THE EVOLUTION OF FISHES
=The Geological Distribution of Fishes.=--The oldest unquestioned
remains of fishes have been very recently made known by Mr. Charles
D. Walcott, from rocks of the Trenton period in the Ordovician or
Lower Silurian. These are from Cañon City in Colorado. Among these
is certainly a small Ostracophore (_Asteraspis desideratus_). With
it are fragments (_Dictyorhabdus_) thought to be the back-bone of a
Chimæra, but more likely, in Dean's view, the axis of a cephalopod,
besides bony, wrinkled scales, referred with doubt to a supposed
Crossopterygian genus called _Eriptychius_. This renders certain
the existence of _Ostracophores_ at this early period, but their
association with _Chimæras_ and Crossopterygians is questionable.
Primitive sharks may have existed in Ordovician times, but thus far no
trace of them has been found.
[Illustration: FIG. 246.--Fragment of Sandstone from Ordovician
deposits, Cañon City, Colo., showing fragments of scales, etc., the
earliest known traces of vertebrates. (From nature.)]
The fish-remains next in age in America are from the Bloomfield
sandstone in Pennsylvania of the Onondaga period in the upper
Silurian. The earliest in Europe are found in the Ludlow shales, both
of these localities being in or near the horizon of the Niagara rocks,
in the Upper Silurian Age.
It is, however, certain that these Lower Silurian remains do not
represent the beginning of fish-life. Probably _Ostracophores_, and
_Arthrodires_, with perhaps Crossopterygians and Dipnoans, existed
at an earlier period, together perhaps with unarmed, limbless forms
without jaws, of which no trace whatever has been left.
[Illustration: FIG. 247.--Fossil fish remains from Ordovician rocks,
Cañon City, Colo. (After Walcott.) _a._ Scale of _Eriptychius
americanus_ Walcott. Family _Holoptychiidæ?_ _b._ Dermal plate
of _Asteraspis desideratus_ Walcott. Family _Asterolepidæ_. _c._
_Dictyorhabdus priscus_ Walcott, a fragment of uncertain nature,
thought to be a chordal sheath of a Chimæra, but probably part of a
Cephalopod (Dean). _Chimæridæ?_]
=The Earliest Sharks.=--The first actual trace of sharks is found
in the Upper Silurian in the form of fin-spines (_Onchus_), thought
to belong to primitive sharks, perhaps Acanthodeans possibly to
Ostracophores. With these are numerous bony shields of the mailed
Ostracophores, and somewhat later those of the more highly specialized
Arthrodires. Later appear the teeth of _Cochliodontidæ_, with Chimæras,
a few Dipnoans, and Crossopterygians.
=Devonian Fishes.=--In the Devonian Age the _Ostracophores_ increase
in size and abundance, disappearing with the beginning of the
Carboniferous. The Arthrodires also increase greatly in variety
and in size, reaching their culmination in the Devonian, but not
disappearing entirely until well in the Carboniferous. These two groups
(often united by geologists under the older name Placoderms) together
with sharks and a few Chimæras made up almost exclusively the rich
fish-fauna of Devonian times. The sharks were chiefly Acanthodean and
Psammodont, as far as our records show. The supposed more primitive
type of _Cladoselache_ is not known to appear before the latter part
of the Devonian Age, while _Pleuracanthus_ and _Cladodus_, sometimes
regarded as still more primitive, are as yet found only in the
Carboniferous. It is clear that the records of early shark life are
still incomplete, whatever view we may adopt as to the relative rank of
the different forms. Chimæroids occur in the Devonian, and with them a
considerable variety of Crossopterygians and Dipnoans. The true fishes
appear also in the Devonian in the guise of the Ganoid ancestors and
relatives of _Palæoniscum_, all with diamond-shaped enameled scales. In
the Devonian, too, we find the minute creature _Palæospondylus_, our
ignorance of which is concealed under the name _Cycliæ_.
