Surviving Extinction

The K-T boundary separates the age of reptiles and the age of mammals, and this was first recognized over one hundred years ago by geologists who realized that there was a dramatic change in the types of fossils deposited on either side of this boundary.  Originally, it was thought that dinosaurs wasted away gradually, probably because of long-term fluctuations in sea level and temperature that altered the amount of land and types of food accessible to them.  This was discounted in 1980 when a better theory emerged from physicist Luis Walter Alvarez and his geologist son Walter that suggested that an asteroid collided with our planet 65 million years ago and caused all non-bird dinosaurs and many other animals to die out.  They were working together on a geology expedition in Italy, when they accidentally discovered a band of sedimentary rock that contained unusually high levels of a rare element, iridium.

The Alvarez hypothesis about iridium is still subject to debate, but they found that it was in a very even, widespread distribution (not just in Italy), and they figured this was the result of a giant asteroid that hit Earth, sending smoke, dust, and iridium into the atmosphere, which eventually settled to the ground.  The metal iridium, which is similar to platinum, is very rare on Earth’s surface but is more common in asteroids and in molten rock deep within the planet.  A smokescreen blocked the sun with dense clouds of dust for months, lowering the earth’s temperature, killing plants (but not seeds or roots), and eventually many species of animals, including the dinosaurs.  The plant-eaters died out first, followed by the meat-eaters who would have eaten them.  Smaller mammals and birds could survive the cold, desolate period because of their fur, feathers, and ability to eat seeds, roots, and decaying vegetation.

This asteroid impact theory is supported by evidence from an identified crater known as the Chicxulub crater, which is centered on the Yucatán Peninsula in Mexico.  The asteroid is thought to have been between 10 and 15 kilometers wide, but the velocity of its collision traveling 30 kilometers per second which is 150 times faster than a jet airliner, caused the creation of a much larger crater, 150 kilometers in diameter.  It’s the second-largest crater on the planet and is exactly the same age as the extinction of the non-bird dinosaurs, which can be tracked in the rock record all around the world.  It is thought that that massive hurricanes and firestorms would have raged across the Earth, adding to the destructive power of the catastrophe and that all of the dinosaurs came to an abrupt “now you see them, now you don’t” end.

Cores drilled into deep-ocean sediments contain geologic data pointing to deadly bursts of gas to the atmosphere, particularly planet-warming carbon dioxide and ocean-acidifying sulfur dioxide.  Some computer simulations determined that the amount of gas spewed into the atmosphere from the volcanos across what’s now western India, was enough to account for the changes in temperature on earth during this time period, so this issue has not been settled yet.  Many scientists believe that the dinosaurs all died because of increased volcanic activity, or a shift in plate tectonics and there is no clear solution in the foreseeable future to resolve this.  All birds today are descendants of the dinosaurs, as they evolved from small, feathery, raptor-like dinosaurs.  Sharks, and crocodiles also share a heritage with dinosaurs.

Written for Reena’s Xploration Challenge #332 where today she asked us what killed the dinosaurs.