Popular Science Monthly/Volume 87/September 1915/Ant-Hill Fossils
| ANT-HILL FOSSILS |
By Professor RICHARD SWANN LULL
YALE UNIVERSITY
LAST summer it was the writer's privilege to lead a small expedition from Yale to the fossil fields of the west in search for the relics of bygone creatures to add to the already extensive collections owned by the university. Our purpose was not solely that of collecting, however, but to get data concerning the distribution in time of certain of the ancient faunas, hoping thereby not only to increase the sum of our knowledge, but to date more accurately some of the wealth of forms collected by the pioneer expeditions which, under the leadership of Professor Marsh, penetrated the unknown west in the early seventies.
The work was partly in Nebraska exploring the Tertiary rocks for the remains of warm-blooded mammals—horses, camels, rhinoceroses, elephants, and their kindred—and partly in eastern Wyoming, where one finds sediments of greater age containing the earthly inhabitants of the closing years of the Age of Reptiles. The mammal collecting is an old, old story, but the work in Wyoming had many novel features and forms the theme of this brief essay.
The Mesozoic rocks, those of the Age of Reptiles, are exposed with their contained fossils in many places in this broad earth of ours, but nowhere to a greater advantage than in the west, and this is particularly true of the states of Wyoming, Colorado and Montana. One of the counties of eastern Wyoming, formerly called Converse county, is now divided into two portions, of which the westernmost retains its ancient name, while the eastern part has been called Niobrara after the long Nebraska river whose source lies here. The latter county includes one of the most notable of Mesozoic localities, the beds lying on either side of the confluent Lance and Lightning creeks. The former of these is a tributary of the Cheyenne river, through which its waters flow into the Missouri on their long journey to the Gulf.
The strata here exposed belong to the ultimate phase of the Cretaceous period, marking the very close of the Reptilian age and possibly its passage into the Age of Mammals. They cover an area of more than sixty square miles, and, because of their geographical locality, have received the name of Lance formation, though they have been called variously Converse County beds and Ceratops beds, the latter name having reference to the most characteristic fossils, the horned dinosaurs or Ceratopsia, whose huge three-horned skulls are the most remarkable features of the formation. Our camp lay near the only available waterhole, in an otherwise dry canyon, tributary to Buck creek, which forms the eastern boundary of the area. From Buck creek the land rises gradually to the summit of the divide, whence it falls away to the level of Lance creek on the west. On the eastern slope the strata, which dip toward the west at an angle of about ten degrees, form a succession of outcrops one above the other as one ascends the hill, so that they may be read in orderly sequence beginning with the oldest in the point of time. Beyond the divide the dip of the strata and the slope of the ground coincide so that the revealing outcrops are absent. To the east of Buck creek, on the other hand, on either side of the little canyon wherein our
Fig. 1. Looking down Spring Creek Canyon toward the Ceratops Beds. In the foreground and middle distance the strata are of marine origin—Pierre and Fox Hills formation—the fresh-water Lance sediments lying beyond.
camp was pitched, the rocks, while still late Cretaceous, are older than the Lance formation and of marine origin, for in them the shells of ancient sea-creatures are abundant.
Geologists tell us that during Cretaceous time the continent of North America was covered in part by an inland sea having its outlet to the north into the Arctic Ocean and on the south into what is now the Gulf of Mexico. Along the western shores of this Cretaceous sea were long stretches of low-lying lands gradually rising toward the west to the region of the Rocky Mountains, then in their nascent state. The shore lands, which rarely extended much above the level of the sea, were subject to occasional inundation when the advancing waters covered what had been land, while their retreat laid bare wide areas which had been submarine. The evidence of this was before our eyes, for as we went westward down the canyons, the sea shells would betray the marine origin of the strata, then as we climbed the slope beyond Buck creek we could see the older sea-borne sediments pass beneath the newer
Lance formation and become replaced by shales and sandstones of fresh-water origin. Occasionally one came across beds of lignite or woody coal which pointed to an abundance of vegetation, either forest or swamp lands wherein plants must have grown luxuriantly.
