The Old Red Sandstone; or, New Walks in an Old Field

by Hugh Miller

Of this upper formation the most characteristic and most abundant ichthyolite, as has been already said, is the Holoptychius. The large scales and plates, and the huge teeth, belong to this genus. It was first introduced to the notice of geologists in a paper read before the Wernerian Society in May, 1830, by Professor Fleming, and published by him in the February of the following year, in Cheek's Edinburgh Journal. Only detached scales and the fragment of a tooth had as yet been found; and these he minutely described as such, without venturing to hazard a conjecture regarding the character or family of the animal to which they had belonged. They were submitted some years after to Agassiz, by whom they were referred, though not without considerable hesitation, to the genus Gyrolepis; and the doubts of both naturalists serve to show how very uncertain a guide mere analogy proves to even men of the first order, when brought to bear on organisms of so strange a type as the ichthyolites of the Old Red Sandstone. At this stage, however, an almost entire specimen of the creature was discovered in the sandstones of Clashbennie, by the Rev. James Noble, of St. Madoes, a gentleman who, by devoting his leisure hours to Geology, has extended the knowledge of this upper formation, and whose name has been attached by Agassiz to its characteristic fossil, now designated the Holoptychius nobilissimus. His specimen at once decided that the creature had been no Gyrolepis, but the representative of a new genus not less strangely organized, and quite as unlike the existences of the present times as any existence of all the past. So marked are the peculiarities of the Holoptychius, that they strike the commonest observer.

The scales are very characteristic. They are massy elliptical plates, scarcely less bulky in proportion to their extent of surface than our smaller copper coin, composed internally of bone, and externally of enamel, and presenting on the one side a porous structure, and on the other, when well preserved, a bright, glossy surface. The upper, or glossy side, is the more characteristic of the two. I have placed one of them before me. Imagine an elliptical ivory counter, an inch and a half in length by an inch in breadth, and nearly an eighth part of an inch in thickness, the larger diameter forming a line which, if extended, would pass longitudinally from head to tail through the animal which the scale covered. On the upper or anterior margin of this elliptical counter, imagine a smooth selvedge or border three eighth parts of an inch in breadth. Beneath this border there is an inner border of detached tubercles, and beneath the tubercles large undulating furrows, which stretch longitudinally towards the lower end of the ellipsis. Some of these waved furrows run unbroken and separate to the bottom, some merge into their neighboring furrows at acute angles, some branch out and again unite, like streams which enclose islands, and some break into chains of detached tubercles. (See [Plate X.], fig. 3.) No two scales exactly resemble one another in the minuter peculiarities of their sculpture, if I may so speak, just as no two pieces of lake or sea may be roughened after exactly the same pattern during a gale; and yet in general appearance they are all wonderfully alike. Their style of sculpture is the same—a style which has sometimes reminded me of the Runic knots of our ancient north country obelisks. Such was the scale of the creature. The head, which was small, compared with the size of the body, was covered with bony plates, roughened after a pattern somewhat different from that of the scales, being tubercled rather than ridged; but the tubercles present a confluent appearance, just as chains of hills may be described as confluent, the base of one hill running into the base of another. The operculum seems to have been covered by one entire plate—a peculiarity observable, as has been remarked, among some of the ichthyolites of the Lower Old Red Sandstone, such as the Diplopterus, Dipterus, and Osteolepis. And it, too, has its fields of tubercles, and its smooth marginal selvedge, or border, on which the lower edges of the upper occipital plates seem to have rested, just as, in the roof of a slated building, part of the lower tier of slates is overtopped and covered by the tier above. The scales towards the tail suddenly diminish at the ventral fins to about one fourth the size of those on the upper part of the body; the fins themselves are covered at their bases, which seem to have been thick and fleshy like the base of the pectoral fin in the cod or haddock, with scales still more minute; and from the scaly base the rays diverge like the radii of a circle, and terminate in a semicircular outline. The ventrals are placed nearer the tail, says Agassiz, than in any other ganoid fish. (See [Plate X.], fig. 2.)

But no such description can communicate an adequate conception to the reader of the strikingly picturesque appearance of the Holoptychius, as shown in Mr. Noble's splendid specimen. There is a general massiveness about the separate portions of the creature, that imparts ideas of the gigantic, independently of its bulk as a whole; just as a building of moderate size, when composed of very ponderous stones, has a more imposing effect than much larger buildings in which the stones are smaller. The body measures a foot across, by two feet and a half in length, exclusive of the tail, which is wanting; but the armor in which it is cased might have served a crocodile or alligator of five times the size. It lies on its back, on a mass of red sandstone; and the scales and plates still retain their bony color, slightly tinged with red, like the skeleton of some animal that had lain for years in a bed of ferruginous marl or clay. The outline of the occipital portion of the specimen forms a low Gothic arch, of an intermediate style between the round Saxon and the pointed Norman. This arch is filled by two angular, pane-like plates, separated by a vertical line, that represents, if I may use the figure, the-dividing astragal of the window; and the under jaw, with its two sweeping arcs, or branches, constitutes the frame. All of the head which appears is that under portion of it which extends from the upper part of the belly to the snout. The belly itself is thickly covered by huge carved scales, that, from their massiveness and regular arrangement, remind one of the flags of an ancient stone roof. The carving varies, as they descend towards the tail, being more in the ridged style below, and more in the tubercled style above. So fairly does the creature lie on its back, that the ventral fins have fallen equally, one on each side, and, from their semicircular form, remind one of the two pouch holes in a lady's apron, with their laced flaps. The entire outline of the fossil is that of an elongated ellipsis, or rather spindle, a little drawn out towards the caudal extremity. The places of all the fins are not indicated, but, as shown by other specimens, they seem to have been crowded together towards the lower extremity, like those of the Glyptolepis, an ichthyolite which, in more than one respect, the Holoptychius must have resembled, and which, from this peculiarity, presents a brush-like appearance—the head and shoulders representing the handle, and the large and thickly clustered fins the spreading bristles.[AP]

[AP] There are now six species of Holoptychius enumerated—H. Andersoni, H. Flemingii, H. giganteus, H. Murchisoni, H. nobilissimus, and H. Omaliusii.

