Popular Science Monthly/Volume 66/December 1904/Nature's Hieroglyphics
MASSACHUSETTS AGRICULTURAL COLLEGE, AMHERST, MASS.
ONE of the loveliest parts of all New England is the broad valley of the Connecticut River, of deep human interest because of its having been the theater of many of the conflicts during the struggle for existence between the white settlers and the aborigines, and occasionally one comes across a monument whose inscription tells of the fierce engagements of colonial days. More numerous still are the records of an earlier race, not in this instance of mankind, but of creatures far antedating man in antiquity, which have left involuntary inscriptions on the rocks.
The long slab shows six successive tracks of a tail-dragging carnivorous dinosaur.
For nearly a century these impressions have been observed by the good folk of the valley, though as many of them had to the uncritical eye the familiar appearance of bird tracks, they were considered as such, and to those who were unaware of their vast antiquity they were undoubtedly of little interest. As soon, however, as they became known to men who could appreciate their full significance, the impressions were at once recognized as being of great scientific interest, and it is to the efforts of the late President Edward Hitchcock, of Amherst College, that we owe the founding and development of a study which soon rose to the dignity of a science. The interest of President Hitchcock ceased only with his death in 1865, and his tireless energy resulted in the bringing together of a magnificent collection of track-bearing slabs and in the description and publication of more than one hundred species of track-making organisms.
The impressions, while mainly of footprints, also give evidence of dragging tails and other portions of the body or of the armoring and texture of the skin. Other attendant phenomena have left their records also, such as the rainprints of a summer shower, ripple and other beach marks, and the shrinkage cracks which are found in sun-dried mud. These are preserved with wonderful fidelity and minuteness of detail.
Classification of the Tracks.
The footprints may be classified under three groups, according to the mode of progression or the posture of the maker. First the impressions of true bipeds, those whose very bird-like imprints gave the popular name of bird tracks to the phenomena as a whole. Here may be, though rarely, the trace of a dragging tail, but with this exception the hind feet alone leave their record on the rocks.
The second group is as truly bipedal as the first in gait, and the footprint is usually as bird-like as before, but there may occur in addition to an occasional tail trace the impressions of little five-fingered hands placed just in front of those of the feet, while from the rear of the footprint often extends that of a long slender heel. These impressions of the hand and heel were only formed when the creature rested, for, except for differences in shape, the footprint of the moving form can not be distinguished from those of the first group.
The third are the true quadrupeds, not alone in resting posture, but during locomotion as well, in some cases with feet whose impressions are full of character, which gives some clue to their maker's affinities; others slender toed and obscure, whose true relationship with known creatures it is difficult to conjecture.
There remains yet a fourth group of imprints, which, while evidently formed by living organisms, are certainly not those of vertebrates or backboned animals, and, if one may judge from their appearance, may have been made by creeping worms or by crab-or centipedelike creatures or possibly by insects.
Interpretation of the Footprints.
Even to the layman the mounted skeleton of an extinct animal is something tangible which the imagination can readily clothe with flesh and endow with life; but the first impression which the mind receives of the footprints is like that of an ancient inscription whose characters are those of an unknown tongue, the meaning of which is an enigma unless one has the key. The earlier students attempted to decipher the footprints by comparing them with animals then known, with the
result that they thought they recognized the impressions of batrachians, reptiles, birds and mammals; in fact every group of creatures which were
capable of making tracks. It will be seen at once that the only true key to these nature's hieroglyphics would be actual skeletal remains associated with the footprints or, if found elsewhere, in beds of equivalent age. In 1818 the first fossil skeleton found in the valley was discovered at New Windsor, Conn., and later, just before the civil war, another was brought to light at Springfield, Mass. These, while fragmentary, were recognized to be reptilian in character, and when the latter specimen came to light it at once cast a grave doubt upon the correctness of the accepted interpretations. Hitchcock himself speaks of these remains as being 'those of fair representatives of the creatures which made the tracks.'
It was not until nearly the last decade of the nineteenth century that further excavations at New Windsor, which resulted in the finding of two more specimens, enabled Professor Marsh, of Yale University, to restore the creature and to give us an adequate knowledge of its organization and affinities and thus to furnish the first true key to the correct interpretation of the footprints. Further discoveries abroad, but more especially in our own great west, have given us a very complete knowledge of the magnificent race of reptiles to which the Connecticut Valley forms belong.