Out of this classic locality have come the remains of a host of creatures, the inhabitants of the shores of the old Cretaceous sea, many of which are now preserved in the Peabody Museum at Yale, the spoil of former expeditions. These are mostly dinosaurs, land-living reptiles which flourished throughout the Mesozoic, but which are here represented in the climax of their evolutionary career. Some were huge headed forms, armed on the snout and above the eyes with horns like those of cattle and with a wide bony crest which protected the neck. These were quadrupeds of rhinocerine proportions, but of much greater size. Others were bipeds with a long tail which served to balance the body when running on land and was a very efficient propelling organ when stress of circumstance made it necessary to take to the waters for retreat. This second group was armor-and weapon-less, with a curious duck-like expansion of jaws, toothless in front but with a wonderful battery of more than two thousand teeth in the rear of the mouth. Occasional mummied carcases have been found which betray the defenseless condition of the scaly skin. Both of these groups were plant feeders, but a third sort, again erect on the hind limbs, bore teeth and claws which can only mean a rapacious flesh-feeder, doubtless the arch-enemy of the other two. These carnivores were represented by small, agile forms, and by others which were truly gigantic.
The dinosaur remains are pretty widely spread over the present area, the horned skulls generally occurring in hard concretionary masses of sandstone, many of which were collected and shipped to New
Haven and later to the National Museum in Washington, where the contained fossils were freed from the investing rock. Most of the dinosaur material from this region was collected by John B. Hatcher, a graduate of Yale, and so thoroughly was his work done that very little sign of them now remains. What we of the present expedition especially desired, however, was not to find more dinosaurs, but to hunt for the remains of the tiny mammals, the forebears of the warm-blooded, furry quadrupeds of to-day, which carried on a precarious existence in the midst of such stalking terrors as the giant reptiles.
These mammal remains are known from but few Mesozoic localities and are valued proportionately. By some lucky chance it was discovered that, although they might be found imbedded in the sediments, the most productive places, curiously enough, were the ant-hills. These are numerous and huge though in no way differing from those I have found in New England except in the contained fossils. The ants are a lusty breed, valiantly uniformed in brown and black, and with very effective stings, as we have good reason to know. In building their formicaries they not only collect material from the surrounding surface, but in excavating their subterranean galleries the sand and other
Fig. 4. A Fossil bearing Ant-hill in which the Bones and Teeth of the tiny Warm-blooded Mammals are found.
particles are deposited on top of the growing pile. This was generally a very symmetrical cone with the entrance almost invariably on the eastern side, part way up the slope, while on the western and northwestern aspects the grains of sand were somewhat coarser. We found, however, on breaking into the nest, that the immediate surface was somewhat hardened so as to be distinctly crust-like, while within, until one came to the well-built tunnels, coarse and fine sands were intermingled. It seemed as though the outer crust and the coarse character of the sand on the exposed surface were merely the result of wind and rain which eroded away the finer particles and compacted the surface, and the evidence of the coarser grains on the westward side simply pointed to the removal of somewhat heavier material here than elsewhere by the prevailing westerly winds. I imagine that these winds were also the determining factor in the placing of the entrance, although the idea that the warmth-loving insects might thus have welcomed the rays of the rising sun did occur to me.
It has been found that in addition to ordinary sand-grains certain of the ant-hills also contain fossils, tiny teeth, bone fragments, and in rare instances perfect bones which the ants had unconsciously collected and which have been the source of much of our knowledge of the smaller forms which were contemporaneous with the giant reptiles. It was toward these ant-hills, therefore, that our attention was turned, indeed, they were the object of our journey. I had a rather imperfect map of the region which was supposed to show the place where each important dinosaur specimen had been found and to indicate in a general way the mammal-producing areas. With its aid a very thorough exploration was made, the great majority of the localities which were marked being searched for ant-hills, and these in turn for fossils.
The older method was to shovel the contents of the formicaries into sacs and leave them until the following day, when the ants would have left the material, and then to sift it very carefully, searching for the minute remains of organic life. Much of the "pay dirt" was shipped directly to New Haven, where it could be investigated more thoroughly than was possible in the field. Our time, however, was very limited, so that such refinement of method was impracticable. Then, too, we were not so intent upon adding to the already great collection at Yale as upon determining the exact stratigraphic sequence of the mammal-bearing beds with reference to the geologic column. We therefore attacked the ants' stronghold at once, going over the surface material very carefully, taking the precaution, however, to put a little earth into the entrance and pat it down, thus reducing to a minimum the number of available defenders. The sand was then passed through a common flour sifter and the residue carefully examined, the tiny teeth and bones being removed with a pair of forceps. We did not come away unscathed, for though we brushed aside all the ants we saw, the task would become so absorbing that before one knew it an ant would fix her jaws into the skin and, turning the abdomen under, insert the short but thoroughly efficient sting. After sifting the material we generally replaced it about as it had been found and, as the ants were uninjured, their fabric was probably soon restored to its former symmetry by the tireless little workers, for whom we had a very respectful sympathy in spite of our wounds.