Some of the occipital bones of the Holoptychius are very curious and very puzzling. There are pieces rounded at one of the ends, somewhat in the manner of the neck joints of our better known quadrupeds, and which have been mistaken for vertebræ; but which present evidently, at the apparent joint, the enamel peculiar to the outer surface of all the plates and scales of the creature, and which belonged, it is probable, to the snout. There are saddle-shaped bones, too, which have been regarded as the central occipital plates of a new species of Coccosteus, but whose style of confluent tubercle belongs evidently to the Holoptychius. The jaws are exceedingly curious. They are composed of as solid bone as we usually find in the jaws of mammalia; and the outer surface, which is covered in animals of commoner structure with portions of the facial integuments, we find polished and japanned, and fretted into tubercles. The jaws of the creature, like those of the Osteolepis of the lower formation, were naked jaws; it is, indeed, more than probable that all its real bones were so, and that the internal skeleton was cartilaginous. A row of thickly-set, pointed teeth ran along the japanned edges of the mouth—what, in fish of the ordinary construction, would be the lips; and inside this row there was a second and widely-set row of at least twenty times the bulk of the other, and which stood up over and beyond it, like spires in a city over the rows of lower buildings in front. A nearly similar disposition of teeth seems also to have characterized the Holoptychius of the Coal Measures, but the contrast in size was somewhat less marked. One of the most singularly-formed bones of the formation will be found, I doubt not, when perfect specimens of the upper part of the creature shall be procured, to have belonged to the Holoptychius. It is a huge ichthyodorulite, formed, box-like, of four nearly rectangular planes, terminating in a point, and ornamented on two of the sides by what, in a work of art, the reader would at once term a species of Chinese fretwork. Along the centre there runs a line of lozenges, slightly truncated where they unite, just as, in plants that exhibit the cellular texture, the lozenge-shaped cells may be said to be truncated. At the sides of the central line, there run lines of half lozenges, which occupy the space to the edges. Each lozenge is marked by lines parallel to the lines which describe it, somewhat in the manner of the plates of the tortoise. The centre of each is thickly tubercled; and what seems to have been the anterior plane of the ichthyodorulite is thickly tubercled also, both in the style of the occipital plates and jaws of the Holoptychius. This curious bone, which seems to have been either hollow inside, or, what is more probable, filled with cartilage, measures, in some of the larger specimens, an inch and a half across at the base on its broader planes, and rather more than half an inch on its two narrower ones.[AQ]

[AQ] This bone has been since assigned by Agassiz to a new genus, of which no other fragments have yet been found, but which has been named provisionally Placothorax paradoxus.

Geologists have still a great deal to learn regarding the contemporaries of the Holoptychius nobilissimus. The lower portion of that upper formation to which it more especially belongs—the portion represented in our second pyramid by the conglomerate and sandstone bar—though unfavorable to the preservation of animal remains, represents assuredly no barren period. It has been found to contain bodies apparently organic, that vary in shape like the sponges of our existing seas, which in general appearance they somewhat resemble, but whose class, and even kingdom, are yet to fix.[AR]

[AR] These organisms, if in reality such, are at once very curious and very puzzling. They occur in some localities in great abundance. A piece of Clashbennie flagstone, somewhat more than two feet in length, by fifteen inches in breadth, kindly sent me for examination by the Rev. Mr. Noble, of St. Madoes, bears no fewer than twelve of them on its upper surface, and presents the appearance of a piece of rude sculpture, not very unlike those we sometimes see in country churchyards, on the tombstones of the times of the Revolution. All the twelve vary in appearance. Some of them are of a pear shape—some are irregularly oval—some resemble short cuts of the bole of a tree—some are spread out like ancient manuscripts, partially unrolled—one of the number seems a huge, though not over neatly formed acorn, an apprentice mason's first attempt—the others are of a shape so irregular as to set comparison and description at defiance. They almost all agree, however, when cut transversely, in presenting flat, elliptical arcs as their sectional lines—in having an upper surface comparatively smooth, and an under surface nearly parallel to it, thickly corrugated—and in being all coated with a greasy, shining clay, of a deeper red than the surrounding stone. I was perhaps rather more confident of their organic character after I had examined a few merely detached specimens, than now that I have seen a dozen of them together. It seems at least a circumstance to awaken doubt, that though they occur in various positions on the slab—some extending across it, some lying diagonally, some running lengthwise—the corrugations of their under surfaces should run lengthwise in all—furrowing them in every possible angle, and giving evidence, not apparently to the influences of an organic law, internal to each, but of the operation of some external cause, acting on the whole in one direction.

It contains, besides, in considerable abundance, though in a state of very imperfect preservation, scales that differ from those of the Holoptychius, and from one another. One of these, figured and described by Professor Fleming in Cheek's Edinburgh Journal, bearing on its upper surface a mark like a St. Andrew's cross, surrounded by tubercled dottings, and closely resembling in external appearance some of the scales of the common sturgeon, "may be referred with some probability," says the Professor, "to an extinct species of the genus Accipenser."[AS]