CHAPTER XV.. Distribution of Fishes in Time.

DISTRIBUTION OF FISHES IN TIME.

Of what kind the fishes were which were the first to make their appearance on the globe; whether or not they were identical with, or similar to, any of the principal types existing at present; are questions which probably will for ever remain hidden in mystery and uncertainty. The supposition that the Leptocardii and Cyclostomes, the lowest of the vertebrate series, must have preceded the other sub-classes, is an idea which has been held by many Zoologists: and as the horny teeth of the Cyclostomes are the only parts of their body which under favourable circumstances might have been preserved, Palæontologists have ever been searching for this evidence.

Indeed, in deposits belonging to the Lower Silurian and Devonian, in Russia, England, and North America, minute, slender, pointed horny bodies, bent like a hook, with sharp opposite margins, have been found and described under the name of Conodonts. More frequently they possess an elongated basal portion, in which there is generally a larger tooth with rows of similar but smaller denticles on one or both sides of the larger tooth, according as this is central or at one end of the base. In other examples there is no prominent central tooth, but a series of more or less similar teeth is implanted on a straight or curved base. Modifications of these arrangements are very numerous, and many Palæontologists entertain still doubts whether the origin of these remains is not rather from Annelids and Mollusks than from Fishes.

The first undeniable evidence of a fish, or, indeed, of a vertebrate animal, occurs in the Upper Silurian Rocks, in a bone-bed of the Downton sandstone, near Ludlow. It consists of compressed, slightly curved, ribbed spines, of less than two inches in length (Onchus); of small shagreen-scales (Thelodus); the fragment of a jaw-like bar with pluricuspid teeth (Plectrodus); the cephalic bucklers of what seems to be a species of Pteraspis; and, finally, the coprolitic bodies of phosphate and carbonate of lime, including recognisable remains of the Mollusks and Crinoids inhabiting the same waters. But no vertebra or other part of the skeleton has been found. The spines and scales seem to have belonged to the same kind of fish, which probably was a Plagiostome. It is quite uncertain whether or not the jaw (if it be the jaw of a fish) belonged to the buckler-bearing Pteraspis, the position of which among Ganoids, with which it is generally associated, is open to doubt.

No detached undoubted tooth of a Plagiostome or Ganoid scale has been discovered in the Ludlow deposits: but so much is certain that those earliest remains in Palæozoic rocks belonged to fishes closely allied to forms occurring in greater abundance in the succeeding formation, the Devonian, where they are associated with undoubted Palæichthyes, Plagiostomes as well as Ganoids.

* * * * *

These fish-remains of the Devonian or Old Red Sandstone, can be determined with greater certainty. They consist of spines or the so-called Ichthyodorulites, which show sufficiently distinctive characters to be referred to several genera, one of them, Onchus, still surviving from the Silurian epoch. All these spines are believed to be those of Chondropterygians, to which order some pluricuspid teeth (Cladodus) from the Old Red Sandstone in the vicinity of St. Petersburg have been referred likewise.

The remains of the Ganoid fishes are in a much more perfect state of preservation, so that it is even possible to obtain a tolerably certain idea of the general appearance and habits of some of them, especially of such as were provided with hard carapaces, solid scales, and ordinary or bony fin-rays. A certain proportion of them, as might have been expected, remind us, with regard to external form, of Teleosteous fishes rather than of any of the few still existing Ganoid types; but it is contrary to all analogy and to all palæontological evidence to suppose that those fishes were, with regard to their internal structure, more nearly allied to Teleosteans than to Ganoids. If they were not true Ganoids, they may be justly supposed to have had the essential characters of Palæichthyes. Other forms exhibit even at that remote geological epoch so unmistakably the characteristics of existing Ganoids, that no one can entertain any doubt with regard to their place in the system. In none of these fishes is there any trace of vertebral segmentation.

The Palæichthyes of the Old Red Sandstone, the systematic position of which is still obscure, are the Cephalaspidæ from the Lower Old Red Sandstone of Great Britain and Eastern Canada; Pterichthys, Coccosteus, and Dinichthys: genera which have been combined in one group--Placodermi; and Acanthodes and allied genera, which combined numerous branchiostegals with chondropterygian spines and a shagreen-like dermal covering.

Among the other Devonian fishes (and they formed the majority) two types may be recognised, both of which are unmistakably Ganoids. The first approaches the still living Polypterus, with which some of the genera like Diplopterus singularly agree in the form and armature of the head, the lepidosis of the body, the lobate pectoral fins, and the termination of the vertebral column. Other genera, as Holoptychius, have cycloid scales; many have two dorsal fins (Holoptychius), and, instead of branchiostegals, jugular scutes; others one long dorsal confluent with the caudal (Phaneropleuron).

