Chapter V The Appearance of Vertebrate Animals
Confessedly the highest style of animal is that which possesses a skull and backbone, with brain and nerve system to match, and which embodies the general plan of structure employed in man himself. Yet among the fishes, which constitute the lowest manifestation of this type, are some so rudimentary that the brain is scarcely developed, and the skeleton is merely a cord of gristle. These are represented in the modern world only by the Lancelot,33 a creature which has sometimes been mistaken for a worm, and by a slightly more advanced type, that of the Lampreys.34 In these animals the Vertebrates make the nearest approach to the lower domains of the animal kingdom, collectively known as Invertebrates. We should naturally expect that since the vertebrates succeed the inferior animals in time, their lower types should appear first, and that these should be aquatic rather than terrestrial. On the other hand, as the oldest fishes that are certainly known are strongly protected with bony armour, and had to contend against formidable Crustaceans and Cuttles, we might suppose that the Lancelot and the Lampreys are rather degraded types belonging to the modern period, than the true precursors of the other fishes.
But if fishes like the Lancelot preceded all others, we may never find in a fossil state any traces of their soft and perishable bodies; and even the Lampreys have no hard parts except small horny teeth, which might easily escape observation. But palæontologists have sharp eyes, and it has not escaped them that certain microscopic tooth-like bodies are somewhat widely distributed in the older rocks. In Russia, Pander has found in the Upper Cambrian and Lower Silurian, and also in the Devonian and Carboniferous, minute conical and comb-like teeth, to which he has given the name of Conodonts (Fig. 102), and which he supposes to be the teeth of ancient Lampreys. Similar teeth have been found by Moore and others in the Carboniferous of England, and by Newberry in Carboniferous shales in Ohio. In point of form, these bodies certainly resemble the teeth of the humble fishes to which they have been referred. In the case of the Carboniferous specimens from Ohio—the only ones I have had an opportunity to examinethe material is calcium phosphate, and the structures are more like those of teeth of Sharks than of Lampreys, so that there can be no doubt that they are really teeth of fishes, and probably of fishes of somewhat higher grade than the Lampreys.35 The Cambrian and Silurian specimens are said to be composed of calcium carbonate, which would render it more probable that, as has been suggested by Prof. Owen, they may have been teeth of some species of Sea-snail destitute of shell. It is, however, possible that they may have originally been horny, and that the animal matter has been replaced by carbonate of lime. Rohon and Zittel have recently shown that many of these are more allied to the teeth of worms than of any other animals.36
If these older Conodonts were really teeth of fishes, they carry the introduction of these nearly as far back as that of the Mollusks and Crustaceans. If they were not, then the earliest known representatives of this class belong to a much later age, that of the Silurian. Here we have undoubted remains of fishes belonging to two of the higher orders of the class; and in the succeeding Devonian these became multiplied and extended exceedingly.
Besides the inferior tribes already referred to, the modern seas and rivers present four leading types of fishes:—first, the ordinary bony fishes (Teleostians), such as the Cod, Salmon, and Herring; secondly, the Ganoid fishes, protected with bony plates on the skin, as the Bony-pike37 and Sturgeon; thirdly, the Sharks and their allies, the Dog-fishes and Rays; fourthly, the peculiar and at present rare group of semi-reptilian fishes to which the name of Dipnoi has been given, on account of their capacity for breathing both in air and in water.
Of these four types the first is altogether modern, and includes the great majority of our present fishes. It does not make its appearance till the Cretaceous age, and then is at once represented by at least three of the modern families, those of the Salmon, Herring, and Perch. The history of the other three groups is precisely the opposite of this. They abound exceedingly at an early period, and dwindle to a much smaller number in the modern time. This is especially the case with the Ganoids and the Dipnoi. It is also remarkable that these groups of old-fashioned fishes38 are in some respects the highest members of the class, approaching the nearest to the reptiles; but this accords with a well-known palæontological law, namely, that the higher members of low groups give way on the introduction of more elevated types, while the lower members may continue. Thus the decadence of these higher fish begins with the incoming of the reptiles, just as the decadence of the higher Mollusks and predaceous Crustaceans began with the incoming of the fishes. Further, the modern Ganoids and Dipnoi are mostly fresh-water animals, though the Sharks are largely pelagic. In the Palæozoic there seem to have been abundance of marine species of all these types; but though marine, they probably flourished most in bays and estuaries and on shallow banks; and the existence of these implies continental masses of land. This explains the curious coincidence that the introduction of fishes and of an abundant land flora synchronise, and that the ocean was still dominated by Invertebrates long after the fishes had become supreme in bays, estuaries, and rivers.
