Animals of the Past

If an animal dies on dry land, where its bones lie exposed to the summer's sun and rain and the winter's frost and snow, it does not take these destructive agencies long to reduce the bones to powder; in the rare event of a climate devoid of rain, mere changes of temperature, by producing expansion and contraction, will sooner or later cause a bone to crack and crumble.

Usually, too, the work of the elements is aided by that of animals and plants. Every one has seen a dog make way with a pretty good-sized bone, and the Hyena has still greater capabilities in that line; and ever since vertebrate life began there have been carnivorous animals of some kind to play the rôle of bone-destroyers. Even were there no carnivores, there were probably then, as now, rats and mice a-plenty, and few suspect the havoc small rodents may play with a bone for the grease it contains, or merely for the sake of exercising their teeth. Now and then we come upon a fossil bone, long since turned into stone, on which are the marks of the little cutting teeth of field mice, put there long, long ago, and yet looking as fresh as if made only last week. These little beasts, however, are indirect rather than direct agents in the destruction of bones by gnawing off the outer layers, and thus permitting the more ready entrance of air and water. Plants, as a rule, begin their work after an object has become partly or entirely buried in the soil, when the tiny rootlets find their way into fissures, and, expanding as they grow, act like so many little wedges to force it asunder.

Thus on dry land there is small opportunity for a bone to become a fossil; but, if a creature so perishes that its body is swept into the ocean or one of its estuaries, settles to the muddy bottom of a lake or is caught on the sandy shoals of some river, the chances are good that its bones will be preserved. They are poorest in the ocean, for unless the body drifts far out and settles down in quiet waters, the waves pound the bones to pieces with stones or scour them away with sand, while marine worms may pierce them with burrows, or echinoderms cut holes for their habitations; there are more enemies to a bone than one might imagine.

Suppose, however, that some animal has sunk in the depths of a quiet lake, where the wash of the waves upon the shore wears the sand or rock into mud so fine that it floats out into still water and settles there as gently as dew upon the grass. Little by little the bones are covered by a deposit that fills every groove and pore, preserving the mark of every ridge and furrow; and while this may take long, it is merely a matter of time and favorable circumstance to bury the bones as deeply as one might wish. Scarce a reader of these lines but at some time has cast anchor in some quiet pond and pulled it up, thickly covered with sticky mud, whose existence would hardly be suspected from the sparkling waters and pebbly shores. If, instead of a lake, our animal had gone to the bottom of some estuary into which poured a river turbid with mud, the process of entombment would have been still more rapid, while, had the creature been engulfed in quicksand, it would have been the quickest method of all; and just such accidents did take place in the early days of the earth as well as now. At least two examples of the great Dinosaur Thespesius have been found with the bones all in place, the thigh bones still in their sockets and the ossified tendons running along the backbone as they did in life. This would hardly have happened had not the body been surrounded and supported so that every part was held in place and not crushed, and it is difficult to see any better agency for this than burial in quicksand.

If such an event as we have been supposing took place in a part of the globe where the land was gradually sinking—and the crust of the earth is ever rising and falling—the mud and sand would keep on accumulating until an enormously thick layer was formed. The lime or silica contained in the water would tend to cement the particles of mud and grains of sand into a solid mass, while the process would be aided by the pressure of the overlying sediment, the heat created by this pressure, and that derived from the earth beneath. During this process the animal matter of bones or other objects would disappear and its place be taken by lime or silica, and thus would be formed a layer of rock containing fossils. The exact manner in which this replacement is effected and in which the chemical and mechanical changes occur is very far from being definitely known—especially as the process of "fossilization" must at times have been very complicated.

In the case of fossil wood greater changes have taken place than in the fossilization of bone, for there is not merely an infiltration of the specimen but a complete replacement of the original vegetable by mineral matter, the interior of the cells being first filled with silica and their walls replaced later on. So completely and minutely may this change occur that under the microscope the very cellular structure of the wood is visible, and as this varies according to the species, it is possible, by microscopical examination, to determine the relationship of trees in cases where nothing but fragments of the trunk remain.