During the Mesozoic age, comparable to the medieval times in human history, reptiles were the dominant forms; they occupied the places in the economy of nature to-day taken by the birds and beasts, both animal and plant feeding, as well as by the whales and other denizens of the air, earth and sea; but among the great reptilian assemblage none were more varied in size, aspect and habits than the dinosaurs or terrible lizards, at that time the peers of the animal realm. These creatures are first known from the rocks of the Triassic, the earliest of the three periods into which the Mesozoic age is divided, reaching their millennium as a race during the close of the second or Jurassic period, at which time they attained their greatest profusion in numbers and their highest development in point of size. In the strata formed toward the close of the Cretaceous, or final period, the dinosaurs reach their maximum of specialization, developing forms among the most weird, grotesque, as well as the most terrifying, the world has ever known. This marks the decadence of the race, the prelude to its extinction, for in the immediately overlying rocks of the Tertiary period not the least vestige of a dinosaur has been found.
At least three great orders of Dinosauria are recognized, of which two, embracing the land forms, were represented in the footprint fauna, while of the third, gigantic quadrupeds, whose vast bulk has won for them the name of Cetiosauria or whale lizards, plant feeding and semi, if not wholly, aquatic in their habits, there, is not a trace.
The remaining orders were sharply differentiated in their habits of feeding, the one being carnivorous, the other herbivorous, in diet; and while the more primitive members of both orders were quite similar, such is the influence of habit upon a race that their evolution was a divergent one, the ultimate representatives differing widely from one another in bodily contour, form and structure of the teeth, and in mode of progression.
The Carnivorous Dinosaurs.
The earliest and most primitive of the carnivorous dinosaurs were those already alluded to as having been found at New Windsor and Springfield in the Connecticut valley. These creatures were somewhat lizard-like in general aspect, with the fore limbs fitted for grasping, while the much larger hind limbs, which were very bird-like, were used for locomotion. A study of the skeleton seems to indicate that the center of gravity came just about the region of the hip socket, so that the weight of the tail counterbalanced that of the forward portion of the body, thus making progression upon the hind limbs the only probable gait. The grasping hand, the structure of which is ill-fitted for locomotion, gives color to this assumption.
An extremely interesting slab in the Amherst College Museum, whose upper surface has been worn smooth by the feet of nearly two generations of men, for it did duty as a paving stone in the streets of Middletown, Connecticut, for more than fifty years, bears on its under surface in high relief numerous perfect tracks, in this instance not the footprints themselves, but the natural casts of the feet which made the prints, formed when the incoming tide deposited its load of sediment over the place where the creatures had walked. This bas relief, for such it may be called, admits of the following interpretation: that the makers of the tracks were true bipeds, as all of the casts are those of hind feet, moderately long of limb, walking with alternate steps, with compact bird-like feet, having three toes directed forward with moderately pointed claws, and evidently another directed backward whose claw only occasionally touched the ground. There is no sign of a caudal trace, showing that the tail if present was used only as a counterpoise. The size, length of limb, number and proportions of the toes, and the absence of hand and tail impressions, together with the fact that they are among the most numerous of the tracks, the makers of which would in consequence be among those most likely to be preserved as fossils, all point conclusively to the New Windsor dinosaur as the creature whose existence is thus recorded.
The carnivorous dinosaurs followed at least two lines of evolution, the more conservative of which simply increased in size and in consequent strength and ferocity throughout their racial career. In this group, perhaps the most remarkable feature is the constantly increasing disparity of size between the fore and hind limbs, for in the later forms the arms were so absurdly small that it is difficult to conjecture their use. As the fore limbs decreased in size, the hind legs, in addition to their duty of supporting their owner's weight, had to assume the grasping function as the hand relinquished it, and the claws became in consequence great talons, differing thus markedly from those of the earlier types.
One seems, therefore, justified in interpreting a second group of tracks, much larger than those of the Middletown slab, sharp clawed and with a very narrow sinuous tail trace, as having been made by a dinosaur of this group. If one should picture an animal about twenty feet in length, weighty in build, with talon-like hind claws, and therefore with small fore limbs and with a fairly erect posture so that the tip of the rather short heavy tail touched the ground, one would have a fair notion of this the maximum form, both in size and in degree of specialization of the Connecticut valley carnivores.
