Thursday, June 18, 2015

Understanding Dinosaurs - Part 9: Dinosaur Extinction

About 66 million years ago, the Cretaceous period, and the Mesozoic era with it, ended. Like the great Permian extinction that began the Mesozoic, it too wiped out a large portion of the life on earth (around 75%). But most notable among these extinctions was the demise of the non-avian dinosaurs, leaving only the birds to continue into the Cenozoic era that we live in today.

For well over a century, scientists have debated about what wiped out these creatures and why none of them continued on to today. It has only been in the last 20-30 years that we have begun to cement a firm theory as to what happened (though many alternative theories do still exist). By looking at the layer of rock known as the K-Pg (or sometimes K-T) boundary, we can look at the moment of extinction in the fossil record. Below it, are the older periods of time, such as the Cretaceous and the Triassic before that. Above this line, is the Paleogene, first period of the Cenozoic era. Within that line, however, exists some rather interesting things, namely the concentration of iridium a metallic element similar to platinum. It exists in this layer in much higher concentrations than in the rock above or below it.

In 1980, a physicist named LuisAlvarez and his son, noticed that this band of odd rock could be found all over the world. Iridium being an incredibly rare element on earth, it lead scientists to the idea that the earth was hit by some foreign body, causing some global catastrophe. This is now known as the Alvarez Impact Hypothesis and is the most widely held theory as to the cause of the Cretaceous-Paleogene Extinction. Thirty-five years of science since then, have lead to further evidence to support this theory. The discovery of the Chicxulub Crater off the coast of Mexico seemed to be the clincher. All evidence pointed to a large astronomical object (probably 10 km wide) smashing into the earth with the force of over 2 million nuclear bombs.

But it wasn't just the impact itself that would have wiped out the majority of life across the globe. While anything near the site would just be incinerated, the rest of the planet suffered through years of horrendous side effects. The cloud of dust and debris from the impact would have blocked out the rays of the sun, killing off plants and phytoplankton that could no longer create food. This would have worked its way up the food chain, killing off herbivores who no longer had plants to eat, and then carnivores who lost things to hunt.

Because dinosaurs made up the greater bulk of the food chain, they would have been hit the hardest. They were large animals who needed an enormous amount of food to survive. On top of that, they would have been more vulnerable to the toxic gases, acid rain, firestorms and shifting weather brought on by the cataclysm . It would be the smallest of things, lizards and early mammals, insects of various kinds, and some birds that would survive by seeking shelter underground, requiring less food, or being able to make long trips to find suitable shelter and nourishment.
This has been the standard model for many years. Often focus shifts on the aftermath as to the leading cause of the long term die off around the world. Sometimes it is a massive global winter, while other times it is something else. But other theories do still exist that play down the importance that even the Chicxulub impact had on the event.

The Deccan Traps Hypothesis points the finger at a massive group of volcanic eruptions leading to a slow die off as toxic gases and dust filled the air. There is a lot of evidence to support this theory as well, which has lead many scientists to add it into the larger Alvarez Hypothesis. It is unlikely any one thing caused the global devastation, but more likely an incalculable chain of events that coalesced into the event that nearly wiped out all life on earth once again. It is often these combination theories that make the most sense for such a wide spread phenomena. Theories of a severe drop in sea levels, multiple impacts, volcanic eruptions, climate changes and radiation are prevalent. But all could easily work together to create a really bad 'wrong place at the wrong time' kind of event.

Regardless of the how, scientists do agree on one thing. The Cretaceous-Paleogene Extinction event saw the end of the non-avian dinosaurs. We find no fossil evidence of them above the K-Pg Boundary. It is unlikely that even the smaller non-avian dinosaurs lived beyond a few hundred thousand years of the event. Evidence found, just outside my door, in the Red Deer River also points to the fact that these dinosaurs were on the decline in the last 10 million years leading up to the event. As their numbers paled, a massive ecology altering event would easily wipe the remaining out.

But every story has to have an ending. Though the event took out all the non-avian dinosaurs, we were left with the avian ones who survive to this day. Sure we have fossils and other traces of dinosaurs that grace the exhibits of a thousand museums, but there is just something awesome about knowing that there are still dinosaurs just outside your window.

Next Time: Further Reading

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