The American Cyclopædia (1879)/Fossil Footprints
FOSSIL FOOTPRINTS, or Ichnolites (Gr. ἴχνος, a track, and λίθος, a stone), in geology, impressions originally made by animals in clay or in sand, and preserved in the shale or sandstone rock resulting from the solidification of those materials. Under these names have been included markings of various forms in rocks of very different geological ages. Some of these markings, though doubtless made by animals, are not the impressions of their feet, but have been produced wholly or in part by their tails or their carapaces; and to these, although truly ichnolites or track-stones, the name of fossil footprints does not therefore apply. It will, however, be convenient to include under this head all the markings of animals found in rocks. Recent impressions of the feet of quadrupeds, birds, and reptiles, and the markings made by crustaceans, mollusks, and worms, may be studied on the shores of tidal waters, where successive layers of mud and sand preserve them in the accumulating sediments; and the careful study of these by Dawson has thrown much light on some of the markings found in earlier rocks. To begin with the ichnolites found in rocks of cenozoic or tertiary age, we may notice those in the eocene of the basin of Paris, where, in the marls which are interstratified with the gypsum beds which there abound, are found a great variety of markings. Prominent among these are the trilobed footprints of several species of palæotherium, a large pachyderm allied to the modern tapir, besides those of anoplotherium, an animal more nearly allied to the ruminants, and of certain carnivorous mammals. In addition to these are tracks of various land and fresh-water tortoises, of a gigantic bird, and of crocodiles, iguanas, and great batrachians or frog-like animals. The bones of many of the mammals whose tracks are here met with occur in the gypsum beds which are inter stratified with the marls; but there were evidently numerous species of which the bones have not been discovered, and which are consequently known to us only by their foot marks. The whole condition of things here shows that there then existed numerous small lakes of fresh water, the shores of which were frequented by great numbers of pachyderms of numerous species, and by beasts of prey which occasionally devoured them, the tooth marks of the carnivora being found on the bones of the former. It will thus be seen that it is only in rare localities that the conditions necessary for the formation and preservation of these foot marks occur, and it is a fortunate chance which exposes them for our inspection. It was not till 1859 that these were discovered in the neighborhood of Paris.—In the mesozoic period the footprints of the trias or new red sandstone are remarkable for their number and variety, and also for the interest which attaches to the history of their discovery in the valley of the Connecticut river, where they are very abundant. Attention was first called to these so-called bird tracks by Mr. Dexter Marsh, and they were subsequently studied by Dr. James Deane and by Prof. Edward Hitchcock, who after a careful examination of them concluded that they were truly the footprints of birds; and they were therefore called by him ornithichnites or bird tracks. He ascertained their existence in numerous localities, and showed that they occur at intervals through a thickness of 1,000 ft. of sandstones and shales. He further remarked that although the beds bearing the tracks are now inclined at angles of from 5° to 30°, they must have been horizontal at the time the impressions were made; and showed that their occurrence throughout so great a thickness of strata could only be accounted for by supposing that the surface was subsiding during the deposition of these rocks. Some of these tracks were of gigantic size, one of them measuring 10 by 16 in., and recurring at intervals of from 4 to 6 ft. along the surface of the rocky bed; these distances indicating the length of the strides made by the animal. A careful study of these markings during many years convinced Prof. Hitchcock that many of them were made not by birds, but by batrachians or huge frog-like animals; and in an elaborate report by him, published by the state of Massachusetts in 1856, he showed that the ichnolites of the red sandstone had been found in not fewer than 38 localities, extending over a length of 90 m., with a breadth of 2 or 3 m., in the Connecticut valley. The markings known to him were referred to as many as 119 species of animals, including quadrupeds, birds, lizards, batrachians, tortoises, fishes, crustaceans, insects, and worms. While most of these markings were made on land, others were apparently produced by animals like fishes, swimming near the bottom. The surfaces of many of the beds bear the marks of waves or ripples, and others are distinctly marked by rain drops. The collection of these ichnolites made by Prof. Hitchcock, and now in the museum of Amherst college, is very great, and shows more than 8,000 individual tracks. A few remains of bones and coprolites have been found in the sandstones of this formation, but they have not thrown much light upon the animals producing the tracks. Footprints have since been met with in the sandstones of the same formation in New Jersey, and in their probable equivalents, the lower triassic sandstones of Lancashire and Cheshire in England, and also at Hildburghausen in Saxony. These European footprints have a rude resemblance to the human hand, and were for some time regarded as the marks of an unknown quadruped, to which was given the name of cheirotherium, a supposed marsupial allied to the kangaroo. The track are of considerable dimensions, and those of the hind and fore feet differ greatly