Fishes seem to have been the master existences of two great geological systems, mayhap of three, ere the age of reptiles began. Now, fishes differ very much among themselves, some ranking nearly as low as worms, some nearly as high as reptiles; and we find in the Old Red Sandstone series of links which are wanting in the present creation, and the absence of which occasions a wide gap between the two grand divisions of fishes, the bony and the cartilaginous.
Of all the organisms of the system one of the most extraordinary is the pterichthys, or winged fish, which the writer had the pleasure of introducing to the acquaintance of geologists. Had Lamarck been the discoverer he would unquestionably have held that he had caught a fish almost in the act of wishing itself into a bird. There are wings which want only feathers, a body which seems to have been as well adapted for passing through the air as through water, and a tail with which to steer.
My first idea regarding it was that I had discovered a connecting link-between the tortoise and the fish. I submitted some of my specimens to Mr. Murchison, and they furnished him with additional data by which to construct the calculations he was then making respecting fossils, and they added a new and very singular link to the chain of existence in its relation to human knowledge. Agassiz confirmed the conclusions of Murchison in almost every particular, deciding at once that the creature must have been a fish.
Next to the pterichthys of the Lower Old Red Sandstone I shall place its contemporary the coccosteus of Agassiz—a fish which in some respects must have resembled it. Both were covered with an armour of thickly tubercled bony plates, and both furnished with a vertebrated tail. The coccosteus seems to have been most abundant. Another of the families of the ichthyolites of the Old Red Sandstone—the cephalaspis—seems almost to constitute a connecting link between fishes and crustaceans. In the present creation fishes are either osseous or cartilaginous, that is, with bony skeletons, or with a framework of elastic, semi-transparent animal matter, like the shark; and the ichthyolites of the Old Red Sandstone unite these characteristics, resembling in some respects the osseous and in others the cartilaginous tribes. Agassiz at once confirmed my suspicion that the ichthyolites of the Old Red Sandstone were intermediate. Though it required skill to determine the place of the pterichthys and coccosteus there could be no mistaking the osteolepis—it must have been a fish, and a handsome one, too. But while its head resembled the heads of the bony fishes, its tail differed in no respects from the tails of the cartilaginous ones. And so through the discovery of extinct species the gaps between existing species have been bridged.
III.—Place-Fixing in the Dim Past
The next step was to fix the exact place of the ichthyolites in the geological scale, and this I was enabled to do by finding a large and complete bed in situ. Its true place is a little more than a hundred feet above the top, and not much more than a hundred yards above the base of the great conglomerate.
The Old Red Sandstone in Scotland and in England has its lower, middle, and upper groups—three distinct formations. As the pterichthys and coccosteus are the characteristic ichthyolites of the Lower Old Red formation, so the cephalaspis distinguishes the middle or coronstone division of the system in England. When we pass to the upper formation, we find the holoptychius the most characteristic fossil.
These fossils are found in a degree of entireness which depends less on their age than on the nature of the rock in which they occur. Limestone is the preserving salt of the geological world, and the conservative qualities of the shales and stratified clays of the Lower Old Red Sandstone are not much inferior to limestone itself; while in the Upper Old Red the beds of consolidated sand are much less conservative of organic remains. The older fossils, therefore, can be described almost as minutely as the existence of the present creation, whereas the newer fossils exist, except in a few rare cases, as fragments, and demand the powers of a Cuvier or an Agassiz to restore them to their original combinations. On the other hand, while the organisms of the Lower Old Red are numerous and well preserved, those of the Upper Old Red are much greater in individual size. In short, the fish of the lower ocean must have ranged in size between a stickleback and a cod; whereas some of the fish of the ocean of the Upper Sandstone were covered with scales as large as oyster shells, and were armed with teeth that rivalled in size those of the crocodile.
IV.—Fish as Nature's Last Word