The other group of carnivorous dinosaurs were of a very different sort, always retaining their agility and the grasping power of their fore limbs, but not increasing very materially in bulk. A beautiful example of this race from the Jurassic beds of Wyoming has recently been mounted at the American Museum of Natural History, New York, and has been given the name of Ornitholestes, the bird robber, in allusion to its supposed habits. It is a slender animal with an extremely long tail. The hind limbs are fitted for locomotion par excellence, while the fore limbs are more slender with but three very long fingers in the hand, admirable for grasping elusive prey.
In studying the footprint slabs one frequently comes across some very small impressions, hardly exceeding three inches in length, three-toed, with no indication of a grasping claw behind, nor of a tail trace. One might be apt to attribute these little footprints to the young of the larger species until one notices the great interval between the successive tracks, a distance six to eight times the length of the foot itself. This of course gives evidence of extremely long limbs, so that the name of Grallator, he who walks upon stilts, which has been given to this group, is not inapt. While no skeletons are known from the same horizon with which the footprint may be compared, it seems safe to consider it one of these aberrant carnivores, of very slender build, agile, and of habits possibly similar to those of the wading birds.
It is but just to the earlier naturalists to say that we have no absolute proof that Grallator was not a true bird, but that it was seems doubtful, as the foot agrees in structure with that of known dinosaurs, and birds are totally unknown from so remote a period.
Plant feeding dinosaurs are known by their skeletal remains only from the Jurassic and Cretaceous periods, but that they existed during the earlier Triassic seems indubitably certain from the fact that their
Drawn from a slab in the Amherst College Museum.
wide variation and distribution in the Jurassic implies an ancestry in the earlier period, and also that their footprints on the Triassic sandstones are unmistakable.
The more primitive herbivores resemble their carnivorous allies in general contour and in mode of progression, though differing in certain internal characters and in the teeth, these being necessarily changed to enable their owner to chop or grind up the plants upon which it lived. The rear part of the jaws became, in the later forms, veritable magazines of teeth, the latter replacing one another in vertical succession,
a new one being always ready to take the place of one lost or worn out in service. The front part of the mouth bore a few teeth in the earlier types, but these soon gave way to a horny upper and lower beak, turtle-like in aspect, probably used for cropping succulent herbage.
The hind feet were still very bird-like, especially in the less specialized forms, but with blunted claws, while the hand always retained its five fingered condition with short rounded nails. The fore limbs could always be used for the support of the forward parts of the body, though perhaps not always for food gathering. It is extremely doubtful whether any of the earlier plant feeders ever walked on all fours, though in the later forms, owing to the great weight of armament which they carried, a four-footed gait was rendered necessary. The herbivorous dinosaurs proved a more plastic race than their carnivorous brethren, and towards the close of their career there arose among them the remarkable types, already alluded to, which marked the decadence of the group.
In the footprint fauna the herbivores were all true bipeds, but did occasionally impress the hand, including thus all the footprints of the second or occasionally quadrupedal group, and probably some of the first as well, though this is open to question.
The footprints give us thus our first recorded evidence of herbivorous dinosaurs in the Triassic, which will some day probably be verified by the finding of their bones.
One extremely interesting specimen in the Amherst collection hears in all about fifty impressions, most of them made either by the same animal walking back and forth along the beach or by several of approximately the same size. In one of his journeys the creature slows down as shown by the fact that the tail begins to drag, whereas it had been
held out stiffly behind to counterbalance the weight of the body. Then the animal stops and comes down on all fours impressing the little hands and long heels, then, having satisfied its purpose, it rises again to its hind feet, touching one hand and the tail tip once more to the ground in regaining its balance, and then goes on its way. This single slab gives us thus a knowledge of the creature's size, proportions, gait, resting posture, feeding habits, for the little hand with its nail like claws could never have been used for grasping prey, and finally of the texture of the skin on the soles of the feet with creases between the joints, like those of the human fingers, and tiny granulations, like
mustard seed covering the entire surface. Footprints of this character are very common and indicate dinosaurs of rather light build, ranging in size from three and a half to seven feet.
