micrerpeton caudatum, gen. et sp. nov. This is a very interesting form, since it shows the impression of the fleshy tail (Fig. 4). On this tail impression are preserved many important structures heretofore unknown for the Branchiosauria to which the form belongs. As such it represents not only the earliest geological evidence of the group, but it is the first appearance of the Branchiosauria in the discoveries in the early quadrupeds in North America. It is a typical branchiosaurian and as such belongs to the family in which the Branchiosaurus of Europe is placed.
Between the years 1865 and 1897 Cope continued his investigations on the early quadrupeds and his results are to be found in the proceedings of several learned societies. He early recognized the unusual characters of the Carboniferous fauna, and his many papers attest his interest in the forms which constitute it. He has described all but two of the known Carboniferous species from the deposits of the United States. Among the many peculiar types of amphibians described by Cope none is more bizarre than the form from the Permian of Texasknown as Diplocaulus magnicornis Cope. This peculiar form has been widely described and commented on. Its affinities are clearly with the Microsauria, and it is one of the latest representatives of that group
Fig. 4. The Type of Micrerpeton caudatum Moodie from the Carboniferous of Mazon Creek, Illinois. In the collection of the University of Chicago. Twice natural size.
known to us. It is peculiar in the wide horn-like expansions of the skull (Fig. 5). There is no pineal foramen in the dorsum of the skull and it lacks a few of the characters of the other amphibians known from this region.
Interest in the early Amphibia has not slackened in the later years and there have been many contributors to the knowledge of the early forms. There are at present nearly seventy-five species of Carboniferous Amphibia known from North America and as many have been recorded from the Carboniferous and Permian of Europe; many more undoubtedly await discovery. The forms do not differ greatly in their structure in the two continents and each has relatively the same amphibian fauna, although the North American fauna is perhaps somewhat older than the European one. Certainly the Branchiosauria are found
Fig. 5. A Skull of Diplocaulus magnicornis Cope from the Permian of Texas. In the collection of the University of Chicago. Three tenths natural size.
here in older rocks than in Europe. Among the many forms known may be mentioned some with a peculiar type of vertebra in several allied genera found in Ireland, western Europe and North America; some with, fish-like scales over the whole body found in Ireland and Ohio; the presence of the snake-like forms in Ireland, England, western Europe and Ohio and the presence on the continents of Europe and North America of the Branchiosauria already referred to. All of the types of amphibians possess the peculiar ventral armature which was arranged in a chevron pattern over the belly, breast, throat, and even extended, in some cases, out on the limbs.
The earliest quadrupeds, as we know them, form highly specialized groups of organisms which had become differentiated into five distinct lines by the close of the Carboniferous. We must await the progress of discovery to unfold for us the nature of the most primitive quadrupeds from which the earliest known forms have sprung.
- Huxley, T. H., 1881, "Science and Hebrew Tradition Essays," p. 29.
- Newberry, J. S., 1889, Monograph U. S. G. S., Vol. XVI., p. 211.