The size of several of the fossil impressions of the Connecticut red sandstone so far exceeds that of any living ostrich, that naturalists at first were extremely adverse to the opinion of their having been made by birds, until the bones and almost entire skeleton of the Dinornis and of other feathered giants of New Zealand were discovered. Their dimensions have at least destroyed the force of this particular objection. The magnitude of the impressions of the feet of a heavy animal, which has walked on soft mud, increases for some distance below the surface originally trodden upon. In order, therefore, to guard against exaggeration, the casts rather than the mould are relied on. These casts show that some of the fossil birds had feet four times as large as the ostrich, but not perhaps larger than the Dinornis.
Some of the quadrupedal footprints which accompany those of birds are analogous to European Chirotheria, and with a similar disproportion between the hind and fore feet. Others resemble that remarkable reptile, the Rhyncosaurus of the English Trias, a creature having some relation in its osteology both to chelonians and birds. Other imprints, again, are like those of turtles.
Among the supposed bipedal tracks, a single distinct example only has been observed of feet in which there are four toes directed forwards. In this case a series of four footprints is seen, each 22 inches long and 12 wide, with joints much resembling those in the toes of birds. Professor Agassiz has suggested that it might have belonged to a gigantic bipedal batrachian; but the evidence on this subject is too defective to warrant such a bold conjecture, and if we were to give the reins to our imagination, we might as well conceive a bird having four toes projecting forwards as a huge two-legged frog. Nor should we forget that some quadrupeds place the hind foot so precisely on the spot just quitted by the fore foot, as to produce a single line of imprints like a biped.
No bones have as yet been met with, whether of reptiles or birds, in the rocks of the Connecticut, but there are numerous coprolites; and an ingenious argument has been derived by Mr. Dana, from the analysis of these bodies, and the proportion they contain of uric acid, phosphate of lime, carbonate of lime, and organic matter, to show that, like guano, they are the droppings of birds, rather than of reptiles.[299-A]
Mr. Darwin, in his "Journal of a Voyage in the Beagle," informs us that the "South American ostriches, although they live on vegetable matter, such as roots and grass, are repeatedly seen at Bahia Blanca (lat. 39° S.), on the coast of Buenos Ayres, coming down at low water to the extensive mud-banks which are then dry, for the sake, as the Gauchos say, of feeding on small fish." They readily take to the water, and have been seen at the bay of San Blas, and at Port Valdez, in Patagonia, swimming from island to island.[300-A] It is therefore evident, that in our times a South American mud-bank might be trodden simultaneously by ostriches, alligators, tortoises, and frogs; and the impressions left, in the nineteenth century, by the feet of these various tribes of animals, would not differ from each other more entirely than do those attributed to birds, saurians, chelonians, and batrachians, in the rocks of the Connecticut.
To determine the exact age of the red sandstone and shale containing these ancient footprints in the United States, is not possible at present. No fossil shells have yet been found in the deposit, nor plants in a determinable state. The fossil fish are numerous and very perfect; but they are of a peculiar type, which was originally referred to the genus Palæoniscus, but has since, with propriety, been ascribed, by Sir Philip Egerton, to a new genus. To this he has given the name of Ischypterus, from the great size and strength of the fulcral rays of the dorsal fin (from ισχὺς; strength, and πτερὸν, a fin). They differ from Palæoniscus, as Mr. Redfield first pointed out, by having the vertebral column prolonged to a more limited extent into the upper lobe of the tail, or, in the language of M. Agassiz, they are less heterocercal. The teeth also, according to Sir P. Egerton, who, in 1844, examined for me a fine series of specimens which I procured at Durham, Connecticut, differ from those of Palæoniscus in being strong and conical.
That the sandstones containing these fish are of older date than the strata containing coal, before described ([p. 284.]) as occurring near Richmond in Virginia, is highly probable. These were shown to be as old at least as the oolite and lias. The higher antiquity of the Connecticut beds cannot be proved by direct superposition, but may be presumed from the general structure of the country. That structure proves them to be newer than the movements to which the Appalachian or Alleghany chain owes its flexures, and this chain includes the ancient coal formation among its contorted rocks. The unconformable position of this New Red with ornithichnites on the edges of the inclined primary or paleozoic rocks of the Appalachians is seen at 4. of the section, [fig. 379.] [p. 327.] The absence of fish with decidedly heterocercal tails may afford an argument against the Permian age of the formation; and the opinion that the red sandstone is triassic, seems, on the whole, the best that we can embrace in the present state of our knowledge.