=Carboniferous Fishes.=--In the Carboniferous Age the sharks increase
in number and variety, the Ostracophores disappear, and the Arthrodires
follow them soon after, the last being recorded from the Permian.
Other forms of Dipnoans, Crossopterygians, and some Ganoids now appear
giving the fauna a somewhat more modern aspect. The _Acanthodei_ and
the _Ichthyotomi_ pass away with the Permian, the latest period of the
Carboniferous Age.
[Illustration: FIG. 248.--_Dipterus valenciennesi_ Agassiz, a Dipnoan.
(After Dean, from Woodward.)]
=Mesozoic Fishes.=--In the Triassic period which follows the Permian,
the earliest types of Ganoids give place to forms approaching the
garpike and sturgeon. The Crossopterygians rapidly decline. The
Dipnoans are less varied and fewer in number; the primitive sharks,
with the exception of certain Cestracionts, all disappear, only the
family of _Orodontidæ_ remaining. Here are found the first true
bony fishes, doubtless derived from Ganoid stock, the allies and
predecessors of the great group of herrings. Herring-like forms become
more numerous in the Jurassic, and with them appear other forms which
give the fish-fauna of this period something of a modern appearance. In
the Jurassic the sharks become divided into several groups, _Notidani_,
Scyllioid sharks, Lamnoid sharks, angel-fishes, skates, and finally
Carcharioid sharks being now well differentiated. Chimæras are still
numerous. The _Acanthodei_ have passed away, as well as the mailed
Ostrachopores and Arthrodires. The Dipnoans and Crossopterygians are
few. The early Ganoids have given place to more modern types, still in
great abundance and variety. This condition continues in the Cretaceous
period. Here the rays and modern sharks increase in number, the Ganoids
hold their own, and the other groups of soft-rayed fishes, as the
smelts, the lantern-fishes, the pikes, the flying-fishes, the berycoids
and the mackerels join the group of herring-like forms which represent
the modern bony fishes. In the Cretaceous appear the first spiny-rayed
fishes, derived probably from herring-like forms. These are allies or
ancestors of the living genus _Beryx_.
[Illustration: FIG. 249.--_Hoplopteryx lewesiensis_ (Mantell),
restored. English Cretaceous. Family _Berycidæ_. (After Woodward.)]
Dr. Woodward observes:
"As soon as fishes with a completely osseous endoskeleton began to
predominate at the dawn of the Cretaceous period, specializations of
an entirely new kind were rapidly acquired. Until this time the skull
of the Actinopterygii had always been remarkably uniform in type. The
otic region of the cranium often remained incompletely ossified and was
never prominent or projecting beyond the roof bones; the supraoccipital
bone was always small and covered with the superficial plates; the
maxilla invariably formed the greater part of the upper jaw; the
cheek-plates were large and usually thick; while none of the head or
opercular bones were provided with spines or ridges. The pelvic fins
always retained their primitive remote situation, and the fin-rays
never became spines. During the Cretaceous period the majority of the
bony fishes began to exhibit modifications in all these characters, and
the changes occurred so rapidly that by the dawn of the Eocene period
the diversity observable in the dominant fish-fauna was much greater
than it had ever been before. At this remote period, indeed, nearly all
the great groups of bony fishes, as represented in the existing world,
were already differentiated, and their subsequent modifications have
been quite of a minor character."
[Illustration: FIG. 250.--A living Berycoid fish, _Paratrachichthys
prosthemius_ Jordan & Fowler. Misaki, Japan. Family _Berycidæ_.]
[Illustration: FIG. 251.--Flying-fish, _Cypsilurus heterurus_
(Rafinesque). Family _Exocætidæ_ Woods Hole, Mass.]
[Illustration: FIG. 252.--The Schoolmaster Snapper, a Perch-like fish.
Family _Lutianidæ_. Key West.]
