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ammonites vs. fish at K-T




John,
Cephalopods suffered terrible losses at K-T. Ammonites were already in serious decline anyway and K-T just finished them off. Cephalopod babies are very dependent on plankton, and only a few of the more generalist feeders among them managed to make it through by scavenging---namely the nautilus and several coleoid groups (cuttlefish, squids and octopei). Probably a minimum of 7 clades (i.e., at least 7 species of cephalopods) made it.
Fish on the other hand were not in decline (like ammonites were). And many baby fishes are comparatively less dependent on plankton (more generalist), so relatively fewer clades of fishes were wiped out at K-T than among cephalopods (probably an absolute minimum of 35 actinoptergyian fish clades survived, probably more). But even the surviving fish groups would have suffered huge population losses (as did almost all groups or organisms except decomposers like bacteria). It may be difficult to figure out why some groups survived and others didn't, but I don't think it is overwhelmingly difficult to find plausible explanations.
-----Ken Kinman
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