The process of fossilization is at best a slow one, and soft substances such as flesh, or even horn, decay too rapidly for it to take place, so that all accounts of petrified bodies, human or otherwise, are either based on deliberate frauds or are the result of a very erroneous misinterpretation of facts. That the impression or cast of a body might be formed in nature, somewhat as casts have been made of those who perished at Pompeii, is true; but, so far, no authentic case of the kind has come to light, and the reader is quite justified in disbelieving any report of "a petrified man."

Natural casts of such hard bodies as shells are common, formed by the dissolving away of the original shell after it had become enclosed in mud, or even after this had changed to stone, and the filling up of this space by the filtering in of water charged with lime or silica, which is there deposited, often in crystalline form. In this way, too, are formed casts of eggs of reptiles and birds, so perfect that it is possible to form a pretty accurate opinion as to the group to which they belong.

Sometimes it happens that shells or other small objects imbedded in limestone have been dissolved and replaced by silica, and in such cases it is possible to eat away the enveloping rock with acid and leave the silicified casts. By this method specimens of shells, corals, and bryozoans are obtained of almost lace-like delicacy, and as perfect as if only yesterday gathered at the sea-shore. Casts of the interior of shells, showing many details of structure, are common, and anyone who has seen clams dug will understand how they are formed by the entrance of mud into the empty shell.

Casts of the kernels of nuts are formed in much the same way, and Professor E. H. Barbour has thus described the probable manner in which this was done. When the nuts were dropped into the water of the ancient lake the kernel rotted away, but the shell, being tough and hard, would probably last for years under favorable circumstances. Throughout the marls and clays of the Bad Lands (of South Dakota) there is a large amount of potash. This is dissolved by water, and then acts upon quartz, carrying it away in solution. This would find its way by infiltration into the interior of the nut. At the same time with this process, carrying lime carbonate in solution was going on, so that doubtless the stone kernels, consisting of pretty nearly equal parts of lime and silica, were deposited within the nuts. These kernels, of course, became hard and flinty in time, and capable of resisting almost any amount of weathering. Not so the organic shell; this eventually would decay away, and so leave the filling or kernel of chalcedony and lime.[1]

"Fossil leaves" are nothing but fine casts, made in natural moulds, and all have seen the first stages in their formation as they watched the leaves sailing to the ground to be covered by mud or sand at the next rain, or dropping into the water, where sooner or later they sink, as we may see them at the bottom of any quiet woodland spring.

Impressions of leaves are among the early examples of color-printing, for they are frequently of a darker, or even different, tint from that of the surrounding rock, this being caused by the carbonization of vegetable matter or to its action on iron that may have been present in the soil or water. Besides complete mineralization, or petrifaction, there are numerous cases of incomplete or semi-fossilization, where modern objects, still retaining their phosphate of lime and some animal matter even, are found buried in rock. This takes place when water containing carbonate of lime, silica, or sometimes iron, flows over beds of sand, cementing the grains into solid but not dense rock, and at the same time penetrating and uniting with it such things as chance to be buried. In this way was formed the "fossil man" of Guadeloupe, West Indies, a skeleton of a modern Carib lying in recent concretionary limestone, together with shells of existing species and fragments of pottery. In a similar way, too, human remains in parts of Florida have, through the infiltration of water charged with iron, become partially converted into limonite iron ore; and yet we know that these bones have been buried within quite recent times.

Sometimes we hear of springs or waters that "turn things into stone," but these tales are quite incorrect. Waters there are, like the celebrated hot springs of Auvergne, France, containing so much carbonate of lime in solution that it is readily deposited on objects placed therein, coating them more or less thickly, according to the length of time they are allowed to remain. This, however, is merely an encrustation, not extending into the objects. In a similar way the precipitation of solid material from waters of this description forms the porous rock known as tufa, and this often encloses moss, twigs, and other substances that are in no way to be classed with fossils.

But some streams, flowing over limestone rocks, take up considerable carbonate of lime, and this may be deposited in water-soaked logs, replacing more or less of the woody tissue and thus really partially changing the wood into stone.

The very rocks themselves may consist largely of fossils; chalk, for example, is mainly made up of the disintegrated shells of simple marine animals called foraminifers, and the beautiful flint-like "skeletons" of other small creatures termed radiolarians, minute as they are, have contributed extensively to the formation of some strata.