Fig. 104.—a, Head-shield of an Upper Silurian fish (Cyathaspis). b, Spine of a Silurian Shark (Onchus tenui-striatus, Agass.). c, d, Scales of Thecodus, enlarged.
The first fishes that we certainly know are the Ganoids and Sharks, which appear near the close of the Upper Silurian, in the English Ludlow for example (Fig. 104). The Ganoids found here all belong to an extinct group, characterised by the covering of the head and anterior part of the body with large bony plates. They are mostly small fishes, and probably fed at the bottom, and used their long or rounded bony snouts for grubbing in the mud for food. In this respect they present a singular resemblance to the Trilobites, so that we seem to have here animals of an entirely new type, the Vertebrate, and with bony instead of shelly coverings, taking up the rôle and, to some extent, the external form of a group about to pass away. Yet I presume that no derivationist would be hardy enough to affirm that the Trilobites could have been the ancestors of these fishes. Nor indeed is any ancestry even hypothetically known for them, for the doubtful Lampreys of the Cambrian Silurian are too remote and uncertain to be used in that way. The head-shield copied in outline in Fig. 104, and the restoration after Lankester in the frontispiece to this chapter, may serve to represent these curious primitive Ganoids, which are continued in the Devonian fishes represented in Figs. 105, 106.
Along with these, and not improbably their enemies, were certain Sharks (Fig. 104), known to us only by the spines which were attached to their fins as weapons of defence, and by detached bony tubercles which protected their skin. These remains are chiefly interesting as indications that two of the great leading divisions of the class of fishes originated together.
In the Devonian age the Ganoids and Sharks, thus introduced in the Silurian, may be said to culminate. The former, more especially, are represented by a great variety of species, some of them nearly allied to their Silurian predecessors (Fig. 106), others of forms and structure not dissimilar to those of the few surviving representatives of the order, or altogether peculiar to the Devonian (Fig. 107). So numerous are these fishes, and of so many genera and species—and this not merely in one region, but in widely separated parts of the world—that the Devonian has not inaptly been called the reign of Ganoids. As an illustration at once of the very peculiar forms of some of these fishes and of their wide distribution, I figure here along with the British species a Cephalaspis (Fig. 105) found in the Lower Devonian of Gaspé, in the same beds with some of the antique Devonian plants described in the last chapter.
a, Ceratodus Fosteri. Australia. b, Lepidosiren annectus. Africa.
A new and interesting light has recently been cast upon some of the most anomalous of the ancient fishes by the study of the now rare and peculiar species of the group of Dipnoi. Two of these, belonging to the genus Lepidosiren, are the “Mud-fishes” of the rivers of tropical Africa and America (Fig. 108, b.) These creatures have an elongated and elegant form, and the body is covered with overlapping horny scales like those of ordinary fishes; but the pectoral and ventral fins are rod-like, and are supported by simple cartilaginous rays, while the tailfin forms a fringe around the posterior part of the body. Unlike ordinary fishes, they have lungs as well as gills, and their mouths are armed with sharp, bony, beak-like teeth (Fig. 115), with which they can inflict terrible bites on the small fishes and frogs which furnish them with food. Their most remarkable habit is that of burying themselves in the mud of dried-up ponds, thus forming a sort of water-chamber or “cocoon,” in which they remain in a torpid state until the return of the rainy season sets them free.