=Tertiary Fishes.=--With the Eocene or first period of the Tertiary
great changes have taken place. The early families of bony fishes
nearly all disappear. The herring, pike, smelt, salmon, flying-fish,
and berycoids remain, and a multitude of other forms seem to
spring into sudden existence. Among these are the globefishes,
the trigger-fishes, the catfishes, the eels, the morays, the
butterfly-fishes, the porgies, the perch, the bass, the pipefishes,
the trumpet-fishes, the mackerels, and the John-dories, with the
sculpins, the anglers, the flounders, the blennies, and the cods.
That all these groups, generalized and specialized, arose at once
is impossible, although all seem to date from the Eocene times.
Doubtless each of them had its origin at an earlier period, and the
simultaneous appearance is related to the fact of the thorough study
of the Eocene shales, which have in numerous localities (London, Monte
Bolca, Licata, Mount Lebanon, Green River) been especially favorable
for the preservation of these forms. Practically fossil fishes have
been thoroughly studied as yet only in a very few parts of the earth.
The rocks of Scotland, England, Germany, Italy, Switzerland, Syria,
Ohio, and Wyoming have furnished the great bulk of all the fish remains
in existence. In some regions perhaps collections will be made which
will give us a more just conception of the origin of the different
groups of bony fishes. We can now only say with certainty that the
modern families were largely existent in the Eocene, that they sprang
from ganoid stock found in the Triassic and Jurassic, that several of
them were represented in the Cretaceous also, that the Berycoids were
earliest of the spiny-rayed fishes, and forms allied to herring the
earliest of the soft-rayed forms. Few modern families arose before the
Cretaceous. Few of the modern genera go back to the Eocene, many of
them arose in the Miocene, and few species have come down to us from
rocks older than the end of the Pliocene. The general modern type of
the fish-faunas being determined in the latter Eocene and the Miocene,
the changes which bring us to recent times have largely concerned
the abundance and variety of the individual species. From geological
distribution we have arising the varied problems of geographical
distribution and the still more complex conditions on which depend the
extinction of species and of types.
[Illustration: FIG. 253.--Decurrent Flounder, _Pleuronichthys
decurrens_ Jordan & Gilbert. San Francisco.]
=Factors of Extinction.=--These factors of extinction have been
recently formulated as follows by Professor Herbert Osborn. He
considers the process of extinction as of five different types:
"(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.
[Illustration: FIG. 254.--An Ostracophore, _Cephalaspis lyelli_
Agassiz, restored. Devonian. (After Agassiz, per Dean.)]
Whatever the Ostracophores may be, they should not be included within
the much-abused term _Ganoidei_, a word which was once used in the
widest fashion for all sorts of mailed fishes, but little by little
restricted to the hard-scaled relatives and ancestors of the garpike of
to-day.
=The Arthrodires.=--Dimly seen in the vast darkness of Paleozoic time
are the huge creatures known as Arthrodires. These are mailed and
helmeted fishes, limbless so far as we know, but with sharp, notched,
turtle-like jaws quite different from those of the fish or those of
any animal alive to-day. These creatures appear in Silurian rocks and
are especially abundant in the fossil beds of Ohio, where Newberry,
Claypole, Eastman, Dean and others have patiently studied the broken
fragments of their armor. Most of them have a great casque on the head
with a shield at the neck and a movable joint connecting the two. Among
them was almost every variation in size and form.
[Illustration: FIG. 255.--An Arthrodire, _Dinichthys intermedius_
Newberry, restored. Devonian, Ohio. (Family after Dean.)]
These creatures have been often called ganoids, but with the true
ganoids like the garpike they have seemingly nothing in common. They
are also different from the Ostracophores. To regard them with Woodward
as derived from ancestral Dipnoans is to give a possible guess as to
their origin, and a very unsatisfactory guess at that. In any event
these have all passed away in competition with the scaly fishes and
sharks of later evolution, and it seems certain that they, like the
mailed Ostracophores, have left no descendants.
=The Sharks.=--Next after the lampreys, but a long way after them in
structure, come the sharks. With the sharks appear for the first time
true limbs and the lower jaw. The upper jaw is, however, formed from
the palate, and the shoulder-girdle is attached behind the skull.
