Postdevonian Evolution Of Fishes

The evolution of modern fishes (Figure 7.13) shows roughly parallel patterns between the chondrichthyans and the actinopterygians. The 'palaeonisciform' radiation is matched by the Carboniferous-Permian shark groups. The semionotids and others of the Triassic and Early Jurassic had their heyday at the same time as the hybodonts, and the greatest radiations of all, of teleosts and neoselachians, seem to go in parallel. The radiation of modern sharks began rather earlier, in the Early Jurassic, than did that of the teleosts.

It has often been assumed that fishes swam and wriggled through the mass extinctions unscathed. This appears to be wrong: new studies find levels of extinction that are entirely comparable with other groups. At the end of the Permian and again at the end of the Triassic, several groups of chondrichthyans and actinopterygians disappeared, and the end-Cretaceous event marked the end of hybodont sharks and numerous basal neopterygian groups. Detailed calculations of extinction rates for sharks (Kriwet and Benton, in press) show that seven out of 39 families (18%) became extinct, corresponding to 34% loss of genera and 45% loss of species. These extinction rates are entirely comparable with other groups that were also affected by the KT event. The extinctions were heavy among both sharks and batoids, but most severe among batoids, which lost almost all species (97% loss). Open marine top predators and shell-crushers from the continental shelf and shallow seas were hard hit, whereas deep-sea forms were apparently little affected.

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