A Guide to the Study of Fishes, Volume 1 (of 2)

by David Starr Jordan

"(1) That extinction which comes from modification or progressive evolution, a relegation to the past as a result of the transmutation into more advanced forms. (2) Extinction from changes of physical environment which outrun the powers of adaptation. (3) The extinction which results from competition. (4) The extinction which results from extreme specialization and limitation to special conditions the loss of which means extinction. (5) Extinction as a result of exhaustion."

Fossilization of a Fish.—When a fish dies he leaves no friends. His body is at once attacked by hundreds of creatures ranging from the one-celled protozoa and bacteria to individuals of his own species. His flesh is devoured, his bones are scattered, the gelatinous substance in them decays, and the phosphate of lime is in time dissolved in the water. For this reason few fishes of the millions which die each year leave any trace for future preservation. At the most a few teeth, a fin-spine, or a bone buried in the clay might remain intact or in such condition as to be recognized.

But now and then it happens that a dead fish may fall in more fortunate conditions. On a sea bottom of fine clay the bones, or even the whole body, may be buried in such a way as to be sealed up and protected from total decomposition. The flesh will usually disappear and leave no mark or at the most a mere cast of its surface. But the hard parts, even the muscles may persist, and now and then they do persist, the salts of lime unchanged or else silicified or subjected to some other form of chemical substitution. Only the scales, the teeth, the bones, the spines, and the fin-rays can be preserved in the rocks of sea or lake bottom. In a few localities, as near Green River in Wyoming, Monte Bolca, near Verona, and Mount Lebanon in Syria, the London clays, with certain quarries in Scotland and lithographic stones in Germany, many skeletons of fishes have been found pressed flat in layers of very fine rock, their structures traced as delicately as if actually drawn on the smooth stone. Fragments preserved in ruder fashion abound in the clays and even the sandstones of the earliest geologic ages. In most cases, however, fossil fishes are known from detached and scattered fragments, many of them, especially of the sharks, by the teeth alone. Fishes have occurred in all ages from the Silurian to the present time and probably the very first lived long before the Silurian.

The Earliest Fishes.—No one can say what the earliest fishes were like, nor do we know what was their real relation to the worm-like forms among which men have sought their presumable ancestors, nor to the Tunicates and other chordate forms, not fish-like, but still degenerate relatives of the primeval fish.

From analogy we may suppose that the first fishes which ever were bore some resemblance to the lancelet, for that is a fish-like creature with every structure reduced to the lowest terms. But as the lancelet has no hard parts, no bones, nor teeth, nor scales, nor fins, no traces of its kind are found among the fossils. If the primitive fish was like it in important respects, all record of this has probably vanished from the earth.

The Cyclostomes.—The next group of living fishes, the Cyclostomes, including the hagfishes and lampreys,—fishes with small skull and brain but without limbs or jaws,—stands at a great distance above the lancelet in complexity of structure, and equally far from the true fishes in its primitive simplicity. In fact the lamprey is farther from the true fish in structure than a perch is from an eagle. Yet for all that it may be an offshoot from the primitive line of fish descent. There is not much in the structure of the lamprey which may be preserved in the rocks. But the cartilaginous skull, the back-bone, fins, and teeth might leave their traces in soft clay or lithographic stone. But it is certain that they have not done so in any rocks yet explored, and it may be that the few existing lampreys owe their form and structure to a process of degradation from a more complex and more fish-like ancestry. The supposed lamprey fossil of the Devonian of Scotland, Palæospondylus, has little in common with the true lampreys.

The Ostracophores.—Besides the lampreys the Devonian seas swarmed with mysterious creatures covered with an armor of plate, fish-like in some regards, but limbless, without true jaws and very different from the true fishes of to-day. These are called Ostracophori, and some have regarded them as mailed lampreys, but they are more likely to be a degenerate or eccentric offshoot from the sharks, as highly modified or specialized lampreys, a side offshoot which has left no descendants among recent forms. Recently Professor Patten has insisted that the resemblance of their head-plates to those of the horseshoe crab (Limulus) is indicative of real affinity.

Among these forms in mail-armor are some in which the jointed and movable angles of the head suggest the pectoral spines of some catfishes. But in spite of its resemblance to a fin, the spine in Pterichthyodes is an outgrowth of the ossified skin and has no more homology with the spines of fishes than the mailed plates have with the bones of a fish's cranium. In none of these fishes has any trace of an internal skeleton been found. It must have retained its primitive gelatinous character. There are, however, some traces of eyes, and the mucous channels of the lateral line indicate that these creatures possessed some other special senses.

Fig. 254.—An Ostracophore, Cephalaspis lyelli Agassiz, restored. Devonian. (After Agassiz, per Dean.)