The Museum of Paleontology at Berkeley contains more than 150,000 curated specimens of nonmarine vertebrate fossils—mammals, dinosaurs, turtles, snakes, and so on—from the Hell Creek and
Tullock formations. David Archibald and Laurie Bryant (both former students of Clemens) took upon themselves the task of sorting through this enormous database to count the number of species that were present both in the Hell Creek formation and in the overlying Tertiary rocks of the Tullock.49 They found that of the 111 species of land-dwelling vertebrates present in the Hell Creek, 35 survived the K-T boundary. This was a percentage survival rate of 32 percent, meaning that 68 percent had become extinct. None of the 20 species of dinosaurs had made it; their percentage survival rate is 0. But what about the mammals? The complete extinction of the fascinating dinosaurs has obscured the devastating blow struck to the smaller and less interesting mammals at the K-T—the Archibald-Bryant study shows that only 1 out of 28 mammal species survived. How lucky we are!
Sheehan and Fastovsky divided the Archibald-Bryant database into those that lived on land and those that lived in freshwater.50 They found that whereas 88 percent of the land dwellers became extinct at the K-T, only 10 percent of the freshwater assemblage did so. What could explain this distinct difference in survival rate? According to Sheehan and Fastovsky it arose because the dinosaurs and other land dwellers were at the top of a food chain based on living plants, some 80 percent of which became extinct, whereas the water dwellers were part of a chain more dependent on organic detritus left behind in lakes, streams, soil, and rotting logs. Our mammalian ancestors may have survived because they were part of, or were able to become part of, the detritus-based food chain.
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