In the second type the principal characters of the Dipnoi are manifest, and some of them, for example Dipterus, Palædaphus, Holodus, approach so closely the Dipnoi which still survive, that the differences existing between them warrant a separation into families only.

Devonian fishes are frequently found under peculiar circumstances, enclosed in the so-called nodules. These bodies are elliptical flattened pebbles, which have resisted the action of water in consequence of their greater hardness, whilst the surrounding rock has been reduced to detritus by that agency. Their greater density is due to the dispersion in their substance of the fat of the animal which decomposed in them. Frequently, on cleaving one of these nodules with the stroke of the hammer, a fish is found embedded in the centre. At certain localities of the Devonian, fossil fishes are so abundant that the whole of the stratum is affected by the decomposing remains emitting a peculiar smell when newly opened, and acquiring a density and durability not possessed by strata without fishes. The flagstones of Caithness are a remarkable instance of this.

* * * * *

The fish-remains of the Carboniferous formation show a great similarity to those of the preceding. They occur throughout the series, but are very irregularly distributed, being extremely scarce in some countries, whilst in others entire beds (the so-called bone-beds) are composed of ichthyolites. In the ironstones they frequently form the nuclei of nodules, as in the Devonian.

Of Chondropterygians the spines of Onchus and others still occur, with the addition of teeth indicative of the existence of fishes allied to the Cestracion-type (Cochliodus, Psammodus): a type which henceforth plays an important part in the composition of the extinct marine fish faunæ. Another extinct Selachian family, that of Hybodontes, makes its appearance, but is known from the teeth only.

Of the Ganoid fishes, the family Palæoniscidæ (Traquair) is numerously represented; others are Cœlacanths (Cœlocanthus, Rhizodus), and Saurodipteridæ (Megalichthys). None of these fishes have an ossified vertebral column, but in some (Megalichthys) the outer surface of the vertebræ is ossified into a ring; the termination of their tail is heterocercal. The carboniferous Uronemus and the Devonian Phaneropleuron are probably generically the same; and the Devonian Dipnoi are continued as, and well represented by, Ctenodus.

* * * * *

The fishes of the Permian group are very similar to those of the Carboniferous. A type which in the latter was but very scantily represented, namely the Platysomidæ, is much developed. They were deep-bodied fish, covered with hard rhomboid scales possessing a strong anterior rib, and provided with a heterocercal caudal, long dorsal and anal, short non-lobate paired fins (when present), and branchiostegals. The Palæoniscidæ are represented by many species of Palæoniscus, Pygopterus and Acrolepis, and Cestracionts by Janassa and Strophodus.

* * * * *

The passage from the Palæozoic into the Mesozoic era is not indicated by any marked change as far as fishes are concerned. The more remarkable forms of the Trias are Shark-like fishes represented by ichthyodorulithes like Nemacanthus, Liacanthus, and Hybodus; and Cestracionts represented by species of Acrodus and Strophodus. Of the Ganoid genera Cœlacanthus, Amblypterus (Palæoniscidæ), Saurichthys persist from the Carboniferous epoch. Ceratodus appears for the first time (Muschel-Kalk of Germany).

Thanks to the researches of Agassiz, and especially Sir P. Egerton, the ichthyological fauna of the Lias is, perhaps, the best known of the Mesozoic era, 152 species having been described. Of the various localities, Lyme Regis has yielded more than any other, nearly all the Liassic genera being represented there by not less than seventy-nine species. The Hybodonts and Cestracionts continue in their fullest development. Holocephales (Ischyodus), true Sharks (Palæoscyllium), Rays (Squaloraja, Arthropterus), and Sturgeons (Chondrosteus) make their first appearance; but they are sufficiently distinct from living types to be classed in separate genera, or even families. The Ganoids, especially Lepidosteoids, predominate over all the other fishes: Lepidotus, Semionotus, Pholidophorus, Pachycormus, Eugnathus, Tetragonolepis, are represented by numerous species; other remarkable genera are Aspidorhynchus, Belonostomus, Saurostomus, Sauropsis, Thrissonotus, Conodus, Ptycholepis, Endactis, Centrolepis, Legnonotus, Oxygnathus, Heterolepidotus, Isocolum, Osteorhachis, Mesodon. These genera offer evidence of a great change since the preceding period, the majority not being represented in older strata, whilst, on the other hand, many are continued into the succeeding oolithic formations. The homocercal termination of the vertebral column commences to supersede the heterocercal, and many of the genera have well ossified and distinctly segmented spinal columns. Also the cycloid form of scales becomes more common: one genus (Leptolepis) being, with regard to the preserved hard portions of its organisation, so similar to the Teleosteous type that some Palæontologists refer it (with much reason) to that sub-class.

