An asteroid killed dinosaurs in spring—which might explain why mammals survived

An international team of scientists used synchrotron radiation to image and analyze fossilized fish from the Tanis deposit in North Dakota.

Some 66 million years ago, a catastrophic event wiped out three-quarters of all plant and animal species on Earth, most notably taking down the dinosaurs. The puzzle of why so many species perished while others survived has long intrigued scientists.

A new paper published in the journal Nature concludes that one reason for this evolutionary selectivity is the timing of the impact. Based on their analysis of fossilized fish killed immediately after the impact, the authors have determined that the extinction event occurred in the spring-at least in the Northern Hemisphere-interrupting the annual reproductive cycles of many species.

As we've reported previously, the most widely accepted explanation for what triggered that catastrophic mass extinction is known as the "Alvarez hypothesis," after the late physicist Luis Alvarez and his geologist son, Walter. In 1980, they proposed that the extinction event may have been caused by a massive asteroid or comet hitting the Earth.

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