Even after an object has become fossilized, it is far from certain that it will remain in good condition until found, while the chance of its being found at all is exceedingly small. When we remember that it is only here and there that nature has made the contents of the rocks accessible by turning the strata on edge, heaving them into cliffs or furrowing them with valleys and canyons, we realize what a vast number of pages of the fossil record must remain not only unread, but unseen. The wonder is, not that we know so little of the history of the past, but that we have learned so much, for not only is nature careless in keeping the records—preserving them mostly in scattered fragments—but after they have been laid away and sealed up in the rocks they are subject to many accidents. Some specimens get badly flattened by the weight of subsequently deposited strata, others are cracked and twisted by the movements of the rocks during periods of upheaval or subsidence, and when at last they are brought to the surface, the same sun and rain, snow and frost, from which they once escaped, are ready to renew the attack and crumble even the hard stone to fragments. Such, very briefly, are some of the methods by which fossils may be formed, such are some of the accidents by which they may be destroyed; but this description must be taken as a mere outline and as applying mainly to vertebrates, or backboned animals, since it is with them that we shall have to deal. It may, however, show why it is that fossils are not more plentiful, why we have mere hints of the existence of many animals, and why myriads of creatures may have flourished and passed away without so much as leaving a trace of their presence behind.

REFERENCES

A very valuable and interesting article by Dr. Charles A. White, entitled "The Relation of Biology to Geological Investigation," will be found in the Report of the United States National Museum for 1892. This comprises a series of essays on the nature and scientific uses of fossil remains, their origin, relative chronological value and other questions pertaining to them. The United States National Museum has published a pamphlet, part K, Bulletin 39, containing directions for collecting and preparing fossils, by Charles Schuchert; and another, part B, Bulletin 39, collecting recent and fossil plants, by F. H. Knowlton.

II

There is a universal, and perfectly natural, desire for information, which in ourselves we term thirst for knowledge and in others call curiosity, that makes mankind desire to know how everything began and causes much speculation as to how it all will end. This may take the form of a wish to know how a millionaire made his first ten cents, or it may lead to the questions—What is the oldest animal? or, What is the first known member of the great group of backboned animals at whose head man has placed himself? and, What did this, our primeval and many-times-removed ancestor, look like? The question is one that has ever been full of interest for naturalists, and Nature has been interrogated in various ways in the hope that she might be persuaded to yield a satisfactory answer. The most direct way has been that of tracing back the history of animal life by means of fossil remains, but beyond a certain point this method cannot go, since, for reasons stated in various places in these pages, the soft bodies of primitive animals are not preserved. To supplement this work, the embryologist has studied the early stages of animals, as their development throws a side-light on their past history. And, finally, there is the study of the varied forms of invertebrates, some of which have proved to be like vertebrates in part of their structure, while others have been revealed as vertebrates in disguise. So far these various methods have yielded various answers, or the replies, like those of the Delphic Oracle, have been variously interpreted so that vertebrates are considered by some to have descended from the worms, while others have found their beginnings in some animal allied to the King Crab.

Every student of genealogy knows only too well how difficult a matter it is to trace a family pedigree back a few centuries, how soon the family names become changed, the line of descent obscure, and how soon gaps appear whose filling in requires much patient research. How much more difficult must it be, then, to trace the pedigree of a race that extends, not over centuries, but thousands of centuries; how wide must be some of the gaps, how very different may the founders of the family be from their descendants! The words old and ancient that we use so often in speaking of fossils appeal to us somewhat vaguely, for we speak of the ancient civilizations of Greece and Rome, and call a family old that can show a pedigree running back four or five hundred years, when such as these are but affairs of yesterday compared with even recent fossils.

Perhaps we may better appreciate the meaning of these words by recalling that, since the dawn of vertebrate life, sufficient of the earth's surface has been worn away and washed into the sea to form, were the strata piled directly one upon the other, fifteen or twenty miles of rock. This, of course, is the sum total of sedimentary rocks, for such a thickness as this is not to be found at any one locality; because, during the various ups and downs that this world of ours has met with, those portions that chanced to be out of water would receive no deposit of mud or sand, and hence bear no corresponding stratum of rock. The reader may think that there is a great deal of difference between fifteen and twenty miles, but this liberal margin is due to the difficulty of measuring the thickness of the rocks, and in Europe the sum of the measurable strata is much greater than in North America.