Another example of these Dipnoi is the Barramunda, or Ceratodus of the Australian rivers (Fig. 108 a). This fish resembles the Lepidosiren in many essential points of structure; but its fins have lateral rays, and are consequently of some breadth, though of peculiar form, and its mouth is armed with flat, pavement-like teeth, wherewith it browses on aquatic grasses.
Fig. 109.—Anterior part of the palate of Dipterus. Showing the dental plates at a, Devonian.—After Traquair.
These modern fishes have enabled us to understand several mysterious forms met with in the older rocks. In the first place, they show the meaning of certain flat-toothed fishes, like Dipterus of the Devonian (Fig. 109), Conchodus of the Carboniferous (Fig. 110), and Ceratodus of the Carboniferous and Trias (Figs. 111, 112), previously of very doubtful character. These must all have been of similar structure and habits with the Barramunda, which is thus the sole survivor, perhaps itself verging on extinction, of a group of herbivorous fishes introduced, it may be, contemporaneously with the first stream affording the requisite vegetable food, and which have continued almost without improvement or deterioration to the present time. These fishes are, however, very closely connected with the Ganoids, and there are some of these, with fringed fins and overlapping scales, which, while regarded as true Ganoids, resemble the Dipnoi very closely.
Fig. 110.—Dental plate of Conchodus plicatus (Dn.). Coal-formation of Nova Scotia. Acadian Geology.
Again, certain huge fishes, whose remains are found in the Devonian of Ohio,39 had jaws on the same plan with those of Lepidosiren, but of enormous size and strength (Figs. 113, 114, 115), so that in this and some other points of structure they may be regarded as colossal Mud-fishes, and they must have had the same destructive powers, but on a far grander scale. They were besides clothed with heavy armour of bony scales, having some resemblance to that of those mailed fishes of smaller size already referred to, and indicating that, huge though they were, and formidable in destructive power, they also had enemies to be dreaded. These plates serve to ally them with the Ganoids, as their jaws do with Lepidosiren.
We are thus enabled to see in the streams, lakes, and bays of the Palæozoic, harmless fishes, of the type of Ceratodus, feeding on plants, and huge precursors of the Mud-fishes darting from the depths, and provided with a dental apparatus more formidable than that of any modern fish, sufficient to pierce the strongest armour of the Ganoids, and to destroy and devour the largest aquatic animals. These huge fishes, armed with shears two or three feet in length, and capable of cutting asunder scale, flesh, and bone, are the beau idéal of destructive monsters of the deep, far surpassing our modern Sharks; and if, by means of supplementary lungs, they could breathe in air as well as in water, they would on that account be all the more vigorous and voracious.
Newberry has well remarked that while in the Devonian the Ganoids and Dipnoi were the real tyrants of the sea, as well as of the streams, in the Carboniferous they already diminish in size, though still abundant as to numbers, and are more limited to estuaries and fresh waters. Thus their departure from power had already begun, and went on until in modern times the proportion of Ganoids to ordinary fishes is, according to Günther, nine out of 9,000. The Carboniferous, indeed, very specially abounds in small Ganoids, though there are many large and formidable species. One of these smaller species, a very beautiful little fish, of fresh-water ponds and streams in the older part of the Carboniferous age, is represented of the natural size in Fig. 116, and is not a restoration, being found preserved entire, though flattened, in a fine bituminous shale, which has perfectly preserved even the most delicate sculpturing of its bony scales.
Fig. 116.—A small Carboniferous Ganoid (Palæoniscus (Rhadinichthys) Modulus Dn.). Lower Carboniferous, New Brunswick.
a, Outline. b, c, d, Sculpture of scales magnified.
a, Diplodus penetrans. b, Psammodus. c, Ctenoptychius cristatus. d, Spine, Gyracanthus magnificus. One-eighth natural size.—Acadian Geology.
