Prehistoric Creatures Life Dinosaurs Genera

Although there has been considerable speculation about what happened to the non-avian dinosaurs at 65.5 Ma, any meaningful hypothesis must operate within the bounds of science: it must be testable and it must explain as much of the data as possible. Given those constraints, only one hypothesis matches what is known about the K/T extinction: the hypothesis that an asteroid hit the Earth 65.5 million years ago and killed many organisms, including the non-avian dinosaurs.

The evidence for a large (10 km in diameter) asteroid striking the Earth at 65.5 Ma in the Yucat√°n region of Mexico is now extensive, and virtually incontrovertible. Its immediate affects are likely to have been, globally, blockage of sunlight for 3-4 months and the propagation of an energy pulse and associated wildfires in the terrestrial realm as well as, locally, tidal waves and severe environmental disruption.

Key among the various biotic conseqences of these events is that the great nutrient cycles that characterize healthy oceans were severely disrupted.

The asteroid impact is consistent with the limited amount that is known of the dinosaur extinction, which is that in the Western Interior of North America, at least (the only place in

Years before present (Ma)

Figure 15.17. Radiation of mammals after the K/T boundary. The vertical axis shows species of mammals; the horizontal axis shows time. The dark blue line is the exact counts of genera at any particular time; the pale blue line is in the inferred, general shape of mammalian diversity. The interpretation (pale blue line) shows a rapid increase in the number of genera of mammals during the first 3 million years or so of the Tertiary, followed by a kind of leveling off of diversity.

Years before present (Ma)

Figure 15.17. Radiation of mammals after the K/T boundary. The vertical axis shows species of mammals; the horizontal axis shows time. The dark blue line is the exact counts of genera at any particular time; the pale blue line is in the inferred, general shape of mammalian diversity. The interpretation (pale blue line) shows a rapid increase in the number of genera of mammals during the first 3 million years or so of the Tertiary, followed by a kind of leveling off of diversity.

the world in which rates of dinosaur extinction have been studied), the dinosaur extinction was as instantaneous as we are able to resolve at a distance of 65.5 Ma.

The patterns of terrestrial vertebrate survivorship are relatively well understood. Analyses show that vertebrates not in aquatic ecosystems were most susceptible to extinction; other inferences, less robust, suggest that larger animals, endotherms, and amniotes also were less likely to survive than smaller animals, ectotherms, and anamniotes. Non-avian dinosaurs were large organisms, and clearly dependent upon primary productivity.

One explanation for the survivorship of organisms living in aquatic food webs is that these are generally not as dependent upon primary productivity as those in land-based food chains. Because primary productivity was clearly perturbed at the K/T boundary, organisms in aquatic food webs were likely protected both by dietary preference by the fact that the aquatic realm provided a refuge from the physical catastrophes caused by the asteroid impact.

The recovery took 2-3 million years in the oceans, and about 5 million years on land.

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