In discussing this subject it will be necessary to say something about
the geological history of the earth. Each geological age had its own
peculiar fauna, and to write about any part of it means that we must
know something about the particular geological age in which the animals
under consideration flourished, and something of the earth's previous
history.
The earth is supposed to be a small, condensed portion of the gaseous
material which astronomers tell us at one time pervaded all space.
The heat given off when the gas was condensing has been largely
converted into mechanical energy which makes the earth revolve once
in twenty-four hours and sends it flying through space. As soon as
the earth decreased to about its present size and became cool enough
for water to be condensed on its surface, it began to write its own
history. Its entire surface may have at one time been covered to a
uniform depth by water. If such was ever the case it did not remain
so long. The interior of the earth was very hot and the crust cooled
very irregularly and portions of it rose above the surface of the
water. Since then there have been two antagonizing forces at work. The
heat has caused the earth's surface to become irregular and the water
has made a strong effort, which has been partially successful, to
reduce all irregularities to the same level. We do not know how long
these forces nearly balanced each other, but sooner or later dry land
appeared in many places on the earth's surface. This was for a long
period of time washed by heavy rains while the shores for some distance
seaward were worn away by action of the tidal waves. Much of the land
area then sank below sea level, and became covered with sand, gravel
and the like. The portion which remained above the level is called the
Archaean. Later a general elevation of the land area brought above sea
level much of this land and gravel, forming around the Archaean an
increased land era, which we call Silurian. The time when the sand and
gravel was deposited forming this land is known as the Silurian age.
Following this came the Devonian age. After this in the following order
came the following geological ages: Carboniferous, Triassic, Jurassic,
Cretaceous, Tertiary, Quaternary and then the present or Recent age,
the one in which we now live. Each of these ages is characterized by
the peculiar animals which then predominated, and these animals are
known only from their remains imbedded in the rocks as fossils.
It may not be out of place here to mention that rocks are usually
placed in two great classes, those which have been subject to great
heat, melted, or partly so, at one time, then cooled and hardened are
called metamorphic or igneous rocks. To these belong such rocks as
our granites. Those which have not been changed by heat are called
sedimentary rocks, such as sandstones, limestones, etc. In the former
class we find no fossils. If fossils ever existed there, the fusing of
the rocks has destroyed them. Sedimentary rocks contain many fossils.
The Archaean area contains no sedimentary rocks, hence no fossils.
Between the close of the Archaean and beginning of the Silurian is a
long interval of which we know nothing. If any rocks were formed during
this interval they are in no place exposed to the surface of the earth
as are portions of all other formations. Life evidently existed then,
for at the close of this interval or rather at the beginning of the
Silurian we find a large number of Invertebrates. There were corals,
crinoids, brachiopods, lamellibranches, gasteropods, cephalopods, worms
and crustaceans. All of these animals flourished during the Silurian.
It was during the latter part of the Silurian that fishes first made
their appearance. If they lived earlier than this they were of low
organization and possessed no hard parts, and when they died they would
entirely decay, leaving nothing to be preserved as fossils. Of course,
no one lived then to give fishes easy common names, and so we only
know them by the long, hard scientific names given by scientific men.
These we will use as little as possible in this article. In classifying
fishes they fall into a few large groups, as follows: The lowest fish
in point of structure is the lancelet, a small, semi-transparent
animal, with no hard parts, as teeth, spines or bones. We would not
expect it to be preserved as a fossil and so we find none. The next
group contains our lampreys and hag fishes. These are parasites. They
vary in length from a few inches to more than three feet. With a mouth
nearly circular they attach themselves to other fishes and feast upon
their blood. The hag fish eats its way into the fish and remains there
until its host is a living hulk of skin and bones. Fishes known as
Pteraspids, thought by some scientists to belong to this group, are
found in the upper part of the Silurian. The lampreys of the present
day have no very hard teeth and their backbone is simply very soft
cartilage. These ancient lampreys, called Pteraspids, had the head and
part of the body covered with a coat of mail. Of these there flourished
in the last days of the Silurian quite a number of species. The next
group of fishes are the sharks, the most blood-thirsty of all the
inhabitants of the sea. Sharks flourished to some extent in the upper
part of the Silurian. The shark has no true bones and its covering
consists of shagreen tentacles. It is provided with hard teeth and
the dorsal fins of the ancient shark were provided each with a hard,
stout spine. The teeth were large, flat and fit for crushing. We know
these ancient sharks only by the spines, shagreen tentacles and the
teeth. These, however, furnished abundant evidence that the sharks
in the upper Silurian were numerous as to individuals and species.
