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Primitive Fishes
Nature Bulletin No. 322-A November 23, 1968
Forest Preserve District of Cook County
Richard B. Ogilvie, President
Roland F. Eisenbeis, Supt. of Conservation
PRIMITIVE FISHES
The history of fish covers such a vast stretch of time that the mind
simply cannot grasp its immensity. The beginnings of fish -- or at least
the earliest known forms -- and of the fish-like animals that existed
before them, are found as fossils in rocks that geologists say were
formed 400 million years ago.
Sea scorpions, worms, mollusks and all of the other main types of lower
animals had already lived in the sea for ages before that. It is a question
which of them, if any, gave rise to fish. These older animals had a
digestive tube and beneath it, on the side next to the ground, was the
brain and nerve cord. The forerunners of fish, however, had the brain
and nerve cord above the digestive tube, with a slender rod of gristle in
between -- something that no other animal ever had had before. The
theory is that this rod later developed into the backbone that is found in
all of their modern descendants: fish, amphibians, reptiles, birds and
mammals.
The earliest of these "new" and different fish-like animals are called
Ostracoderms, meaning "shell-skins", because each had its head and
trunk covered with a hard bony shell, or shield. They were only a few
inches long and probably very sluggish, spending most of their time on
or near the bottom. Without a streamlined shape, and only a lopsided
tail for power, they were poor swimmers, like tadpoles. They had no
real jaws but sucked in floating food particles by a pumping action of
the throat. Some kinds, however, were more active and almost fishlike.
Two sorts of present day animals, while lacking any hard covering,
have simple primitive bodies much like those early ostracoderms. One
is the little two-inch Lancelet that lies buried in the sand of warm seas
with only its jawless mouth sticking up to strain bits of food from the
water. If disturbed, it zigzags wildly for a few seconds and then plunges
into the sand again. The other is the Lamprey, an eel-like degenerate
cousin of the earliest fish, with a round jawless sucking mouth and a
rasping tongue for drawing blood and gathering food. The arrangement
of the mouth, brain and gills in its young is remarkably like that of the
ostracoderm.
Millions and millions of years went by. Then the first real fish
appeared. Although they still had heavy armor, and no paired fins like
modern fish, they did have jaws. Much later, as fishes developed, the
bony shield was replaced by a lighter smoother covering, the body
became streamlined, and finally they got two pairs of fins which
correspond to the forelegs and hindlegs of higher animals.
Before
that ancient period of the earth's history, when vast swamps and
forests were piling up stores of vegetation later transformed into coal,
fish of many kinds had spread throughout both fresh and salt water.
Huge sharks, possibly 60 feet long, and their flattened relatives, the rays
so plentiful then, have left many descendants almost unchanged since
that time. The ancestors of our sturgeons, gars, and the strange lung-
fishes, came into existence and we find their fossils in rocks. Most
interesting of all were the "Lobe-fins" because some of their offspring
came out of water, colonized the land, and it is claimed that all other
backboned animals descended from them. The last one was supposed to
have died in the Age of Dinosaurs, 60 million years ago, but in 1938
one was netted near the coast of Africa, a savage fighting "fish-beast"
about five feet long and weighing 127 pounds -- truly a living fossil.
No wonder he was mad!
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