Magazine article Natural History

Fossil Fishes: So Much Diversity, So Little Change

Magazine article Natural History

Fossil Fishes: So Much Diversity, So Little Change

Article excerpt

Fishes embody one of the greatest paradoxes of vertebrate evolution: While they are phenomenally diverse--accounting for more than half the world's total vertebrate species--they have retained essentially the same anatomical plan for 450 million years. If we could go fishing in the Devonian period, between 360 and 400 million years ago, we would still recognize our catch as fishes, although we would notice several curious features. Familiar categories, such as sharks and rayfinned bony fishes (which are distant relatives of today's paddlefishes, gars, salmon, and perch), would be comparatively rare. But we would discover far greater diversity and ahundance among lobe-finned relatives of the modern coelacanth and lungfishes.

We would also find completely unfamiliar and now-extinct fishes, including small, spiny ones, known as acanthodians, and bone-encrusted monsters, called placoderms, which ould represent the most diverse part ot our catch. Finally, among the finny throng we would also find a few peculiar gilled creatures with toed limbs instead of paired fins, and with no fins at all on their backs. These are the aquatic ancestors of modern tetrapods, or four-limbed creatures, including ourselves. A netful of Devonian fishes would therefore reveal something of our own distant origins, as well as the tandem evolution of fishes and tetrapods.

Since the Devonian, two major groups of fishes--placoderms and acanthodians--have gone extinct, and a third group--the limbed fishes, or tetrapods, including all our terrestrial ancestors--has been displaced onto land. Lungfishes and coelacanths have barely survived, and only two groups, the sharks and the rayfinned bony fishes, are actually more abundant and diverse today than they were in the Devonian. In a very general sense, therefore, the basic variety of form seen among fishes is far less than it used to be. Their diversity today is mostly confined to a single group of ray-finned fishes, the teleosts, which first evolved about 200 million years ago. There are about 23,500 extant species of teleost fishes, including tarpons, herrings, eels, piranhas, catfishes, salmon, and perch.

Fossils reveal that modern teleost diversity did not evolve gradually, but rather exploded between 75 and 50 million years ago. Two groups of teleosts have shown the greatest diversification: ostariophysans, comprising the carps, characins, and catfishes, with about 6,000 species; and acanthomorphs, with more than 15,000 species of perchlike fishes. …

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