"Little is known," says Professor Dean, "of the primitive stem of the
sharks, and even the lines of descent of the different members of the
group can only be generally suggested. The development of recent forms
has yielded few results of undoubted value to the phylogenist. It
would appear as if paleontology alone could solve the puzzles of their
descent."
Of the very earliest sharks in the Upper Silurian Age the remains are
too scanty to prove much save that there were sharks in abundance
and variety. Spines, teeth, fragments of shagreen, show that in some
regards these forms were highly specialized. In the Carboniferous Age
the sharks became highly varied and extensively specialized. Of the
Paleozoic types, however, all but a single family seems to have died
out, leaving Cestraciontes only in the Permian and Triassic. From these
the modern sharks one and all may very likely have descended.
=Origin of the Sharks.=--Perhaps the sharks are developed from the
still more primitive shark imagined as without limbs and with the teeth
slowly formed from modification of the ordinary shagreen prickles. In
determining the earliest among the several primitive types of shark
actually known we are stopped by an undetermined question of theory.
What is the origin of paired limbs? Are these formed, like the unpaired
fins, from the breaking up of a continuous fold of skin, in accordance
with the view of Balfour and others? Or is the primitive limb, as
supposed by Gegenbaur, a modification of the bony gill-arch? Or again,
as supposed by Kerr, is it a modification of the hard axis of an
external gill?
If we adopt the views of Gegenbaur or Kerr, the earliest type of limb
is the jointed _archipterygium_, a series of consecutive rounded
cartilaginous elements with a fringe of rays along its length.
Sharks possessing this form of limb (_Ichthyotomi_) appear in the
Carboniferous rocks, but are not known earlier. It may be that from
these the Dipnoans, on the one hand, may be descended and, on the
other, the true sharks and the Chimæras; but there is no certainty that
the jointed arm or archipterygium of the Dipnoans is derived from the
similar pectoral fin of the _Ichthyotomi_.
On the other hand, if we regard the paired fins as parts of a lateral
fold of skin, we find primitive sharks to bear out our conclusions.
In _Cladoselache_ of the Upper Devonian, the pectoral and the ventral
fins are long and low, and arranged just as they might be if Balfour's
theory were true. _Acanthoessus_, with a spine in each paired fin and
no other rays, might be a specialization of this type or fin, and
_Climatius_, with rows of spines in place of pectorals and ventrals,
might be held to bear out the same idea. In all these the tail is
less primitive than in the _Ichthyotomi_. On the other hand, the vent
in _Cladoselache_ is thought by Dean to have been near the end of
the tail. If this is the case, it should indicate a very primitive
character. On the whole, though there is much to be said in favor of
the primitive nature of the _Ichthyotomi_ (_Pleuracanthus_) with the
tapering tail and jointed pectoral fin of a dipnoan, and other traits
of a shark, yet, on the whole, _Cladoselache_ is probably nearer the
origin of the shark-like forms.
The relatively primitive sharks called _Notidani_ have the weakly
ossified vertebræ joined together in pairs and there are six or seven
gill-openings. This group has persisted to our day, the frilled shark
(_Chlamydoselachus_) and the genera _Hexanchus_ and _Heptranchias_
still showing its archaic characters.
Here the sharks diverge into two groups, the one with the vertebræ
better developed and its calcareous matter arranged star-fashion.
This forms Hasse's group of _Asterospondyli_, the typical sharks. The
earliest forms (_Orodontidæ_, _Heterodontidæ_) approach the _Notidani_,
and so far as geological records go, precede all the other modern
sharks. One such ancient type, _Heterodontus_, including the bullhead
shark, and the Port Jackson shark, still persists. The others diverge
to form the three chief groups of the cat-sharks (_Scyliorhinus_,
etc.), the mackerel-sharks (_Lamna_, etc.), and the true sharks
(_Carcharhias_, etc.).
[Illustration: FIG. 256.--Mackerel-shark or Salmon-shark, _Lamna
cornubica_ (Gmelin). Santa Barbara, Cal.]
