Extinction Of Dinosaurs

Although the fossil record provides many clues about what dinosaurs were like, paleontologists who study dinosaurs still have many unanswered questions. For example, why did the dinosaurs and many other species become extinct 66 million years ago, at the end of the Cretaceous period?

Asteroid Impact Hypothesis

Until recently, most scientists thought that a single catastrophic event was responsible for the mass extinction of dinosaurs. The asteroid impact hypothesis suggests that a huge asteroid hit Earth and formed a crater on the Yucatán Peninsula in southern Mexico. The impact sent so much dust into the atmosphere that the amount of sunlight reaching the Earth's surface was greatly reduced. The reduced sunlight caused severe climatic changes that led to the mass extinction. The asteroid impact hypothesis is supported by the fact that sediments from the end of the Cretaceous period contain unusually high concentrations of iridium. Iridium is a metal that is very rare in the Earth's crust but more abundant in asteroids and other meteroids. According to this hypothesis, the dinosaurs would have become extinct very quickly.

Multiple Impact Hypothesis

Another hypothesis, called the multiple impact hypothesis, proposes that asteroid impacts began before the mass extinction took place. Along with other unfavorable environmental conditions, multiple asteroid impacts, including the impact on the Yucatán peninsula, led to a decline in numbers of organisms among species. In 2004, paleontologist Gerta Keller and her colleagues provided evidence to support this hypothesis. Results of their research suggest that conditions hostile to dinosaur survival were spread over many years. It is likely that these reptiles were not wiped out by a single asteroid impact.

figure 41-4

(a) Pterosaurs ranged in size from the smallest, which were only the size of sparrows, to the largest, which were about the size of a small airplane, with wingspans of 12 m (about 39 ft).

(b) Like dolphins, ichthyosaurs were probably fast swimmers and fed on fish.

figure 41-4

(a) Pterosaurs ranged in size from the smallest, which were only the size of sparrows, to the largest, which were about the size of a small airplane, with wingspans of 12 m (about 39 ft).

(b) Like dolphins, ichthyosaurs were probably fast swimmers and fed on fish.

Word Roots and Origins crocodile from the Greek krokodilos, meaning "lizard of the Nile"

figure 41-5

This phylogenetic diagram represents hypotheses for the relationship among reptiles, birds, and mammals. For updates on phylogenetic information, visit go.hrw.com. Type in the keyword HM6 Phylo.

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