Animals that move through the water primarily by
swimming are nektons, and free-swimming pelagic animals
are said to be nektonic. They are distinguished from other
pelagic organisms (plankton) that float in the water. The
most important nektons in the water today are the fish,
whereas in the Paleozoic several other forms were common.
The ammonoids of the Devonian were coiled cephalopod
mollusks that evolved from earlier nautilids, and these existed
with the free-swimming scorpion-like eurypterids. Fish first
appeared in the marine record in the Cambrian-Ordovician
and included the early bony-skinned fish known as ostracoderms,
followed in the Late Silurian by the finned acanthodians.
Heavily armored large-jawed fish known as placoderms
are found in many Late Devonian deposits, as are lungfish,
ray-finned fish, and lobe-finned fish that include the coelacanths,
one species of which survives to this day. Lobe-finned
fish are the ancestors of all terrestrial vertebrates. Sharks
were very common in the marine realm by the Late Paleozoic.
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