The Chimera, a fish much resembling the sharks, was also abundant in
the upper Silurian. A group of fishes usually known as ganoids and
which comprise the lung fishes of the Nile, of Australia, the garpikes
of North America and the sturgeons, were very abundant during the
closing days of the Silurian. The fishes of this group are especially
well preserved as fossils, their covering consisted of bony plates
or bucklers or of scales covered with a coat of enamel. Their outer
covering was well suited to become fossilized, and so we know this
group much better than we do any other found in the Silurian.
The next and last group of fishes is known as Teleosts, or bony fishes.
To this group belong our typical fishes, such as black bass, sun
fishes, suckers, cat fishes and the like. None of this group lived
during the Silurian.
[Illustration: HOME OF THE SEA BIRDS.
CHICAGO:
A. W. MUMFORD PUBLISHER.]
Following the Silurian came the Devonian, which is called the age of
fishes. In no time in the world's history have fishes been so large
and so abundant as during this age. They outclassed in every respect
all other animals. The same general types flourished as those which
existed in the latter part of the Silurian. There were more species
and more individuals and some grew to an enormous size. Fishes ruled
the Devonian seas. The crustaceans, such as trilobites which greatly
predominated during the Silurian age, diminished greatly during the
Devonian. In the struggle for existence they decreased in size and in
numbers, and were obliged to seek safety in less favorable places.
The Devonian fishes were largely sharks and ganoids, especially the
latter. These were covered, with hard, enamel-coated scales or bony
plates. Some were short and heavy and entirely encased in a covering of
large bony plates. They were evidently anything but pretty and their
movements in the water must have been extremely awkward. Others were
formed much after our own ideas of fishes. These bore much resemblance
to our garpikes, the lung fish of the Nile and the lung fish of
Australia, and the worthless dog fish of our own fresh waters. Anglers
and fishermen all despise these fishes now, yet in Devonian times the
fishes most nearly like them were evidently the most handsome and
graceful of all fishes then living. It appears as if fishes in those
days did not fight each other. They found abundant sea room and plenty
of food in the form of invertebrates. Of course it is quite probable
that many fish-like animals existed at this time, but possessing no
hard parts and were not preserved as fossils; these could not become at
all important for the sea was too full of large animals of all classes
which were so well protected with a coat of mail and so hostile that
those less favorably situated could not exist in any great numbers. At
the close of the Devonian many changes took place. The rocks of this
formation, which now form a portion of the earth's surface, rose out of
the water, the land area thus considerably increased, the seasons, such
as they were then, became more marked; many inland seas were formed.
These changes were more or less gradual, but not so much so that the
fishes living then could not suit themselves to the new conditions.
Those fishes which had flourished for generations had become accustomed
to easy living and certain fixed ways, could not adapt themselves
to changing conditions, and so became extinct. The Pteraspids, the
earliest forms to appear; the Pterichthys, in fact, all forms which
bear any resemblance to our present lampreys, or which may prove a
close relative of the earlier ganoids, became extinct at close of the
Devonian. The early Chimeras which flourished from close of Silurian
also became extinct. Many ganoids became extinct, but other ganoids
came into existence to take their places. The ganoids most nearly
like our modern sturgeons increased during the last of the Devonian
and retained their prominence to the close of the Carboniferous. The
slow-moving, heavily plated ganoids passed away. They ruled during the
Devonian age, but could not suit themselves to the new conditions at
beginning of the Carboniferous. While fishes were numerous and large
in the Devonian, throughout the Carboniferous they began to decline.
By this time the land area had much increased, land plants became very
abundant, there were immense forests of tropical vegetation, great
swamps and peat bogs--all of which later sank below sea level--became
covered up and changed into coal. Immense lizards lived in these
forests and along the sea shores; these were the first land animals.
At the close of the Carboniferous great changes took place; greater
changes than at any time since the close of the Archaean. So marked
were the changes at this time that it marks a new era in the geological
history of the earth. All preceding the close of the Carboniferous is
regarded as ancient geology; all since then as modern geology. It was
at this time that plants and animals were represented by new forms more
like those now living. The geological age following the Carboniferous
is the Triassic. With this age began our modern sharks and fishes.