The earliest traces of animal life are found deeper still, beneath something like eighteen to twenty-five miles of rock, while below this level are the strata in which dwelt the earliest living things, organisms so small and simple that no trace of their existence has been left, and we infer that they were there because any given group starts in a modest way with small and simple individuals.

At the bottom, then, of twenty miles of rocks the seeker for the progenitor of the great family of backboned animals finds the scant remains of fish-like animals that the cautious naturalist, who is much given to "hedging," terms, not vertebrates, but prevertebrates or the forerunners of backboned animals. The earliest of these consist of small bony plates, and traces of a cartilaginous backbone from the Lower Silurian of Colorado, believed to represent relatives of Chimæra and species related to those better-known forms Holoptychius and Osteolepis, which occur in higher strata. There are certainly indications of vertebrate life, but the remains are so imperfect that little more can be said regarding them, and this is also true of the small conical teeth which occur in the Lower Silurian of St. Petersburg, and are thought to be the teeth of some animal like the lamprey.

A little higher up in the rocks, though not in the scale of life, in the Lower Old Red Sandstone of England, are found more numerous and better preserved specimens of another little fish-like creature, rarely if ever exceeding two inches in length, and also related (probably) to the hag-fishes and lampreys that live to-day.

These early vertebrates are not only small, but they were cartilaginous, so that it was essential for their preservation that they should be buried in soft mud as soon as possible after death. Even if this took place they were later on submitted to the pressure of some miles of overlying rock until, in some cases, their remains have been pressed out thinner than a sheet of paper, and so thoroughly incorporated into the surrounding stone that it is no easy matter to trace their shadowy outlines. With such drawbacks as these to contend with, it can scarcely be wondered at that, while some naturalists believe these little creatures to be related to the lamprey, others consider that they belong to a perfectly distinct group of animals, and others still think it possible that they may be the larval or early stages of larger and better-developed forms.

Still higher up we come upon the abundant remains of numerous small fish-like animals, more or less completely clad in bony armor, indicating that they lived in troublous times when there was literally a fight for existence and only such as were well armed or well protected could hope to survive. A parallel case exists to-day in some of the rivers of South America, where the little cat-fishes would possibly be eaten out of existence but for the fact that they are covered—some of them very completely—with plate-armor that enables them to defy their enemies, or renders them such poor eating as not to be worth the taking. The arrangement of the plates or scales in the living Loricaria is very suggestive of the series of bony rings covering the body of the ancient Cephalaspis, only the latter, so far as we know, had no side-fins; but the creatures are in no wise related, and the similarity is in appearance only.

Pterichthys, the wing fish, was another small, quaint, armor-clad creature, whose fossilized remains were taken for those of a crab, and once described as belonging to a beetle. Certainly the buckler of this fish, which is the part most often preserved, with its jointed, bony arms, looks to the untrained eye far more like some strange crustacean than a fish, and even naturalists have pictured the animal as crawling over the bare sands by means of those same arms. These fishes and their allies were once the dominant type of life, and must have abounded in favored localities, for in places are great deposits of their protective shields jumbled together in a confused mass, and, save that they have hardened into stone, lying just as they were washed up on the ancient beach ages ago. How abundant they were may be gathered from the fact that it is believed their bodies helped consolidate portions of the strata of the English Old Red Sandstone. Says Mr. Hutchinson, speaking of the Caithness Flagstones, "They owe their peculiar tenacity and durability to the dead fishes that rotted in their midst while yet they were only soft mud. For just as a plaster cast boiled in oil becomes thereby denser and more durable, so the oily and other matter coming from decomposing fish operated on the surrounding sand or mud so as to make it more compact."

