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.
[Illustration: Fig. 101. Right dental plate of Myxine affinis.]
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.
[See G. J. Hinde, in “Quarterly Journal of the Geological
Society,” 1879.]
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[16]) 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.
[See _E. Sauvage_, Essai sur la Faune Ichthyologique de la
période Liasique. In “Bibl. de l’école des hautes études,” xiii.
art. 5. Paris 1875. 8^o.]
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.
[See _E. Sauvage_, Mémoire sur la Faune Ichthyologique de
la période Tertiaire. Paris 1873. 8°.]
[Illustration: Fig. 102.--_Pycnodus rhombus_, a Ganoid from
the Upper Oolite.]