|
Some of the early fish
were quite different from the graceful, agile swimmers that we see
today. They were covered with a thick, bony armour, to protect them
from sea scorpions. The pincers of the sea scorpions could easily
crack open any protection, save those that the fish were endowed
with. The armour would weigh the fish down and they would swim
rather slowly and often clumsily, using their fins and flippers.
Many
of the earliest fish did not have jaws with which to bite. To find
and eat food, they would move along the surface. The cephalaspis was
a species of fish with a mass of solid bone for a head. It measured
about twenty to thirty centimeters in length. On its head, it had
sensory areas covered with tiny bones. It used to plough along the
bottom and swallow the mud, as also some worms and snails.
The
Pteraspis was yet another of the earliest varieties. It was a small
sized fish, measuring only fifteen centimeters in length and lived
in lakes and streams. It had no fins to guide it when swimming. It
is believed that Pteraspis must have scraped plant off rocks for
food. The Climatius was luckier. It had a cover of thick diamond
shaped scales, but it also had a powerful swimming tail and a series
of fins to help steer itself thorough the water.
For
better swimming, some fish evolved lighter bodies in which the
skeleton of the heavy bones was replaced by one made of cartilage.
The cartilage or gristle covered the joints between the bones in all
backboned animals, or the vertebrates. The Xenacanthus was an early
fish with a skeleton of cartilage. It had paddle like fins and many
sharp teeth. It measured seventy-five centimeters in length.
Among the twenty five thousand varieties of fish, said to be
existent in the world now, the coelacanth has an interesting
history. It dates from the Devonian Period, about three million
years ago. It was thought the coelacanth had become extinct since
the last seventy million years. However, it was sighted in the sea
off the coast of East Africa. The fins of the coelacanth are
different from those of the others; it has curious fins on stalks. |