It may not be easy to explain how it came to pass that fishes dwelling in salt water, as these undoubtedly did, were thus deposited in great numbers, but we may now and then see how deposits of fresh-water fishes may have been formed. When rivers flowing through a stretch of level country are swollen during the spring floods, they overflow their banks, often carrying along large numbers of fishes. As the water subsides these may be caught in shallow pools that soon dry up, leaving the fishes to perish, and every year the Illinois game association rescues from the "back waters" quantities of bass that would otherwise be lost. Mr. F. S. Webster has recorded an instance that came under his observation in Texas, where thousands of gar pikes, trapped in a lake formed by an overflow of the Rio Grande, had been, by the drying up of this lake, penned into a pool about seventy-five feet long by twenty-five feet wide. The fish were literally packed together like sardines, layer upon layer, and a shot fired into the pool would set the entire mass in motion, the larger gars as they dashed about casting the smaller fry into the air, a score at a time. Mr. Webster estimates that there must have been not less than 700 or 800 fish in the pool, from a foot and a half up to seven feet in length, every one of which perished a little later. In addition to the fish in the pond, hundreds of those that had died previously lay about in every direction, and one can readily imagine what a fish-bed this would have made had the occurrence taken place in the past.

From the better-preserved specimens that do now and then turn up, we are able to obtain a very exact idea of the construction of the bony cuirass by which Pterichthys and its American cousin were protected, and to make a pretty accurate reconstruction of the entire animal. These primitive fishes had mouths, for eating is a necessity; but these mouths were not associated with true jaws, for the two do not, as might be supposed, necessarily go together. Neither did these animals possess hard backbones, and, while Pterichthys and its relatives had arms or fins, the hard parts of these were not on the inside but on the outside, so that the limb was more like the leg of a crab than the fin of a fish; and this is among the reasons why some naturalists have been led to conclude that vertebrates may have developed from crustaceans. Pteraspis, another of these little armored prevertebrates, had a less complicated covering, and looked very much like a small fish with its fore parts caught in an elongate clam-shell.

The fishes that we have so far been considering—orphans of the past they might be termed, as they have no living relatives—were little fellows; but their immediate successors, preserved in the Devonian strata, particularly of North America, were the giants of those days, termed, from their size and presumably fierce appearance, Titantichthys and Dinichthys, and are related to a fish, Ceratodus, still living in Australia.

We know practically nothing of the external appearance of these fishes, great and fierce though they may have been, with powerful jaws and armored heads, for they had no bony skeleton—as if they devoted their energies to preying upon their neighbors rather than to internal improvements. They attained a length of ten to eighteen feet, with a gape, in the large species called Titanichthys, of four feet, and such a fish might well be capable of devouring anything known to have lived at that early date.

Succeeding these, in Carboniferous times, came a host of shark-like creatures known mainly from their teeth and spines, for their skeletons were of cartilage, and belonging to types that have mostly perished, giving place to others better adapted to the changed conditions wrought by time. Almost the only living relative of these early fishes is a little shark, known as the Port Jackson Shark, living in Australian waters. Like the old sharks, this one has a spine in front of his back fins, and, like them, he fortunately has a mouthful of diversely shaped teeth; fortunately, because through their aid we are enabled to form some idea of the manner in which some of the teeth found scattered through the rocks were arranged. For the teeth were not planted in sockets, as they are in higher animals, but simply rested on the jaws, from which they readily became detached when decomposition set in after death. To complicate matters, the teeth in different parts of the jaws were often so unlike one another that when found separately they would hardly be suspected of having belonged to the same animal. Besides teeth these fishes, for purposes of offence and defence, were usually armed with spines, sometimes of considerable size and strength, and often elaborately grooved and sculptured. As the soft parts perished the teeth and spines were left to be scattered by waves and currents, a tooth here, another there, and a spine somewhere else; so it has often happened that, being found separately, two or three quite different names have been given to one and the same animal. Now and then some specimen comes to light that escaped the thousand and one accidents to which such things were exposed, and that not only shows the teeth and spines but the faint imprint of the body and fins as well. And from such rare examples we learn just what teeth and spines go with one another, and sometimes find that one fish has received names enough for an entire school.

