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What Is a Dinosaur? | Anatomy & Evolution | General Behavior | Where Did They Live? | Why Did They Go Extinct?

Why Did They Go Extinct?
Introduction | Alvarez Hypothesis: Origin and Evidence | Effects of the Asteroid Impact | Other Extinction Hypotheses | Deep-sea Evidence for the Impact Hypothesis | Post Extinction Recovery | References  

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Post-Extinction Recovery
It has been estimated that the planet took 1-2 million years to fully recover from the asteroid impact. In deep-sea sediments, several very small sized species of foraminifera with simple, unadorned shells appeared within several thousand years after the extinction event, but several million years elapsed before species diversity, shell ornamentation, and shell sizes increased to values comparable to those that occurred before the impact event. The small sized planktonic foraminifera are considered opportunistic species that had rapid rates of reproduction and higher tolerances to changing environmental conditions.

A similar pattern of extinction and recovery has been observed in the North American fossil land plant record. In southwestern North Dakota, where the fossil record of land plants is most complete and best studied, abrupt extinction of 70 to 90% of plant species was immediately followed by a dramatic increase in the abundance of ferns. Because the North American forests were decimated by the asteroid impact, ferns were able to rapidly disperse and dominate much of the newly cleared land surface for hundreds to thousands of years afterwards. Full recovery of North American forests, resulting from appearance of new species and repopulation by surviving species, took from several hundred thousand to over a million years.

Parrish Illustration of life post-extinction

Of the many long-term effects produced by the global devastation at the K/T boundary, the most obvious is the disappearance of all non-avian dinosaurs. Yet the close of the Age of Dinosaurs meant the start of the Age of Mammals. Although mammals had existed alongside the dinosaurs for hundreds of millions of years, they had remained small and comparatively rare. The extinction of the dinosaurs allowed mammals to come into dominance, as they evolved into new and larger forms throughout the Tertiary Period.

Within the first five million years of the Paleocene Epoch, large mammals had appeared for the first time. Some of them were the earliest members of modern groups, including primitive carnivorans and ungulates. The first primates (members of the mammalian order that includes humans) appeared about 10 million years after the K/T boundary event. Modern bird groups diversified as well, in the absence of pterosaurs (which had also gone extinct). Perhaps without the extinction of the dinosaurs, the evolution of mammals and the subsequent rise of humans would have never happened. And although recent history might well be called the Human Age, the time that the human race has dominated planet Earth is but a blink in geologic terms. It is certain that the world will change again. Indeed, we may be in the midst of another mass extinction event right now.

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