Early Fish Evolution And Mass Extinction

Fishes arose in the Early Cambrian, at least based on the evidence of the myllokunmingiids, and radiated by the

Late Cambrian, as shown by the conodonts and Ana-tolepis. The Ordovician once seemed to be almost barren of fish fossils, until several well preserved Early Ordovician astraspids and arandaspids came to light. Renewed efforts in searching for more isolated remains have turned up evidence of a wide array of Late Ordovi-cian fish groups: thelodonts, shark-like fishes, possible acanthodians, conodonts and osteostracans.

In the Silurian, fishes became more diverse and more abundant (Figure 3.25), with the radiation of the armoured jawless fishes and the acanthodians. Further, in the Devonian, the seven major placoderm orders arose, as well as the first sharks, and six important groups of bony fishes, including their derivatives, the tetrapods.

The first extinctions of fishes occurred at the end of the Early Devonian, with the loss of cyathaspids, acan-thothoracids and others. Further fish groups, including some 'agnathan' and placoderm families, and some acanthodians, disappeared during the Mid-Devonian.

During the Late Devonian, a time span of 15 Myr (377-362 Myr ago), further groups disappeared, most of the 'agnathans', including many conodont families, and placoderms, as well as most remaining acanthodi-

'AGNATHA'

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DEVONIAN

SILURIAN

ORDO-VICIAN

CAMBRIAN

'AGNATHA'

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SARCOPTERYGII

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