They did not become abundant until the Jurassic and Cretaceous. All of
the earlier sharks had strong spines in front of each dorsal fin and
broad teeth made for crushing. One form of these known as Cestracionts
were very abundant till the end of the Cretaceous. In the early
Triassic they began to decline and the sharks, with pointed teeth,
increased. These sharks, with pointed teeth, but rounded on the edges,
commenced back in the Carboniferous. During the Triassic the sharks,
with lancet-shaped teeth, such as are now possessed by nearly all our
sharks, commenced in small numbers. One of the important differences
between the ganoids and the teleosts or true fishes is in the tail
vertebrae. In the ganoids the tail vertebrae decrease gradually in size
and curve upwards in the upper lobe of the tail. In the teleosts the
tail vertebrae ends a short distance in front of ends of the middle
fin rays of the tail fin. In the ganoids the upper lobe of the tail
fin is the largest. In the teleosts both lobes are nearly the same
size. The tail of the ganoid fish is called heterocercal, that of our
modern or teleost fishes is homocercal. The tail of all early ganoids
was strongly heterocercal. In the Triassic and Jurassic its lobes
in many cases became nearly equal, approaching the homocercal tail.
The tails of all sharks are heterocercal, of all modern fishes it is
homocercal except in a few families, as the cod and related fishes, it
is Isocercal; that is, the vertebrae decrease in size, but do not form
an upward curve. So far as we know the Shad family is the first of our
teleosts or true fishes to appear, and these were quite abundant in the
early part of the Triassic.
The rays, fish-like animals much like Sharks, but with the body and
fins flattened or spread out in a broad flat disc, appeared in the
Jurassic. The Chimeras, so abundant in the Devonian and which died
out apparently at the close of the Devonian, also reappeared at the
beginning of the Jurassic. These did not belong to the same families
as did the more ancient Chimeras. The Chimeras no doubt flourished in
the Carboniferous and Triassic, but migrated to some portion of the sea
where now perhaps their remains lie buried in rocks below the bottom of
the sea. Their survivors, which were able to modify their structure and
habits to become suited to new conditions, returned in modified forms
in the Jurassic, where in time their remains come to the surface as
fossils.
At the end of the Cretaceous or beginning of the Tertiary we find
all of our modern types of sharks and all of the important orders of
teleosts. The sturgeons and ganoids decreased throughout the Tertiary
or Quaternary until at present we have but few living species. The
sturgeons are the more abundant. Of the large group of Ganoids so
abundant during all these geological ages but few forms are living
to-day. These are the Ceratodus, lung fish of Australia; the Polypterus
of the Nile, the Protopterus of Western Africa, the Dogfish and the
three Garpike of North America. These few species are but the remnants
of a once large and extensive group of fishes.
In the study of fishes we notice that some are highly specialized
so far as their structures are concerned; the teeth of some become
especially fitted for a peculiar kind of food, and as a result quite
unfit for any other kind. Some, to be protected from their enemies,
develop a heavy armor, which only retards their activity. Other fishes
are more generalized; that is, are of medium size, omnivorous habits,
are not hampered in their movements by a too heavy coat of mail, etc.
When any change of conditions came to modify their habits of living
the specialized were always the first to disappear. Being particularly
fitted for one mode of life made them all the more unfitted for any
other, and so when conditions changed they perished. All of our modern
fishes except the few ganoids are more or less specialized. The trout
lives in cool running water and some varieties can live in no other,
while some fishes have become accustomed to warm, stagnant water and
cannot live with the trout. What is true in this respect of fishes
is true of land animals as well. The large, ponderous, slow-moving
reptiles of the Triassic, Jurassic and the Cretaceous, and the large
mammals of the Tertiary and Quarternary could not exist except under
the peculiar conditions of that time, and sooner or later had to give
way to the smaller, more active and more resourceful animals of their
class.
In tracing the history of fishes from their earliest existence to the
present one is struck with the myriad forms he finds. It would seem
that all possible effort was made by them to modify their structure
to suit their environment; when this changed all their efforts came to
naught, and they were destined to give way to the more favored kinds.
Seth E. Meek.