As already mentioned, the Oolithic formations show a great similarity of their fish-fauna to that of the Lias; but still more apparent is its approach to the existing fauna. Teeth have been found which cannot even generically be distinguished from Notidanus. The Rays are represented by genera like Spathobatis, Belemnobatis, Thaumas; the Holocephali are more numerous than in the Lias (Ischyodus, Ganodus). The most common Ganoid genera are Caturus, Pycnodus, Pholidophorus, Lepidotus, Leptolepis, all of which had been more or less fully represented in the Lias. Also Ceratodus is continued into it.

* * * * *

The Cretaceous group offers clear evidence of the further advance towards the existing fauna. Teeth of Sharks of existing genera Carcharias (Corax), Scyllium, Notidanus, and Galeocerdo, are common in some of the marine strata, whilst Hybodonts and Cestracionts are represented by a small number of species only; of the latter one new genus, Ptychodus, appears and disappears. A very characteristic Ganoid genus, Macropoma, comprises homocercal fishes with rounded ganoid scales sculptured externally and pierced by prominent mucous tubes. Caturus becomes extinct. Teeth and scales of Lepidotus (with Sphærodus as subgenus), clearly a freshwater fish, are widely distributed in the Wealden, and finally disappear in the chalk; its body was covered with large rhomboidal ganoid scales. Gyrodus and Aspidorhynchus occur in the beds of Voirons, Coelodus and Amiopsis (allied to Amra), in those of Comen, in Istria. But the Palæichthyes are now in the minority; undoubted Teleosteans have appeared, for the first time, on the stage of life in numerous genera, many of which are identical with still existing fishes. The majority are Acanthopterygians, but Physostomes and Plectognaths are likewise well represented, most of them being marine. Of Acanthopterygian families the first to appear are the Berycidæ, represented by several very distinct genera: Beryx; Pseudoberyx with abdominal ventral fins; Berycopsis with cycloid scales; Homonotus, Stenostoma, Sphenocephalus, Acanus, Hoplopteryx, Platycornus with granular scales; Podocys with a dorsal extending to the neck; Acrogaster, Macrolepis, Rhacolepis from the chalk of Brazil. The position of Pycnosterynx is uncertain, it approaches certain Pharyngognaths. True Percidæ are absent, whilst the Carangidæ, Sphyrænidæ, Cataphracti, Gobiidæ, Cottidæ, and Sparidæ are represented by one or more genera. Somewhat less diversified are the Physostomes, which belong principally to the Clupeidæ and Dercetidæ, most of the genera being extinct; Clupea is abundant in some localities. Scopelidæ (Hemisaurida and Saurocephalus) occur in the chalk of Comen in Istria, and of Mæstricht. Of all cretaceous deposits none surpass those of the Lebanon with regard to the number of genera, species, and individuals; the forms are exclusively marine, and the remains in the most perfect condition.

* * * * *

In the Tertiary epoch the Teleosteans have almost entirely replaced the Ganoids; a few species only of the latter make their appearance, and they belong to existing genera, or, at least, very closely allied forms (Lepidosteus, Amia, Hypamia, Acipenser). The Chondropterygians merge more and more into recent forms; Holocephali continue, and still are better represented than in the present fauna. The Teleosteans show even in the Eocene a large proportion of existing genera, and the fauna of some localities of the Miocene (Oeningen) is almost wholly composed of them. On the whole, hitherto more than one-half have been found to belong to existing genera, and there is no doubt that the number of seemingly distinct extinct genera will be lessened as the fossils will be examined with a better knowledge of the living forms. The distribution of the fishes differed widely from that of our period, many of our tropical genera occurring in localities which are now included within our temperate zone, and being mixed with others, which nowadays are restricted to a colder climate: a mixture which continues throughout the Pliocene.

* * * * *

A few families of fishes, like the freshwater Salmonidæ, seem to have put in their appearance in Post-pliocene times; however, not much attention has been paid to fish-remains of these deposits; and such as have been incidentally examined offer evidence of the fact that the distribution of fishes has not undergone any further essential change down to the present period.