The Story of a Boulder; or, Gleanings from the Note-book of a Field Geologist

by Archibald Geikie

But perhaps the most remarkable and characteristic parts of the carboniferous holoptychius were its jaws and teeth. As we might readily conjecture from the great size and strength of the scales and cranial plates of this fish, its dentition was of a correspondingly massive type. The under jaw, with the usual corrugated ornament, frequently exceeded a foot in length, and displayed along its upper edge a thick-set group of teeth. Of these there were two kinds one of a smaller size and more blunted form, with short indented furrows at their base; the other of a greatly more formidable size, grouped at intervals among the smaller ones. The front end of each under jaw bore one of these long conical tusks, serving as it were to guard the entrance of the mouth. Each of the larger teeth had a base strongly marked with longitudinal furrows, and sank deep into the jaw, with the bone of which it sometimes anchylosed.[47] The part of the tooth above this socket had an oval form, so flattened as to present two cutting edges, one facing the front, the other the back of the mouth, and meeting at the upper end of the tooth which was sharp and pointed. Such large conical tusks may frequently be obtained, having a length of two or three inches, while occasionally they range as high as six or seven, the smaller teeth seldom reaching so much as an inch. It is difficult to see how, with such a formidable dentition, the jaws could readily close. In some specimens I have seen deep hollows beside the bases of the teeth, which may possibly have received those of the opposite jaw, but the gigantic tusks at the entrance of the mouth seem to have stood high over the jaw, passing outside like those of the wild-boar. If this be correct, the jaw of the holoptychius would unite the mechanism of both the alligator and the crocodile—its recipient hollows being analogous to the tooth-pits in the former tribe, and its protruded teeth to the similarly exposed teeth of the latter.

[47] I have seen detached teeth, wherein the length of the root, or part imbedded in the jaw, tripled that of the exposed part, sinking four or five inches into the bone without any trace of anchylosis. Whether these huge tusks belonged to the upper or under maxillary, I do not pretend to say, though no specimen of the under jaw, which has ever come under my notice, would accommodate half of such a deep-sunk base.

Fig. 31.—Jaw of Holoptychius (Rhizodus.—Owen) from Gilmerton, one-fourth nat. size; the large teeth along the middle part of the jaw are here wanting.

When we bring the microscope to bear upon the elucidation of the structure of these ancient teeth, it seems as if our labour had but just begun; and that so far from having by an external scrutiny exhausted all that they have to show us, our knowledge of them can be but scanty and superficial until we have studied them carefully under a magnifying power. Microscopic sections of such organic remains are prepared in the same way as those of the fossil woods already noticed; and a more interesting or beautiful series of objects cannot be conceived than a set of slices of these fossil-teeth.

Viewed, then, in longitudinal section from base to point, the part above the fluted root of one of the large teeth of the holoptychius is seen to consist of minute hair-like fibres of extreme tenuity, which proceed in straight lines from the outer surface to the interior. At right angles to these, and parallel with the outer edges, there is a set of dark widely-placed lines conforming to the outline of the tooth, like so many long sugar-loaf shaped caps, placed within each other. When this part is cut across, and viewed in transverse section, the tooth is observed to be of a flattened oval form, with the same fine fibres or tubes radiating from the centre, and traversed by the same dark bands which now assume the form of concentric rings. The appearance thus presented reminds one at once of a cross section of some dicotyledonous tree, the dark bands resembling the annual layers of growth, and like these resulting from a similar thickening of the internal tissue. The upper part of the tooth is solid and the concentric rings few; the middle exhibits an increase of the rings, and possesses, moreover, a hollow centre or pulp-cavity,[48] with the usual diverging fibres. Here the oval form is well shown, and the encircling rings are considerably flattened at the ends of the long axis.

[48] This hollow centre may be seen occasionally filled up by a sharp conical tooth like the phragmocone of a belemnite.

The lower portion of the tooth exhibits a much more complicated texture. Externally it is marked by deep longitudinal furrows, that run down the enamelled sides and sink into the jaw. When cut across at this ribbed part, the tooth is found to present the most complex and graceful internal structure. The prominent ridges between the furrows are seen to be produced by crumpled folds of the substance of the tooth, which roll inwards towards the centre, coalescing with each other, and forming intricate groups of circling knots and folds. In some places they seem all but separated from each other into little circles, pierced with a central aperture, and recall the aspect of the upper layer in the scale of megalichthys. Each of these loops and folds presents a texture exactly similar to that of the upper part of the tooth. The same minute hair-like tubes, darkened and thickened in the long axis, radiate towards the centre; the same concentric bands run from centre to circumference; so that the lower part of the tooth seems, as it were, made up of a bundle of smaller teeth partially melted into each other. Between these loops and folds circular meshes frequently occur, and add to the complexity as well as the beauty of the whole structure. One of these sections, with all its twisting crumples, and folds, and knots, and coloured meshes, and encircled rings, bears no small resemblance to an antique polished table that has been cut out of the gnarled roots of a venerable oak. This complex structure arose from the mode of growth of the tooth; each prominent external ridge continually turning inwards down the furrow on either side, and mingling in freakish knots with the folds that had gone before.[49]

[49] For an acquaintance with the remarkable teeth of this ancient fish, more minute than it had been my good fortune to possess before, I am indebted to a most interesting series of microscopical preparations kindly lent me from his extensive collection by my friend Mr. Alexander Bryson of Edinburgh.

The internal bones of the holoptychius were of great size and strength, as befitted such a bulky ganoid. Some of them had a singular style of surface ornament, that somewhat resembled a frosted widow on a December morning. Their internal structure was loose and cancellated; the endo- being usually of a less compact texture than the exo-skeleton. Judging from the size of such bones, the carboniferous holoptychius must have been one of the bulkiest and most formidable denizens of the deep, reaching sometimes to a length of twenty feet or even more. Such an animal would have been, perhaps, quite a match for our hugest crocodile or alligator, for it must have swum about with a litheness and agility possessed by none of the saurian reptiles. Like that leviathan chosen by the Almighty, in an age long subsequent, as an illustration of His power and greatness, the holoptychius must have been king over all the inhabitants of the sea, and the magnificent language of Job, descriptive of the living animal, applies not less graphically to the extinct one: "Who can open the doors of his face? his teeth are terrible round about. His scales are his pride, shut up together as with a close seal. One is so near to another, that no air can come between them. He maketh the deep to boil like a pot: he maketh the sea like a pot of ointment. He maketh a path to shine after him; one would think the deep to be hoary."