These ancient sharks were not the large and powerful fishes that we have to-day—these came upon the scene later—but mostly fishes of small size, and, as indicated by their spines, fitted quite as much for defence as offence. Their rise was rapid, and in their turn they became the masters of the world, spreading in great numbers through the waters that covered the face of the earth; but their supremacy was of short duration, for they declined in numbers even during the Carboniferous Period, and later dwindled almost to extinction. And while sharks again increased, they never reached their former abundance, and the species that arose were swift, predatory forms, better fitted for the struggle for existence.

REFERENCES

The early fishes make but little show in a museum, both on account of their small size and the conditions under which they have been preserved. The Museum of Comparative Zoölogy has a large collection of these ancient vertebrates, and there is a considerable number of fine teeth and spines of Carboniferous sharks in the United States National Museum.

Hugh Miller's "The Old Red Sandstone" contains some charming descriptions of his discoveries of Pterichthys and related forms, and this book will ever remain a classic.

III

The Rev. H. N. Hutchinson commences one of his interesting books with Emerson's saying, "that Everything in nature is engaged in writing its own history;" and, as this remark cannot be improved on, it may well stand at the head of a chapter dealing with the footprints that the creatures of yore left on the sands of the sea-shore, the mud of a long-vanished lake bottom, or the shrunken bed of some water-course. Not only have creatures that walked left a record of their progress, but the worms that burrowed in the sand, the shell-fish that trailed over the mud when the tide was low, the stranded crab as he scuttled back to the sea—each and all left some mark to tell of their former presence. Even the rain that fell and the very wind that blew sometimes recorded the direction whence they came, and we may read in the rocks, also, accounts of freshets sweeping down with turbid waters, and of long periods of drouth, when the land was parched and lakes and rivers shrank beneath the burning sun.

All these things have been told and retold; but, as there are many who have not read Mr. Hutchinson's books and to whom Buckland is quite unknown, it may be excusable to add something to what has already been said in the first chapter of these impressions of the past.

The very earliest suggestion we have of the presence of animal life upon this globe is in the form of certain long dark streaks below the Cambrian of England, considered to be traces of the burrows of worms that were filled with fine mud, and while this interpretation may be wrong there is, on the other hand, no reason why it may not be correct. Plant and animal life must have had very lowly beginnings, and it is not at all probable that we shall find any trace of the simple and minute forms with which they started,[2] though we should not be surprised at finding hints of the presence of living creatures below the strata in which their remains are actually known to occur.

Worm burrows, to be sure, are hardly footprints, but tracks are found in Cambrian rocks just above the strata in which the supposed burrows occur, and from that time onward there are tracks a-plenty, for they have been made, wherever the conditions were favorable, ever since animals began to walk. All that was needed was a medium in which impressions could be made and so filled that there was imperfect adhesion between mould and matrix. Thus we find them formed not only by the sea-shore, in sands alternately dry and covered, but by the river-side, in shallow water, or even on land where tracks might be left in soft or moist earth into which wind-driven dust or sand might lodge, or sand or mud be swept by the mimic flood caused by a thunder shower.

So there are tracks in strata of every age; at first those of invertebrates: after the worm burrows the curious complicated trails of animals believed to be akin to the king crab; broad, ribbed, ribbon-like paths ascribed to trilobites; then faint scratches of insects, and the shallow, palmed prints of salamanders, and the occasional slender sprawl of a lizard; then footprints, big and little, of the horde of Dinosaurs and, finally, miles above the Cambrian, marks of mammals. Sometimes, like the tracks of salamanders and reptiles in the carboniferous rocks of Pennsylvania and Kansas, these are all we have to tell of the existence of air-breathing animals. Again, as with the iguanodon, the foot to fit the track may be found in the same layer of rock, but this is not often the case.

Although footprints in the rocks must often have been seen, they seem to have attracted little or no notice from scientific men until about 1830 to 1835, when they were almost simultaneously described both in Europe and America; even then, it was some time before they were generally conceded to be actually the tracks of animals, but, like worm burrows and trails, were looked upon as the impressions of sea-weeds.

