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Mass Extinctions

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Ken Tapping, March 12, 2004

In the sky this week...

> Venus dominates the western sky after sunset.
> Mars is fading.
> Saturn is high in the eastern sky during the evening, and Jupiter comes up a couple of hours later.
> The Moon will reach Last Quarter on March 12.

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A popular theory is that dinosaurs were wiped out 65 million years ago by the impact of an asteroid. However, is that the whole story? There is evidence that there is a lot more to it. When we started to decipher the history of the Earth, we divided story into four eras. First there was the Cryptozoic Era (Hidden life: until 544 million years ago. We used to think there was no life back then, but now we are finding scattered evidence that there were some simple creatures and plants). Then there was the Palaeozoic Era (Old Life: 544-245 million years ago), the Mesozoic Era (Middle Life: 245 - 65 million years ago), and the Cainozoic Era (The Age of Man: 65 million years ago to present. This is a bit pretentious since we have been here for less than a million years). The Palaeozoic and Mesozoic Eras ended with environmental catastrophes, when most forms of life vanished. We call these events "Mass Extinctions". Then, when we look inside these eras, we find more extinctions. The Burgess Shales, rocks in British Columbia and Alberta, are loaded with the remains of creatures that lived at the beginning of the Palaeozoic Era. Most of these creatures are really bizarre. One of them, called "hallucigenia", looks like something out of a dream, and there are continuing arguments about whether it walked on its spines and waved its tentacles, or walked on its tentacles while waving its spines. Then there was a mass extinction, most creatures died out, including hallucigenia, and the survivors diversified and took over. Let's look at the extinction we've all heard about: the end of the dinosaur age.

The Mesozoic Era is divided into three Periods: the Triassic, Jurassic, and the Cretaceous. In the Triassic, the dinosaurs were becoming dominant. Their high point was in the Jurassic. Then, as we entered the Cretaceous, there were growing signs that things were going wrong, possibly due to massive environmental change. The best evidence comes from the ammonites, octopus-like creatures that lived in tight spiral shells, like the modern nautilus. Through the Triassic and Jurassic ages ammonites were all tight spirals. Then, in the Cretaceous, they started to change rapidly. There were straight ones, slightly curved ones and open spirals. Then they went extinct, along with many other species, including the dinosaurs. It seems that, for environmental, genetic or other reasons, species go through rises, peaks and declines. When declining, a species is more vulnerable to environmental catastrophe. It looks as though environmental change and other factors had the dinosaurs and many other species already in trouble. The asteroid was just the last straw.

Thanks mainly to us, the biggest extinction in the history of the Earth is happening now. Many species are in trouble, perhaps even humans. That is why we are looking at the asteroid impact threat very seriously. There are now telescopes dedicated to finding objects that might hit us. That's Phase 1. Phase 2 is more difficult. Having found a threatening object, what do we do about it?


Ken 
TappingKen Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,
E-mail: ken.tapping@nrc-cnrc.gc.ca