The origin of Vertebrates, and how far back in time they extend, is
unknown. The earliest fishes were in all probability devoid of hard
parts and traces of their existence can scarcely be expected to be
found. The hypothesis that they may be derived from the early
Crustaceans, or Arachnids, is chiefly based on the somewhat striking
resemblance which the mailed fishes of the Silurian period
(Ostracodermi) bear to the Arthropods of that remote time, a
resemblance, however, very superficial and regarded by most
morphologists as an interesting example of mimetic resemblance--whatever
this term may be taken to mean. The minute denticles known as conodonts,
which first appear in the Ordovician, were once looked upon as teeth of
Cyclostomes, but their histological structure does not afford any
support to the identification and they are now generally dismissed
altogether from the Vertebrates. As a compensation the Lower Silurian of
Russia has yielded small teeth or spines which seem to have really
belonged to fishes, although their exact affinities are not known
(_Palaeodus_ and _Archodus_ of J. V. Rohon).
It is not until we reach the Upper Silurian that satisfactory remains of
unquestionable fishes are found, and here they suddenly appear in a
considerable variety of forms, very unlike modern fishes in every
respect, but so highly developed as to convince us that we have to
search in much earlier formations for their ancestors. These Upper
Silurian fishes are the _Coelolepidae_, the _Ateleaspidae_, the
_Birkeniidae_, the _Pteraspidae_, the _Tremataspidae_ and the
_Cephalaspidae_, all referred to the Ostracophori. The three last types
persist in the Devonian, in the middle of which period the Osteolepid
Crossopterygii, the Dipneusti and the Arthrodira suddenly appear. The
most primitive Selachian (_Cladoselache_), the Acanthodian Selachians
(_Diplacanthidae_), the Chimaerids (_Ptyctodus_), and the Palaeoniscid
ganoids (_Chirolepis_) appear in the Upper Devonian, along with the
problematic _Palaeospondylus_.
In the Carboniferous period, the Ostracophori and Arthrodira have
disappeared, the Crossopterygii and Dipneusti are still abundant, and
the Selachians (_Pleuracanthus_, Acanthodians, truesharks) and
Chondrostean ganoids (_Palaeoniscidae_ and _Platysomidae_) are
predominant. In the Upper Permian the Holostean ganoids
(_Acanthophorus_) make their appearance, and the group becomes dominant
in the Jurassic and the Lower Cretaceous. In the Trias, the
Crossopterygii and Dipneusti dwindle in variety and the _Ceratodontidae_
appear; the Chondrostean and Holostean ganoids are about equally
represented, and are supplemented in the Jurassic by the first,
annectant representatives of the Teleostei (_Pholidophoridae_,
_Leptolepidae_). In the latter period, the Holostean ganoids are
predominant, and with them we find numerous Cestraciont sharks, some
primitive skates (_Squatinidae_ and _Rhinobatidae_), Chimaerids and
numerous Coelacanthid crossopterygians.
The fish-fauna of the Lower Cretaceous is similar to that of the
Jurassic, whilst that of the Chalk and other Upper Cretaceous formations
is quite modern in aspect, with only a slight admixture of Coelacanthid
crossopterygians and Holostean ganoids, the Teleosteans being abundantly
represented by _Elopidae_, _Albulidae_, _Halosauridae_, _Scopelidae_ and
_Berycidae_, many being close allies of the present inhabitants of the
deep sea. At this period the spiny-rayed Teleosteans, dominant in the
seas of the present day, made their first appearance.
With the Eocene, the fish-fauna has assumed the essential character
which it now bears. A few Pycnodonts survive as the last representatives
of typically Mesozoic ganoids, whilst in the marine deposits of Monte
Bolca (Upper Eocene) the principal families of living marine fishes are
represented by genera identical with or more or less closely allied to
those still existing; it is highly remarkable that forms so highly
specialized as the sucking-fish or remoras, the flat-fish
(_Pleuronectidae_), the Pediculati, the Plectognaths, &c., were in
existence, whilst in the freshwater deposits of North America
_Osteoglossidae_ and _Cichlidae_ were already represented. Very little
is known of the freshwater fishes of the early Tertiaries. What has been
preserved of them from the Oligocene and Miocene shows that they
differed very slightly from their modern representatives. We may
conclude that from early Tertiary times fishes were practically as they
are at present. The great hiatus in our knowledge lies in the period
between the Cretaceous and the Eocene.
At the present day the Teleosteans are in immense preponderance,
Selachians are still well represented, the Chondrostean ganoids are
confined to the rivers and lakes of the temperate zone of the northern
hemisphere (_Acipenseridae_, _Polyodontidae_), the Holostean ganoids are
reduced to a few species (_Lepidosteus_, _Amia_) dwelling in the fresh
waters of North America, Mexico and Cuba, the Crossopterygians are
represented by the isolated group _Polypteridae_, widely different from
any of the known fossil forms, with about ten species inhabiting the
rivers and lakes of Africa, whilst the Dipneusti linger in Australia
(_Neoceratodus_), in South America (_Lepidosiren_), and in tropical
Africa (_Protopterus_). The imperfections of the geological record
preclude any attempt to deal with the distribution in space as regards
extinct forms, but several types, at present very restricted in their
habitat, once had a very wide distribution. The _Ceratodontidae_, for
instance, of which only one species is now living, confined to the
rivers of Queensland, has left remains in Triassic, Rhaetic, Jurassic
and Cretaceous rocks of Europe, North America, Patagonia, North and
South Africa, India and Australia; the _Amiidae_ and _Lepidosteidae_
were abundant in Europe in Eocene and Miocene times; the
_Osteoglossidae_, now living in Africa, S.E. Asia and South America,
occurred in North America and Europe in the Eocene.
In treating of the geographical distribution of modern fishes, it is
necessary to distinguish between fresh-water and marine forms. It is,
however, not easy to draw a line between these categories, as a large
number of forms are able to accommodate themselves to either fresh or
salt water, whilst some periodically migrate from the one into the
other. On the whole, fishes may be roughly divided into the following
categories:--
I. Marine fishes. A. shore-fishes; B. pelagic fishes; C. deep-sea
fishes.