Fish are cold blooded creatures that inhabit all kinds of water
bodies from small ponds to the deep sea. They are vertebrates,
with a back-bone made of cartilage or bone and a braincase protecting
the brain; they swim using fins; and take oxygen from the water
by means of gills (although some also have lungs).
These features distinguish them from amphibians and reptiles,
which have lungs and limbs rather than gills and fins; and whales
and dolphins which are warm blooded mammals with lungs. Since
the Late Cambrian, fish have evolved into thousands of species
and today they form over half of all living vertebrates.
Jawless fish, which resemble living lampreys, first appeared
in the early Cambrian and later evolved into many types, including Loganellia, above right and Jamoytius.
Some (e.g. Cephalaspis, below right) were protected by thick scales
or by bony head-shields. They did not have bones within their
bodies but they did have a notochord, or primitive spinal column, and so fossils are rare.
One group, the conodonts, were for decades known only from
teeth found in Palaeozoic rocks, but very rare fossils of their
soft, eel-like body have now been found. Jawless fish ate tiny
organisms and food particles from the sea floor and some may
have sucked blood.
The last of the coelacanths were believed to have become extinct
in the late Cretaceous, about 70 million years ago. But in 1938,
to the amazement of scientists, a living specimen, called Latimeria, was
caught off the Comoro Islands, in the Indian Ocean.
More recently these 'living fossils' have also been found off
Indonesia. Islanders have known of these fish for many years,
and used to use the rough skin as paper.
