Some Salient Points in the Science of the Earth

by Sir John William Dawson

I thus found that the common King-crab could produce a considerable variety of tracks and burrows comparable with those which have been named Protichnites, Climactichnites, Bilobites, Cruziana, Rusichnites, etc.; and that the kind of markings depended partly on the differences of gait in the animal, and partly on the circumstances in which it was placed; so that different kinds of tracks do not always prove diversity in the animals producing them.

The interest of this investigation as applied to Limulus is increased by the fact that this creature is the near ally of Trilobites, Eurypterids and other Crustaceans which were abundant in the earlier geological ages, and whose footprints are probably among the most common we find on the rocks.

Rusichnites Grenvillensis, Billings a "Bilobite." Probably the Cast of a Crustacean burrow.

Lastly, on this part of the subject, it is to be observed that many other marine animals, both crustaceans and worms, make impressions resembling in general character those of Limulus. In addition to those already mentioned, Nathorst and Bureau have shown that various kinds of shrimps and lobster-like Crustaceans, when swimming rapidly by successive strokes of the tail, make double furrows with transverse ridges resembling those of Bilobites, and there are even some mollusks which by the undulations of the foot or the hook-like action of its anterior part, can make similar trails. A question arises here as to the value of such things as fossils. This depends on the fact that many creatures have left their marks on the rocks when still soft on the sea bottom, of which we have no other indications, and it also depends on our ability to understand the import of these unconscious hieroglyphics. They will certainly be of little use to us so long as we persist in regarding them as vegetable forms, and until we have very carefully studied all kinds of modern markings.[150] Nor does it seem of much use to assign to them specific names. The same trail often changes from one so-called species, or even genus, to another in tracing it along, and the same animal may in different circumstances make very different kinds of tracks. There will eventually, perhaps, arise some general kind of nomenclature for these markings under a separate sub-science of Ichnology or the doctrine of Footprints.

[150] Geologists are greatly indebted to Dr. Nathorst of Stockholm for his painstaking researches of this kind.

I have said nothing of true Algæ or seaweeds, of which there are many fossil species known to us by their forms, and also by the carbonaceous or pyritous matter, or discharge of colour from the matrix, which furnishes evidence of the presence of organic material; nor of the marks and trails left by seaweeds and land plants drifting in currents, some of which are very curious and fantastic; nor of those singular trails referred to the arms of cuttle-fishes and the fins of fishes, or to sea jellies and starfishes. These might form materials for a treatise. My object here is merely to indicate the mode of dealing with such things, and the kind of information to be derived from them.

When we come to the consideration of actual footprints of vertebrate animals having limbs, the information we can obtain is of a far more definite character. This has already been referred to in treating of the first Air-breathers in a previous chapter. One very curious example we may close with. It is that of the celebrated "bird tracks" of the sandstone quarries in the Trias of Connecticut and Massachusetts. These tracks, of immense size, as much as eighteen inches in length, and so arranged as to indicate the stride of a long-legged biped, were naturally referred to gigantic birds, allied to modern waders. But when it was found that some of them showed a central furrow indicating a long tail trailing behind, this conclusion was shaken, and when in tracing them along, places were found where the animal had sat down on its haunches and the end of its tail, and had brought down to the ground a pair of small fore feet with four or five fingers, it was discovered that we had to deal with biped reptiles; and when the tracks were correlated with the bones of the extinct reptiles known as Dinosaurs, we found ourselves in the presence of a group of the most strange and portentous reptilian forms that the earth has ever known. Marsh has been enabled, by nearly perfect skeletons of some allied reptilian bipeds found in the West, to reproduce them in their exact forms and proportions, so that we can realize in imagination their aspect, their gait, and their gigantic proportions. Examples of this putting together of footprints and osseous remains of vertebrate animals are not rare in the history of geology, and show us how the monsters of the ancient world, equally with their human successors, could leave "footprints on the sands of time."

The Dinosaurs which have left their footprints on the sandstones of Connecticut and Massachusetts are, however, greatly more numerous than those known to us by osseous remains. Thus footprints have the further use of filling up the imperfections of our geological record, or at least of pointing out gaps which but for them we might not have suspected. The remarkable inferences of Matthew already referred to, respecting cuttle-fishes in the Cambrian period, constitute a case in point. Footprints of Batrachians in the Carboniferous rocks were known before their bones. The strange hand-like tracks in the Trias were known before we knew the Labyrinthodon that produced them. We are still ignorant of the animals whose tracks in the old Potsdam sandstones we name Protichnites.