in size. They have since, however, been referred with greater probability to the labyrinthodon, an animal allied to the crocodiles, to which may be due some of the footprints of the Connecticut valley. But besides these five-toed and four-footed animals, were those which made the three-toed biped impressions at first regarded as the tracks of birds, and very abundant in the Connecticut sandstones. Prof. Hitchcock finally recognized the fact that some of these animals had huge tails, which had left their impressions, and smaller fore feet or paws, which they sometimes put to the ground; and he then referred them to a kind of bird-like lizards. More recent studies of the fossil remains of these animals, which have been carefully made by various naturalists, and especially by Cope, have made us acquainted with that curious class of animals, the dinosaurs. These creatures constituted numerous genera and species, some of gigantic size, others comparatively small; some feeding on plants, and others carnivorous; but all remarkable for presenting a higher type of reptilian organization than any now existing, and approaching in some respects to the birds and in others to the mammalia. Among the vegetable feeders of this group was hadrosauros, a gigantic animal, 20 ft. or more in height, with huge bird-like legs and feet, a lizard-like tail, a diminutive head, and small fore feet or hands, feeding on plants; while lælaps was an equally huge carnivorous animal of somewhat similar organization. The animals which made the so-called bird tracks in the sandstones of the Connecticut valley were probably similar to these.—If we go backward to the palæozoic period, we find in its upper portion, in the rocks of the coal formation in Pennsylvania, footprints which probably belong to an air-breathing frog-like animal related to the labyrinthodon of the mesozoic. Footprints, apparently of batrachian reptiles, are also found in the carboniferous formation of Nova Scotia. These, so far as we know, are the oldest air-breathers, and the remains of animals of this kind which abound in the rocks of this region have been described and figured by Dawson. In the great series of palæozoic rocks beneath the coal, comprising the Devonian, Silurian, and Cambrian, we have numerous ichnolites, but, so far as we know, belonging, unlike those which we have described, solely to fishes or to invertebrate animals. The sandstones at the base of the coal in Nova Scotia are marked with the tracks of a crustacean allied to the limulus or king crab; and to an animal of that kind are ascribed those curious markings found in the beds of the Potsdam sandstone at several localities in the St. Lawrence valley near Montreal, to which the name of protichnites has been given. These tracks, at first supposed to be the footprints of a large tortoise-like animal, show the presence of several pairs of walking feet and of a flexible tail. In the same sandstone beds are singular ladder-like markings, which have been called climactichnites. Dr. Dawson has in this connection studied carefully the habits of the king crab, and has shown that when walking on the sands it produces impressions very like protichnites, and when using its swimming feet, markings like climactichnites were the result. In the Chazy and Clinton divisions of the lower palæozoic in New York and in Canada are curious bilobate markings, which were supposed to be the impressions of a marine plant, and received the name of rusophycus, but according to Dawson are really casts of burrows, connected with footprints consisting of a double series of transverse markings, so that a comparison of them with the trails and burrows of limulus justifies the conclusion that they were produced by trilobites. To these markings he has given the name of rusichnites, and has recognized the existence of similar forms in the carboniferous, which he refers to the trilobites of the genus Phillipsia found in these beds. The curious markings which have been called cruziana, from the lower Cambrian rocks, were probably produced by crustaceans not dissimilar to those which made rusichnites. Curious parallel notched grooves in pairs, found in the carboniferous of Nova Scotia, have been described and figured by Dawson under the name of diplichnites, and referred by him, with great probability, to fishes having pectoral or ventral fins armed with spines; while in rocks of the same age and still older, down to the base of the Cambrian, are numerous grooved and striated markings, some of which may have been produced by the feet or spinous tails of swimming animals. Other markings are with probability ascribed to lingula, which, as Prof. Morse has shown, crawls in a worm-like manner over the surface; while others still are perhaps produced by the trailing of seaweeds drifting with tides or currents. Certain markings of this kind have been regarded as impressions of the stems of plants, and, occurring in the oldest Cambrian rocks, have received the name of eophyton. According to Dawson, however, they are more probably the grooves produced by swimming crustaceans; and he includes under the name of rabdichnites all those rod-like markings. Various imitative markings are met with in rocks, which are probably not due to any organic bodies. Such are the rill marks produced by running water on the surface of soft argillaceous layers, which sometimes simulate fronds of ferns or seaweeds, or the tracks of worms.—The literature of this subject is considerable, and, as will be seen from the facts already given, the study of ichnolites is one of much geological interest. Besides the publications of Hitchcock, see Lyell's “Student's Manual of Geology,” and a paper on the subject by Dawson in the “American Journal of Science” for January, 1873.