The Story of the Earth and Man

by Sir John William Dawson

Out of the rich and varied life of the Devonian I may select for illustration its corals, its crustaceans, its fishes, its plants, and its insects.

Fig. 11.—CORALS, FISHES, AND CRUSTACEANS OF THE DEVONIAN

In the foreground are Corals of the genera Favosites, Michelina, Phillipsatrea, Zaphrentis, Blothrophyllum, and Syringopora, and the seaweed Spirephyton; also Fishes of the genera Cephalaspis and Pterichthys. Above are Pterygotus and Dinichtys, with Fishes of the genera Diplacanthus, Osteolepis, Holoptychius, Pteraspis, Coccosteus, etc. The distant land had Lepidodendra, Pines and Tree-ferns.

The central limestones of the Devonian may be regarded as the head-quarters of the peculiar types of coral characteristic of the Palæozoic age. Here they were not only vastly numerous, but present some of their grandest and also their most peculiar forms. Edwards and Haime, in their “Monograph of British Fossil Corals” in 1854, enumerate one hundred and fifty well-ascertained species, and the number has since been largely increased; I have no doubt that my friend Dr. Bigsby, in his forth-coming “Thesaurus Devonicus,” will more than double it. In the Devonian limestones of England, as for instance at Torquay, the specimens, though abundant and well preserved as to their internal structure, are too firmly imbedded in the rock to show their external forms. In the Devonian of the continent of Europe much finer specimens occur; but, perhaps, in no part of the world is there so clear an exhibition of them as in the Devonian limestones of the United States and Canada. Sir Charles Lyell thus expresses his admiration of the exposure of these corals, which he saw at the falls of the Ohio, near Louisville. He says, "Although the water was not at its lowest, I saw a grand display of what may be termed an ancient coral-reef, formed by zoophytes which flourished in a sea of earlier date than the Carboniferous period. The ledges of horizontal limestone, over which the water flows, belong to the Devonian group, and the softer parts of the stone have decomposed and wasted away, so that the harder calcareous corals stand out in relief. Many branches of these zoophytes project from their erect stems precisely as if they were living. Among other species I observed large masses, not less than five feet in diameter, of Favosites Gothlandica, with its beautiful honeycomb structure well displayed. There was also the cup-shaped Cyathophyllum, and the delicate network of Fenestella, and that elegant and well-known European species of fossil, the chain coral, Catenipora escharoides, with a profusion of others which it would be tedious to all but the geologist to enumerate. Although hundreds of fine specimens have been detached from these rocks to enrich the museums of Europe and America, another crop is constantly working its way out under the action of the stream, and of the sun and rain in the warm season when the channel is laid dry."[K] These limestones have been estimated to extend, as an almost continuous coral reef, over the enormous area of five hundred thousand square miles of the now dry and inland surface of the great American continental plateau. The limestones described by Sir Charles are known in the Western States as the “Cliff limestone.” In the State of New York and in Western Canada the “Corniferous limestone,” so called from the masses of hornstone, like the flint of the English chalk, contained in it, presents still more remarkable features. The corals which it contains have been replaced by the siliceous or flinty matter in such a manner that, when the surrounding limestone weathers away, they remain projecting in relief in all the beauty of their original forms. Not only so, but on the surface of the country they remain as hard siliceous stones, and may be found in ploughing the soil and in stone fences and roadside heaps, so that tons of them could often be collected over a very limited space. When only partly disengaged from the matrix, the process may be completed by immersing them in a dilute acid. The beauty of these specimens when thus prepared is very great not at all inferior to that of modern corals, which they often much resemble in general form, though differing in details of structure. One of the most common forms is that of the Favosites, or honeycomb coral, presenting regular hexagonal cells with transverse floors or tabulæ. Of these there are several species, usually flat or massive in form; but one species, F. polymorpha, branches out like the modern stag-horn corals. Another curious form, Michelina, looks exactly like a mass of the papery cells of the great American hornet in a petrified state, and the convex floors simulate the covers of the cells, so that it is quite common to find them called fossil wasps' nests. Some of the largest belong to the genus Phillipsastrea or Smithia, which Hugh Miller has immortalized by comparing its crowded stars, with confluent rays, to the once-popular calico pattern known as “Lane’s net”—a singular instance of the accidental concurrence of a natural and artificial design. Another very common type is that of the conical Zaphrentis, with a deep cut at top to lodge the body of the animal, whose radiating chambers are faithfully represented by it’s delicate lamellæ. Perhaps the most delicate of the whole is the Syringopora, with its cylindrical worm-like pipes bound together by transverse processes, and which sometimes can be dissolved out in all its fragile perfection by the action of an acid on a mass of Corniferous limestone filled with these corals in a silicified state.

[K] “Travels in North America.” second series.

These Devonian corals, like those of the Silurian, belong to the great extinct groups of Tabulate and Rugose corals; groups which present, on the one hand, points of resemblance to the ordinary coral animals of the modern seas, and, on the other, to those somewhat exceptional corals, the Millepores, which are produced by another kind of polyp, the Hydroids. Some of them obviously combine properties belonging to both, as, for example, the radiating partitions with the arrangement of the parts in multiples of four, the horizontal floors, and the external solid wall; and this fact countenances the conclusion that in these old corals we have a group of high and complex organization, combining properties now divided between two great groups of animals, neither of them probably, either in their stony skeletons or the soft parts of the animal, of as high organization as their Paleozoic predecessors. This sort of disintegration of composite types, or dissolution of old partnerships, seems to liave been no unusual occurrence in the history of life.[L]

[L] Verril has suggested that the Tabulata may be divided into two groups, one referable to Actinoids, the other to Hydroids.