In the second group the vertebræ have their calcareous matter arranged
in rings, one or more about the notochordal center. In all these the
anal fin is absent, and in the process of specialization the shark
gradually gives place to the flattened body and broad fins of the ray.
This group is called _Tectospondyli_. Those sharks of this group with
one ring of calcareous matter in each vertebra constitute the most
primitive extreme of a group representing continuous evolution.
From _Cladoselache_ and _Chlamydoselachus_ through the sharks to the
rays we have an almost continuous series which reaches its highest
development in the devil rays or mantas of the tropical seas, _Manta_
and _Mobula_ being the most specialized genera and among the very
largest of the fishes. However different the rays and skates may appear
in form and habit, they are structurally similar to the sharks and have
sprung from the main shark stem.
[Illustration: FIG. 257.--Star-spined Ray, _Raja stellulata_ Jordan &
Gilbert. Monterey, Cal.]
=The Chimæras.=--The most ancient offshoot from the shark stem, perhaps
dating from Silurian times and possibly separated at a period earlier
than the date of any known shark, is the group of _Holocephali_ or
Chimæras, shark-like in essentials, but differing widely in details.
Of these there are but few living forms and the fossil types are known
only from dental plates and fin-spines. The living forms are found
in the deeper seas the world over, one of the simplest in structure
being the newly discovered _Rhinochimæra_ of Japan. The fusion of the
teeth into overlapping plates, the covering of the gills by a dermal
flap, the complete union of the palato-quadrate apparatus or upper jaw
with the skull and the development of a peculiar clasping spine on the
forehead of the male are characteristic of the Chimæras. The group is
one of the most ancient, but it ends with itself, none of the modern
fishes being derived from Chimæras.
[Illustration: FIG. 258.--A Deep-sea Chimæra, _Harriotta raleighiana_
Goode & Bean. Gulf Stream.]
[Illustration: FIG. 259.--An extinct Dipnoan, _Dipterus valenciennesi_
Agassiz. Devonian. (After Pander.)]
=The Dipnoans.=--The most important offshoot of the primitive sharks is
not the Chimæras, nor even the shark series itself, but the groups of
Crossopterygians and Dipnoans, or lung-fishes, with the long chain of
their descendants. With the Dipnoan appears the lung or air-bladder,
at first an outgrowth from the ventral side of the oesophagus, as
it still is in all higher animals, but later turning over, among
fishes, and springing from the dorsal side. At first an arrangement
for breathing air, a sort of accessory gill, it becomes the sole
organs of respiration in the higher forms, while in the bony fishes
its respiratory function is lost altogether. The air-bladder is a
degenerate lung. In the Dipnoans the shoulder-girdle moves forward to
the skull, and the pectoral limb, a jointed and fringed archipterygium,
is its characteristic appendage. The shark-like structure of the mouth
remains.
The few living lung-fishes resemble the salamanders in many regards,
and some writers have ranged the class as midway between the primitive
sharks and the amphibians. These forms show their intermediate
characters in the development of lungs and in the primitive character
of the pectoral and ventral limbs. Those now extant give but little
idea of the great variety of extinct Dipnoans. The living genera are
three in number--_Neoceratodus_ in Australian rivers, _Lepidosiren_ in
the Amazon, and _Protopterus_ in the Nile. These are all mudfishes,
some of them living through most of the dry season encased in a cocoon
of dried mud. Of these forms _Neoceratodus_ is certainly the nearest to
the ancient forms, but its embryology, owing to the shortening of its
growth stages due to its environment, has thrown little light on the
question of its ancestry.
From some ally of the Dipnoans the ancestry of the amphibians, and
through them that of the reptiles, birds, and mammals may be traced,
although a good deal of evidence has been produced in favor of
regarding the primitive crossopterygian or fringe fin as the point of
divergence. It is not unlikely that the Crossopterygian gave rise to
Amphibian and Dipnoan alike.