Near Buck creek itself the ant-hills contained only small sharks' teeth, showing that they were still built over marine beds. Then as one mounted the divide and with it crossed higher and higher strata there was a long interval of totally unproductive formicaries, until the principal bed of lignite was reached, and above this, at a vertical distance of ten to fifteen feet, the best of the ant-hills were found, which contained among other remains the bones, scales and teeth of fresh-water fishes allied to the gar-pikes living to-day in southern rivers. These were carefully preserved for comparison with the mammal-bearing sands at Yale collected by the older expeditions, and which were found later to include, in addition to the mammals, fossils exactly similar to those which we secured at this time. That we had found an old locality was further proved by indications of a former camp, fire wood, rusted tin cans, and in an adjacent ant-hill tiny, worn fragments of an unfortunate investigator's spectacles!
The mammalian relics which were thus found were of very small size, consisting of individual teeth or a fragment of a jaw with one or more teeth still in position, or other portions of the bones. Mammal teeth are fortunately highly indicative of habit and relationships; but on the other hand, in collections of this sort, not one bone can be associated with another. Thus, while some idea of the general size and method of feeding may be obtained, no attempt to restore the skeleton or even the entire skull is possible. In size none of the ant-hill mammals was larger than a rat, and many were much smaller. Those whose teeth bore few sharp-pointed cusps were carnivorous, feeding on such feeble folk—insects, worms, and possibly other mammals—as they could overcome. Others with chisel-shaped incisors and many-cusped grinding teeth were plant-feeders, probably living not only on tender leaves, but on berries and the seeds of berries and other fruits.
The little glimpse of a former geologic age which this trip gave us was full of suggestive detail. I have already described the scene as the scientific imagination conjures it up, the broad, savanna-like, low-lying lands over which wandered the huge reptiles then at the very culmination of their evolution. But what of the mammals? Where were they and how did they survive the competition with the dinosaurs? Possibly their very insignificance was their chief safeguard, just as countless small rodents and insect-eating creatures live to-day in the lion-and buffalo-haunted African jungles. Possibly they were tree-inhabitants, and the location of the mammal-bearing ant-hills near the lignite certainly suggests an abundance of near-by vegetation. However we may interpret it, the evidence is clear that these mammals and the dinosaurs were contemporaries in the same general locality though the exact environmental conditions under which they lived may have differed.
Passing back in time some millions of years further, one finds in rocks which mark the beginning of Cretaceous time as they are exposed in another Wyoming region one hundred miles or so to the southwest, this same association of mammals and dinosaurs, but many of the latter are very unlike their successors and include mighty forms which soon became entirely extinct. But the mammals are practically the same, with little evidence of change, while the reptiles are undergoing their most remarkable evolution.
Again we find the mammals at the close of the preceding Triassic period, this time far to the eastward in South Carolina and then in Germany and South Africa, not in direct association with dinosaurs, but nevertheless in contemporary strata with some of the most primitive of the race. Thus these forebears of modern beasts and men live long ages without measurable progress, while reptilian dynasties wax and wane. But the mammals, with the tenacity of their race, are merely awaiting their opportunity, although so effective is the check laid upon them by their cold-blooded contemporaries that for them evolution practically ceases while the march of time goes on. At last comes the day of reckoning when, due to some cause or causes of which we have not yet learned the nature, although they were doubtless conditioned upon the mountain-making revolution which closed the Age of Reptiles, the dinosaurs, after their multi-millennial career, are blotted out and the Age of Mammals is begun. Now from their fastnesses stream the furry hosts, impelled by age-long earth hunger, to fill every station in the economy of nature which the reptiles had possessed, and now the evolutionary mill, turning faster and faster, grinds out the beasts both small and great which become in their turn the rulers of the earth until their place is usurped by humanity.