- Ancient Jawless Fish, The Agnatha,
The first fishes
Jawless fishes’ (agnathans) fossil record spans nearly
500 million years, the oldest identifiable fragmentary remains
coming from central Australia. Shortly after Ordovician agnathans
radiated into many diverse groups, most characterised by bizarre
armoured bony shields covering the head, each with unique shape
and surface sculpture. The early evolution of agnathans involved
many of the great advances in vertebrate history, such as the
development of cellular bone, paired limbs, intricate sensory-line
systems, dentine-like tissues, complex eye muscle patterns, and
the inner ear with two semicircular canals.
In spite of the agnathans’ great radiation during the
Silurian and Devonian, only the naked skinned lampreys (petromyzontiformes)
and hagfishes (myxiniformes) survived, and these constitute today’s
only living relicts. Very successful groups extinct by end of
Devonian included Thelodonti, Heterostraci, Galeaspida, and Osteostraci.
The latter group are thought to have given rise to the first
jawed fishes.
Summary by Dr. Irwin Haydock of the book entitled The Rise
of Fishes, 500 million years of evolution, by John A. Long, Johns
Hopkins University Press, 1995.
If you would like to learn more about the evolution of fishes,
we recommend Long's excellent book. It's loaded information,
well written and easy to understand.
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