In the process of development we next reach the characteristic fish
mouth in which the upper jaw is formed of maxillary and premaxillary
elements distinct from the skull. The upper jaw of the shark is part
of the palate, the palate being fused with the quadrate bone which
supports the lower jaw. That of the Dipnoan is much the same. The
development of a typical fish mouth is the next step in evolution, and
with its appearance we note the decline of the air-bladder in size and
function.
=The Crossopterygians.=--The fish-like mouth appears with the group
of Crossopterygians, fishes which still retain the old-fashioned
type of pectoral and ventral fin, the archipterygium. In the archaic
tail, enameled scales, and cartilaginous skeleton the Crossopterygian
shows its affinity with its Dipnoan ancestry. Thus these fishes unite
in themselves traits of the shark, lung-fish, and Ganoid. The few
living Crossopterygians, _Polypterus_ and _Erpetoichthys_, are not
very different from those which prevailed in Devonian times. The
larvæ possess external gills with firm base and fringe-like rays,
suggesting a resemblance to the pectoral fin itself, which develops
from the shoulder-girdle just below it and would seem to give some
force to Kerr's contention that the archipterygium is only a modified
external gill. In _Polypterus_ the archipterygium has become short and
fan-shaped, its axis made of two diverging bones with flat cartilage
between. From this type it is thought that the arm of the higher
forms has been developed. The bony basis may be the humerus, from
which diverge radius and ulna, the carpal bones being formed of the
intervening cartilage.
[Illustration: FIG. 260.--An extinct Crossopterygian, _Holoptychius
giganteus_ Agassiz (1835). (After Agassiz, per Zittel.)]
=The Actinopteri.=--From the Crossopterygians springs the main branch
of the true fishes, known collectively as _Actinopteri_, or ray-fins,
those with ordinary rays on the paired fins instead of the jointed
archipterygium. The transitional series of primitive _Actinopteri_ are
usually known as Ganoids. The Ganoid differs from the Crossopterygian
in having the basal elements of the paired fins small and concealed
within the flesh. But other associated characters of the Crossopterygii
and Dipnoans are preserved in most of the species. Among these are the
mailed head and body, the heterocercal tail, the cellular air-bladder,
the presence of valves in the arterial bulb, the presence of a spiral
valve in the intestine and of a chiasma in the optic nerves. All these
characters are found in the earlier types so far as is known, and all
are more or less completely lost or altered in the teleosts or bony
fishes. Among these early types is every variety of form, some of
them being almost as long as deep, others arrow-shaped, and every
intermediate form being represented. An offshoot from this line is the
bowfin (_Amia calva_), among the Ganoids the closest living ally of the
bony fishes, showing distinct affinities with the great group to which
the herring and salmon belong. Near relatives of the bowfin flourished
in the Mesozoic, among them some with a forked tail, and some with a
very long one. From Ganoids of this type the vast majority of recent
fishes may be descended.
[Illustration: FIG. 261.--An ancient Ganoid fish, _Platysomus gibbosus_
Blainville. Family _Platysomidæ_. (After Woodward.)]
[Illustration: FIG. 262.--A living Ganoid fish, the Short-nosed Gar,
_Lepisosteus platystomus_ Rafinesque. Lake Erie.]
Another branch of Ganoids, divergent from both garfish and bowfin and
not recently from the same primitive stock, included the sturgeons
(_Acipenser_, _Scaphirhynchus_, _Kessleria_) and the paddle-fishes
(_Polyodon_ and _Psephurus_). All these are regarded by Woodward as
degenerate descendants of the earliest Ganoids, _Palæoniscidæ_, of
Devonian and Carboniferous time.
[Illustration: FIG. 263.--A primitive Ganoid fish, _Palæoniscum
macropomum_ (Agassiz), restored. Permian. Family _Potaconiscidæ_.
(After Traquair.)]
[Illustration: FIG. 264.--A fossil Herring, _Diplomystus humilis_
Leidy. (From a specimen obtained at Green River, Wyo.) The scutes along
the back lost in the specimen. Family _Clupeidæ_.]