One of the most remarkable of all of the footprints measures twenty inches in length, with four toes directed forward bearing broad rounded claws. The foot was bear-like, in that the entire sole and heel rested upon the ground and bore around its margin a broad web-like flange of skin, the probable function of which was to prevent the creature from sinking too deeply in the soft mud. The hand very rarely impresses, and but one instance is known of a dragging tail. What this uncouth brute looked like, one can not even imagine, for no skeletal remains are known which it in the least resembles.
One very numerous group of large bird-like tracks unaccompanied by hand or tail impressions seems from the blunted claws to have been those of plant feeders, but of this we can not be sure, for the condor of the Andes is also blunt clawed, and suggests the possibility that the makers of these tracks may also have been carrion feeding, which would place them among the carnivores. Certain it is that they were dinosaurs, and among the largest of the valley forms, though probably but half the bulk of their successors in the later rocks. One huge footprint from Northampton, Mass., measures twenty inches in length and holds four quarts of water.
There must have been quite a host of quadrupedal forms in the Triassic days, mainly of small size, but while they were probably of amphibian or reptilian origin nothing was really known of them until very recently. Professor Marsh found, some years ago, the remains of one animal, but unfortunately only the impression of the armor of the back remained, and, as the limbs were lacking, nothing could be learned of its probable footprints. A second specimen has just been brought to light, found in the village of Longmeadow, Mass., sufficient of which is preserved to show that the creature was long of limb and was probably a rapid runner. From its size and proportions, it corresponds very closely with one of the most numerous of the quadrupedal tracks. The Longmeadow specimen belongs to a group of primitive crocodile-like reptiles, to which Professor Huxley has given the name of Parasuchia. The footprints are small, but with a long interval between the successive tracks, with sharp claws, with four toes on the foot and five on the much smaller hand. Thus far only may we interpret the quadrupedal footprints with any assurance, for beyond this we are in the realm of almost pure conjecture, which in footprint interpretation has thus far nearly always proved wrong.
To summarize briefly, the footprint fauna contains amphibians, reptiles, and possibly birds; of the first we as yet know nothing, but we may be reasonably sure that they occurred; of the reptiles we have identified numerous dinosaurs, representatives of both great land-inhabiting orders, and we have also found indications of early crocodile-like forms. Other reptilian orders were doubtless present, but what they were we have as yet no means of knowing. Finally the only creatures which could have been birds could as readily have been dinosaurs and such in all probability they were.
Occurrence and Means of Preservation.
Fossil footprints have been found in various parts of the world, as in England, Germany, France, and, in our own country, in the Grand Canon of the Colorado, as well as in association with Jurassic dinosaurs in the northwest; but it is in the valley of the Connecticut and in rocks of the same formation in New Jersey that they occur in an abundance and perfection of preservation which is unrivaled elsewhere.
In this region sandstone beds of great thickness occur which every now and then exhibit impressions with generally something of an interval between the track-bearing layers.
Geologists have been led to suppose that the broad valley of the Connecticut was during Triassic times a tidal estuary extending from the village of Northfield, Mass., to New Haven, a distance of one hundred and ten miles, with an average width of twenty miles. In places in this estuary were mud flats, some well out in the ancient bay, others nearer the shore, which were left bare by the receding tides. Here the animals loved to congregate, possibly they came for food, but it seems more likely that the dinosaurs here assembled at certain seasons for mating as the seals do in the Alaskan rookeries.
The means of preservation were threefold, of which the first was the fierce heat of a tropical sun, for plant remains indicate that such climatic conditions prevailed, while the second and third are really attributable to one cause, volcanic activity. This resulted in the formation of the Holyoke and Mt. Tom ranges and in broad lava flows. Upon these sheets the sand and silt were deposited, forming thus the tidal flats where the creatures congregated. The heat of the cooling lava added its baking effect to that of the sun, while the decomposing lava liberated an iron cement which completed the task of solidifying the overlying material into rock.
The impressions made when the tide had ebbed were thus somewhat hardened before the incoming flood bearing its burden of sediment gently buried the traces without the least injury, thus preserving for our enlightenment these monuments of the past.