Remarks on some fossil impressions in the sandstone rocks of Connecticut River

by John Collins Warren

President Hitchcock has distinguished more than thirty species of birds, four of lizards, three of tortoises, and six of batrachians.

The great difference in the characters of many fossil animals from those of existing genera and species, in the opinion of Prof. Agassiz, makes it probable that in various instances the traces of supposed birds may be in fact traces of other animals, as, for example, those of the lizard or frog. And he supports this opinion, among other reasons, by the disappearance of the heel in a great number of Ornithichnites.

D'Orbigny, to whom we are indebted for the most ample and systematic work on Paleontology ("Cours Elémentaire de Paléontologie et de Géologie," 5 vols. 1849-52), does not accept the arrangement of President Hitchcock. He objects to the term Ornithichnites, and proposes what he considers a more comprehensive arrangement into organic, physiological, and physical impressions. Organic impressions are those which have been produced by the remains of organized substances, such as vegetable impressions from calamites, &c. Physiological impressions are those produced by the feet and other parts of animals. Physical impressions are those from rain-drops and ripple-marks; and to these may be added coprolites in substance. This plan of D'Orbigny seems to exclude the curious and interesting distinctions of groups, genera, and species; in this way diminishing the importance of the science of Ichnology.

Fossil impressions have been found on this continent in the carboniferous strata of Nova Scotia, and of the [Alleghenies]; in the sandstone of New Jersey, and in that of the Connecticut Valley in a great number of places, from the town of Gill in Massachusetts to Middletown in Connecticut, a distance of about eighty miles.

A slab from Turner's Falls, obtained for me by Dr. Deane in 1845, measuring two feet by two and a half, and two inches in thickness, contains at least ten different sets of impressions, varying from five inches in length to two and a half, with a proportionate length of stride from thirteen inches to six. All these are tridactylous, and represent at least four different species. In most of them the distinction of articulation is quite clear. The articulations of each toe can readily be counted, and they are found to agree with the general statement made above as to number. The impressions are singularly varied as to depth; some of them, perfectly distinct, are superficial, like those made by the fingers laid lightly on a mass of dough, while others are of sufficient depth nearly to bury the toes; some of the tracks cross each other, and, being of different sizes, belong to animals of different ages or different species. There is one curious instance of the tracks of a large and heavy bird, in which, from the softness of the mud, the bird slipped in a lateral direction, and then gained a firm footing; the mark of the first step, though deep, is ill-defined and uncertain; the space intervening between the tracks is superficially furrowed; in the settled step, which is the deepest, the toes are very strongly indicated. On the same surface are impressions of nails, which may have belonged to birds or chelonians.

The inferior surface of the same slab exhibits appearances more superficial, less numerous, but generally regular. There are three sets of tracks entirely distinct from each other; two of them containing three tracks, and one containing two,—the latter being much the largest in size. In addition, there is one set of tracks, which are probably those of a tortoise. These marks present two other points quite observable and interesting. One is that they are displayed in relief, while those on the upper surface are in depression. The relief in this lower surface would be the cast of a cavity in the layer below; so the depressions in the upper surface would be moulds of casts above. The second point is the non-correspondence of the upper and lower surfaces; i.e. the depressions in the upper surface have not a general correspondence with the elevations on its inferior surface. The tracks above were made by different individuals and different species from those below. This leads to another interesting consideration, that in the thickness of this slab there must be a number of different layers, and in each of them there may be a different series of tracks.

To these last remarks there is one exception: the deep impression in which the bird slipped in a lateral direction corresponds with an elevation on the lower surface, in which the impression of these toes is very distinctly displayed, and even the articulations. Moreover, one of the tracks on the inferior surface interferes with the outer track in the superior, and tends in an opposite direction, so that this last-described footstep must have been made before the other. It is also observable, that, while all the other tracks are superficial, this last penetrates the whole thickness of the slab; thus showing that the different deposits continued some time in a soft state.

On the surfaces of this slab, particularly on the upper, there are various marks besides those of the feet, some of which seem to have been made by straws, or portions of grass, or sticks; and there is a curved line some inches in length, which seems to have arisen from shrinkage.

In the collection of Mr. Marsh,[B] there were two slabs of great size, each measuring ten by six feet, having a great number of impressions of feet, and about the same thickness as the slab under examination. One of these presented depressions; and the other, corresponding reliefs. These very interesting relations were necessarily parted in the sale of Mr. Marsh's collection; one of them being obtained for the Boston Society of Natural History, and the other for the collection of Amherst College.

The Physical Impressions, according to Professor D'Orbigny, are of three kinds, viz.: 1st, Rain-drops; 2d, Ripple-marks; and 3d, Coprolites. I have a slab which exhibits two leptodactylous tracks very distinct, about an inch and a half long, surrounded by impressions of rain-drops and ripple-marks. Another specimen exhibits the impressions of rain in a more distinct and remarkable manner. The imprints are of various sizes, from those which might be made by a common pea to others four times its diameter; some are deep, others superficial and almost imperceptible. They are generally circular, but some are ovoid. Some have the edge equally raised around, as if struck by a perpendicular drop; and others have the edge on one part faintly developed, while another part is very sharp and well defined, as if the drop had struck obliquely. It has been suggested, that these fossil rain-drops may have been made by particles of hail; but I think the variety of size and depth of depression would have been more considerable if thus made.