The now famous tracks in the "brown stone" of the Connecticut Valley seem to have first been seen by Pliny Moody in 1802, when he ploughed up a specimen on his farm, showing small imprints, which later on were popularly called the tracks of Noah's raven. The discovery passed without remark until in 1835 the footprints came under the observation of Dr. James Deane, who, in turn, called Professor Hitchcock's attention to them. The latter at once began a systematic study of these impressions, publishing his first account in 1836 and continuing his researches for many years, in the course of which he brought together the fine collection in Amherst College. At that time Dinosaurs were practically unknown, and it is not to be wondered at that these three-toed tracks, great and small, were almost universally believed to be those of birds. So it is greatly to the credit of Dr. Deane, who also studied these footprints, that he was led to suspect that they might have been made by other animals. This suspicion was partly caused by the occasional association of four and five-toed prints with the three-toed impressions, and partly by the rare occurrence of imprints showing the texture of the sole of the foot, which was quite different from that of any known bird.

In the light of our present knowledge we are able to read many things in these tracks that were formerly more or less obscure, and to see in them a complete verification of Dr. Deane's suspicion that they were not made by birds. We see clearly that the long tracks called Anomœpus, with their accompanying short fore feet, mark where some Dinosaur squatted down to rest or progressed slowly on all-fours, as does the kangaroo when feeding quietly;[3] and we interpret the curious heart-shaped depression sometimes seen back of the feet, not as the mark of a stubby tail, but as made by the ends of the slender pubes, bones that help form the hip-joints. Then, too, the mark of the inner, or short first, toe, is often very evident, although it was a long time before the bones of this toe were actually found, and many of the Dinosaurs now known to have four toes were supposed to have but three.

It seems strange, and it is strange, that while so many hundreds of tracks should have been found in the limited area exposed to view, so few bones have been found—our knowledge of the veritable animals that made the tracks being a blank. A few examples have, it is true, been found, but these are only a tithe of those known to have existed; while of the great animals that strode along the shore, leaving tracks fifteen inches long and a yard apart pressed deeply into the hard sand, not a bone remains. The probability is that the strata containing their bones lie out to sea, whither their bodies were carried by tides and currents, and that we may never see more than the few fragments that were scattered along the seaside.

That part of the Valley of the Connecticut wherein the footprints are found seems to have been a long, narrow estuary running southward from Turner's Falls, Mass., where the tracks are most abundant and most clear. The topography was such that this estuary was subject to sudden and great fluctuations of the water-level, large tracts of shore being now left dry to bake in the sun, and again covered by turbid water which deposited on the bottom a layer of mud. Over and over again this happened, forming layer upon layer of what is now stone, sometimes the lapse of time between the deposits being so short that the tracks of the big Dinosaurs extend through several sheets of stone; while again there was a period of drouth when the shore became so dry and firm as to retain but a single shallow impression.

Something of the wealth of animal life that roamed about this estuary may be gathered from the number of different footprints recorded on the sands, and these are so many and so varied that Professor Hitchcock in two extensive reports enumerated over 150 species, representing various groups of animals. One little point must, however, be borne in mind, that mere size is no sure indication of differences in dealing with reptiles, for these long-lived creatures grow almost continuously throughout life, so that one animal even may have left his footprints over and over in assorted sizes from one end of the valley to the other.

The slab shown in Fig. 7 is a remarkably fine example of these Connecticut River footprints; it shows in relief forty-eight tracks of the animal called Brontozoum sillimanium and six of a lesser species. It was quarried near Middletown, in 1778, and for sixty years did duty as a flagstone, fortunately with the face downwards. When taken up for repairs the tracks were discovered, and later on the slab, which measures three by five feet, was transferred to the museum of Amherst College.

There is an interesting parallel between the history of footprints in England and America, for they were noticed at about the same time, 1830, in both countries; in each case the tracks were in rocks of Triassic age, and, in both instances, the animals that made them have never been found. In England, however, the tracks first found were those ascribed to tortoises, though a little later Dinosaur footprints were discovered in the same locality. Oddly enough these numerous tracks all run one way, from west to east, as if the animals were migrating, or were pursuing some well-known and customary route to their feeding grounds.