=The Bony Fishes.=--All the remaining fishes have ossified instead
of cartilaginous skeletons. The dipnoan and ganoid traits one by one
are more or less completely lost. Through these the main line of fish
development continues and the various groups are known collectively as
bony fishes or teleosts.
[Illustration: FIG. 265.--A primitive Herring-like fish, _Holcolepis
lewesiensis_ Mantell, restored. Family _Elopidæ_. English Chalk. (After
Woodward.)]
[Illustration: FIG. 266.--Ten-pounder, _Elops saurus_ L. An ally of the
earliest bony fishes. Virginia.]
The earliest of the true bony fishes or teleosts appear in Mesozoic
times, the most primitive forms being soft-rayed fishes with the
vertebræ all similar in form, allied more or less remotely to the
herring of to-day, but connected in an almost unbroken series with
the earliest ganoid forms. In these and other soft-rayed fishes the
pelvis still retains its posterior insertion, the ventral fins being
said to be abdominal. The next great stage in evolution brings the
pelvis forward, attaching it to the shoulder-girdle so that the ventral
fins are now thoracic as in the perch and bass. If brought to a point
in front of the pectoral fins, a feature of specialized degradation,
they become jugular as in the codfish. In the abdominal fishes the
air-bladder still retains its rudimentary duct joining it to the
oesophagus.
From the abdominal forms allied to the herring, the huge array
of modern fishes, typified by the perch, the bass, the mackerel,
the wrasse, the globefish, the sculpin, the sea-horse, and the cod
descended in many diverging lines. The earliest of the spine-rayed
fishes with thoracic fins belong to the type of _Berycidæ_, a group
characterized by rough scales, the retention of a primitive bone
between the eyes, and the retention of the primitive larger number
of ventral rays. These appear in the Cretaceous or chalk deposits,
and show various attributes of transition from the abdominal to the
thoracic type of ventrals.
[Illustration: FIG. 267.--Cardinal-fish, a perch-like fish, _Apogon
semilineatus_ Schlegel. Misaki, Japan.]
[Illustration: FIG. 268.--Summer Herring, _Pomolobus æstivalis_
(Mitchill). Potomac River. Family _Clupeidæ_.]
Another line of descent apparently distinct from that of the
herring and salmon extends through the characins to the loach,
carps, catfishes, and electric eel. The fishes of this series have
the anterior vertebræ coossified and modified in connection with the
hearing organ, a structure not appearing elsewhere among fishes. This
group includes the majority of fresh-water fishes. Still another great
group, the eels, have lost the ventral fins and the bones of the head
have suffered much degradation.
[Illustration: FIG. 269.--Fish with jugular ventral fins, _Bassozetus
catena_ Goode & Bean. Family _Brotulidæ_. Gulf Stream.]
[Illustration: FIG. 270.--A specialized bony fish, _Trachicephalus
uranoscopus_. Family _Scorpænidæ_. From Swatow, China.]
The most highly developed fishes, all things considered, are doubtless
the allies of the perch, bass, and sculpin. These fishes have lost
the air-duct and on the whole they show the greatest development of
the greatest number of structures. In these groups their traits one
after another are carried to an extreme and these stages of extreme
specialization give way one after another to phases of degeneration.
The specialization of one organ usually involves degeneration of some
other. Extreme specialization of any organ tends to render it useless
under other conditions and may be one step toward its final degradation.
[Illustration: FIG. 271.--An African Catfish, _Chlarias breviceps_
Boulenger. Congo River. Family _Chlariidæ_. (After Boulenger.)]
[Illustration: FIG. 272.--Silverfin, _Notropis whipplii_ (Girard).
White River, Indiana. Family _Cyprinidæ_.]
