I think this is covered in Tim Flannery's excellent book, The Eternal Frontier: An Ecological History of North America and its Peoples (New York: Atlantic Monthly Press, 2001).
Since this is a list about North American fishes, not dinosaurs, let me redirect the discussion:
Why did gars, sturgeons, paddlefish, bowfin, and other aquatic creatures such as turtles and hellbenders survive the catastrophic asteroid that struck the Gulf of Mexico 65 millions years ago, whereas dinosaurs and many other terrestrials creatures did not? According to Flannery, the answer lies partly in the fact that freshwater aquatic environments are somewhat independent of plants and photosynthesis; their chain of life is in part detritus-fed. When the immense dust cloud from the asteroid's impact blocked the sun and halted photosynthesis, plant-based marine and terrestrial ecosystems crashed. Dead leaves and other plant matter, however, continued to flood into ancient rivers and lakes, feeding the bacteria that formed the foundation of a still-functioning food web. In addition, water absorbs heat, and creatures living in deep pools may have been shielded from the intense temperature of the initial impact.
Chris Scharpf
Baltimore
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