The first scientific publication concerning fossil foot-marks is contained in the Transactions of the Royal Society of Edinburgh in 1828. Six years later, Prof. Kaup described the tracks of the Cheirotherium, a beast with hands, upon Triassic sandstones in Germany. The animal must have equalled an ox in size, with hind-feet shaped like the human hand, which were about three times larger than the front-feet. He is generally supposed to have been a batrachian. The earliest description of the American ichnites appeared in 1836.
The Triassic formations on the Atlantic slope are disposed in long and narrow areas. These may correspond with the spaces occupied by estuaries before the deposition of the strata. We may suppose that an arm of the sea extended northerly from Long Island Sound to New Hampshire along the Connecticut Valley, possibly connecting, beneath the Sound and North River, with a similar estuary running southerly to Virginia. If we transport ourselves in imagination to these ancient shores, we shall see that the animals left their hiding-places and were traversing the soft mud laid bare by the ebbing tide, in search of food. The heat of a tropical sun quickly hardens the mud, so that the returning tide, in bringing a fresh deposit of mud, does not wash away the impressions already made on the lower layer, but carefully covers them over. The imprints have, therefore, become a species of mould into which another muddy fluid is poured, and by hardening is made to copy the foot-mark like a plaster cast. Hence, when artificially cleared, no matter how many ages subsequently, the strata will present to view the depressed print below and the cast of the foot above, both as perfect as the respective fineness of the mud and its degree of rapid induration by the sun will permit. This process of deposition may have been repeated, just as it may now be studied, in the Bay of Fundy, till the whole estuary was filled up, partly with fine mud and clay, partly with beds of sand and gravel, all more or less marked by the feet of animals, interspersed with volcanic beds of lava, tufa, and conglomerate, and rare chemical deposits of carbonate of lime, salt, and gypsum.
The Ichnozoa, or the animals who made the tracks on stones in Triassic times, may be referred to several prominent divisions of the animal kingdom. The first, and highest in the scale, is a group of five species, remotely allied to marsupials, which, from their osseous remains found in Europe, we know must have flourished in that period. The most characteristic is a five-toed quadruped, about the size of a lion, whose foot is not unlike that of a carnivorous animal. The others had unequal feet, larger behind and smaller in front. The most important groups are those referred to birds, embracing thirty-four species; equally divided between those related to the ostrich family—thick-toed—and those with long, slender toes, like the crane and heron. These are the impressions chiefly relied upon to prove the ornithic character of any of the Ichnozoa, as they show distinctly the phalangeal