We have thus seen, in hasty review, that the fish-like vertebrates
spring from an unknown and possibly worm-like stock, that from this
stock, before it became vertebrate, degenerate branches have fallen
off, represented to-day by the _Tunicates_ and _Enteropneustans_. We
have seen that the primitive vertebrate was headless and limbless
and without hard parts. The lancelet remains as a possible direct
offshoot from it; the cyclostome with brain and skull is a possible
derivative from archaic lancelets. The earliest fishes leaving
traces in the rocks were mailed ostracophores. From an unknown but
possibly lamprey-like stock sprang the sharks and chimæras. The sharks
developed into rays in one right line and into the highest sharks along
another, while by a side branch through lost stages the primitive
sharks passed into Crossopterygians, into Dipnoans, or lung-fishes,
and perhaps into Ostracophores. All these types and others abound
in the Devonian Age and the early records were lost in the Silurian.
From the Crossopterygians or their ancestors or descendants by the
specialization of the lung and limbs, the land animals, at first
amphibians, after these reptiles, birds, and mammals, arose.
[Illustration: FIG. 273.--Moray, _Gymnothorax moringa_ Bloch. Family
_Murænidæ_ Tortugas.]
[Illustration: FIG. 274.--Amber-fish, _Seriola lalandi_ (Cuv. & Val.).
Family _Carangidæ_. Woods Hole.]
In the sea, by a line still more direct, through the gradual emphasis
of fish-like characters, we find developed the Crossopterygians with
archaic limbs and after these the Ganoids with fish-like limbs but
otherwise archaic; then the soft-rayed and finally the spiny-rayed bony
fishes, herring, mackerel, perch, which culminate in specialized and
often degraded types, as the anglers, globefishes, parrot-fishes, and
flying gurnards; and from each of the ultimate lines of descent radiate
infinite branches till the sea and rivers are filled, and almost every
body of water has fishes fitted to its environment.
[Illustration: Geological Distribution of the Families of Elasmobranchs.
A=Pliocene
B=Miocene
C=Eocene
D=Cretaceous
E=Jurassic
F=Triassic
G=Permian
H=Coal Measures
I=Sub-Carbonifero
J=Devonian
K=Silurian
L=Ordovician
+-+-+-+-+-+-+-+-+-+-+-+-+
|A|B|C|D|E|F|G|H|I|J|K|L|
|-|-|-|-|-|-|-|-|-|-|-|-|
Cladoselachidæ | | | | | | | | |X|X| | |
Acanthodii | | | | | | |X|X|X|X| | |
Pleuracanthidæ | | | | | | |X|X|X|X| | |
Cladodontidæ | | | | | | |X|X|X|X| | |
Petalodontidæ | | | | | | |X|X|X| | | |
Psammodontidæ | | | | | | | |X|X|X| | |
Cochliodontidæ | | | | | | | |X|X|X| | |
Orodontidæ | | | | | | | |X|X| | | |
Heterodontidæ |X|X|X|X|X|X|X| | | | | |
Tamiobatidæ | | | | | | | | | |X| | |
Hexanchidæ |X|X|X|X|X| | | | | | | |
Lamnidæ |X|X|X|X|X| | | | | | | |
Mitsukurinidæ |X|X|X|X| | | | | | | | |
Odontaspidæ |X|X|X|X| | | | | | | | |
Scyliorhinidæ |X|X|X|X|X| | | | | | | |
Carchariidæ |X|X|X|X| | | | | | | | |
Squalidæ |X|X|X|X| | | | | | | | |
Dalatiidæ |X|X|X| | | | | | | | | |
Squatinidæ |X|X|X|X|X| | | | | | | |
Rhinobatidæ |X|X|X|X|X| | | | | | | |
Pristididæ |X|X|X|X| | | | | | | | |
Rajidæ |X|X|X|X| | | | | | | | |
Narcobatidæ |X|X|X| | | | | | | | | |
Dasyatidæ |X|X|X|X| | | | | | | | |
Myliobatidæ |X|X|X|X|X| | | | | | | |
Ptychodontidæ | | | |X| | | | | | | | |
Chimæridæ |X|X|X|X|X| | | | | | | |]