by Raqi Haas

Created on: April 10, 2009   Last Updated: April 13, 2009

Ever since the first dinosaur fossils were discovered and the world came to know about these magnificent former rulers of our planet, one question that has constantly baffled us is: Just how did dinosaurs became extinct? They were quite literally the most dominant terrestrial animals to have ever walked on Earth, and apart from other dinosaurs, they had no natural predators. Not to mention they had ruled the Earth for over 165 million years during which they have evolved, diversified, and spread all over the globe like no other creature before or after them. Scientists have long tried to come up with theories that would explain the catastrophic demise of these great reptiles. It must be kept in mind however, that along with dinosaurs, many other groups of animals such as Plesiosaurs, Pterosaurs, Ammonites, much of the Crocodilian and Aves (birds) families, and many species of Turtles, Sharks and Mammals also became extinct. It is considered to be one of the 5 big extinction events in the history of planet Earth and it also took place in a relatively short span of time from a geological point of view. Now however, it is generally accepted that the meteor impact towards the end of the Cretaceous period was the root cause and combined with very volatile geo-activity during the time, drove much of life on Earth to mass extinction. However there is debate on other reasons behind extinction as well. Let's take a look at some of the most generally accepted theories behind the K-Pg (Cretaceous-Paleogene, and not K-T or Cretaceous-Tertiary according to popular belief) extinction event.

1. Maastrichtian Sea-Level Regression: The Maastrichtian was the final part of the Cretaceous period and it took place between 72 to 66 million years ago. The Maastrichtian rock layers that have been exposed do not show the kind of erosion, tilting, distortion and other geologic patterns that are so common with the formation of mountains. This is solid evidence that sea levels fell dramatically during this time. Scientists are not quite sure what exactly caused this regression, however, currently the most plausible of the theories that explains this drop in sea levels is the mid-ocean ridges became less active and thus sunk under their own weight. This would explain the mass extinction of the marine lifeforms as a drop in sea level would greatly expose the continental shelves which is where most of the marine life usually live. This change in the sea-level would also result in considerable

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