For some reason Triassic rocks are particularly rich in footprints; for from strata of this same age in the Rhine Valley come those curious examples so like the mark of a stubby hand that Dr. Kaup christened the beast supposed to have made them Cheirotherium, beast with a hand, suggesting that they had been made by some gigantic opossum. As the tracks measure five by eight inches, it would have been rather a large specimen, but the mammals had not then arisen, and it is generally believed that the impressions were made by huge (for their kind) salamander-like creatures, known as labyrinthodonts, whose remains are found in the same strata.

Footprints may aid greatly in determining the attitude assumed by extinct animals, and in this way they have been of great service in furnishing proof that many of the Dinosaurs walked erect. The impressions on the sands of the old Connecticut estuary may be said to show this very plainly, but in England and Belgium is evidence still more conclusive, in the shape of tracks ascribed to the Iguanodon. These were made on soft soil into which the feet sank much more deeply than in the Connecticut sands, and the casts made in the natural moulds show the impression of toes very clearly. If the animals had walked flat-footed, as we do, the prints of the toes would have been followed by a long heel mark, but such is not the case; there are the sharply defined marks of the toes and nothing more, showing plainly that the Iguanodons walked, like birds, on the toes alone. More than this, had these Dinosaurs dragged their tails there would have been a continuous furrow between the footprints; but nothing of this sort is to be found; on the contrary, a fine series of tracks, uncovered at Hastings, England, made by several individuals and running for seventy-five feet, shows footprints only. Hence it may be fairly concluded that these great creatures carried their tails clear of the ground, as shown in the picture of Thespesius, the weight of the tail counterbalancing that of the body. Where crocodilians or some of the short-limbed Dinosaurs have crept along there is, as we should expect, a continuous furrow between the imprints of the feet. This is what footprints tell us when their message is read aright; when improperly translated they only add to the enormous bulk of our ignorance.

Some years ago we were treated to accounts of wonderful footprints in the rock of the prison-yard at Carson City, Nev., which, according to the papers, not only showed that men existed at a much earlier period than the scientific supposed, but that they were men of giant stature. This was clearly demonstrated by the footprints, for they were such as might have been made by huge moccasined feet, and this was all that was necessary for the conclusion that they were made by just such feet. For it is a curious fact that the majority of mankind seem to prefer any explanation other than the most simple and natural, particularly in the case of fossils, and are always looking for a primitive race of gigantic men.

Bones of the Mastodon and Mammoth have again and again been eagerly accepted as those of giants; a salamander was brought forward as evidence of the deluge (homo diluvii testis); ammonites and their allies pose as fossil snakes, and the "petrified man" flourishes perennially. However, in this case the prints were recognized by naturalists as having most probably been made by some great ground sloth, such as the Mylodon or Morotherium, these animals, though belonging to a group whose headquarters were in Patagonia, having extended their range as far north as Oregon. That the tracks seemed to have been made by a biped, rather than a quadruped, was due to the fact that the prints of the hind feet fell upon and obliterated the marks of the fore. Still, a little observation showed that here and there prints of the fore feet were to be seen, and on one spot were indications of a struggle between two of the big beasts. The mud, or rather the stone that had been mud, bears the imprints of opposing feet, one set deeper at the toes, the other at the heels, as if one animal had pushed and the other resisted. In the rock, too, are broad depressions bearing the marks of coarse hair, where one creature had apparently sat on its haunches in order to use its fore limbs to the best advantage. Other footprints there are in this prison-yard; the great round "spoor" of the mammoth, the hoofs of a deer, and the paws of a wolf(?), indicating that hereabout was some pool where all these creatures came to drink. More than this, we learn that when these prints were made, or shortly after, a strong wind blew from the southeast, for on that face of the ridges bounding the margin of each big footprint, we find sand that lodged against the squeezed-up mud and stuck there to serve as a perpetual record of the direction of the wind.

REFERENCES

Almost every museum has some specimen of the Connecticut Valley footprints, but the largest and finest collections are in the museums of Amherst College, Mass., and Yale University, although, owing to lack of room, only a few of the Yale specimens are on exhibition. The collection at Amherst comprises most of the types described by Professor E. Hitchcock in his "Ichnology of New England," a work in two fully illustrated quarto volumes. Other footprints are described and figured by Dr. J. Deane in "Ichnographs from the Sandstone of the Connecticut River."