The Sharks in the Carboniferous increase in number and importance. Fig. 117 shows a few examples of their teeth and spines. In the Carboniferous, however, there is a great preponderance of those species with flat, crushing teeth fitted for grinding shells,40 which in diminishing numbers continue up to the present time, when they are represented by the Port Jackson Shark and a few other species. The increase toward the modern time of the true Sharks41 with sharp cutting teeth, is obviously related to the increase of the ordinary fishes which furnish them with food. Another curious difference, connected probably with the same circumstance, is the fact that in the sharp toothed Sharks of the Carboniferous the two side fangs of each tooth are the largest, or are exclusively developed (Fig. 117, a), while in later periods the central point becomes dominant, or is developed to the exclusion of the others (Figs. 118, 119).
The Ganoids and Dipnoi still, however, occupy a very important place through the Mesozoic ages (Fig. 120), and it is only at the close of the Cretaceous that they finally give place to the Teleosts, or common fishes, which, though perhaps more fully specialised in purely ichthyic features, have dropped the reptilian characteristics of their predecessors (Fig. 121). It is interesting to observe that these old-fashioned fishes had culminated before the advent of air-breathing Vertebrates, which appear for the first time in the Carboniferous. It is further to be observed that groups of fishes furnished with means of aiding their gills by rudimentary lungs were especially suited to waters more charged with carbonic acid, and less with free oxygen, than those of more recent times. This remark especially applies to the mephitic and sluggish streams and lagoons of the Carboniferous swamps, where, in the midst of a rank vegetation and reeking masses of decaying organic matter, the half air-breathing fishes and the amphibious reptilian animals met with each other and found equally congenial abodes. Thus, independently of the fact that some of these fishes were probably vegetable feeders, it is not altogether an accident, but a wise adaptation, that caused the culmination of the reptilian fishes and batrachian reptiles to coincide with the enormous development of the lower forms of land-plants in the Devonian and Carboniferous. Another curious illustration of the diminishing necessity for air-breathing to the fishes, is the change of the tail from the unequally-lobed or heterocercal form, which prevailed in the Palæozoic, to the more modern equally-lobed (homocercal) style in the Mesozoic. The former is better suited to animals which have to rise rapidly to the surface for air, and is still continued in some modern fishes, which for other reasons need to ascend and descend, or to turn themselves in the water; but the homocercal form is best suited to the ordinary fish, whether Ganoids or Teleosts (Fig. 122). It is curious also to find the beginning of the dominancy of the ordinary fish to coincide with that of the broad-leaved exogenous trees in the later Cretaceous, and to precede immediately the appearance of the mammals on the land; all these changes being related to the purer air, the clearer waters, and the more varied continental profiles of the later geological periods. Thus physical improvement and the changes of animal and vegetable life are linked together by correlations which imply not only design, but prescience, whether we attribute these qualities to a spiritual Creator or to mere atoms and forces.
a, Beryx Lewesiensis. English chalk. b, Portheus molossus (Cope). A large fish from the American Cretaceous. One twenty-eighth natural size.
The history of fishes extends further through geological time than that of any other Vertebrates, and is perhaps more completely known to us, in consequence of the greater facilities for the preservation of their remains in aqueous deposits. If we receive Pander’s Conodonts as indicating a low type of cartilaginous fishes, these must have continued for vast ages without any elevation, and struggling for a bare existence amidst formidable Cuttle-fishes and Crustaceans, before, under more favourable conditions, they suddenly expanded into the high and perfect types of Ganoids and Sharks. If we reject the early Conodonts, then the two last-mentioned types spring together and suddenly into existence, like the armed men from the dragon’s teeth of Cadmus. They rapidly attain to numbers and grandeur unexampled in later times, and become the lords of the waters at the time when there was probably no Vertebrate life on the land. As the reptiles establish themselves on the land and in the waters, the Ganoids diminish, but the Sharks hold their own. At length the reign of reptiles is over, but the Ganoids, instead of resuming their pristine numbers, give place to the Teleosts, and become reduced to insignificance; while the Sharks, profiting by the decadence of the great marine reptiles, remain the tyrants of the seas. This history is strangely unlike a continuous evolution; but we are anticipating facts which will fall to be discussed in a subsequent chapter.
A Microsaurian of the Carboniferous Period (Hylonomus Lyelli).
Restored from the skeleton and dermal appendages found in an erect Sigillaria. Half natural size.