Paleontology

Why Dinosaurs became Extinct the k Pg Extinction Theories



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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 climate changes, specially due to the change of ocean currents combined with different wind patterns, and the reduced albedo (reflection of sunlight) effect which would have resulted in increased temperatures worldwide. Furthermore this regression greatly reduced shallow continental seas, thus many species of animals which had homes along coastal areas lost their habitats. However freshwater lifeform was considerably expanded due to the fact that rivers and other water bodies had to run off longer distances before reaching the seas. Although this meant freshwater life would flourish, marine life suffered.

Although this theory explains the extinction event to a certain extent, it does not account for the fact that there was extinction on a mass level beyond the seas, oceans and coastal plains. That is why this theory is not generally seen as a sole contributor to the K-T extinction event.

2. Deccan Traps: The Deccan Traps formed towards the end of the Cretaceous period, between 62 to 68 million years ago. They are one of the largest volcanic and igneous rock formations in the world. Most of the volcanic eruption took place around 66 million years ago during which time they were thought to have covered a lava flow area of upto 2 million square Km, and were active for as less as 30,000 years. The Deccan Traps were thought to have contributed to the extinction through the release of greenhouse gases (mostly Sulfur and Carbon particles), dust and ashes from the eruptions. This would have created a thin layer of ash and dust in the upper stratosphere resulting in less sunlight reaching the Earth's surface which would mean plants would die from reduced photosynthesis. Herbivores would be badly affected by this, and then the carnivores which rely on the herbivores, and the whole ecosystem would basically collapse and trigger an extinction event. This theory however would explain a gradual extinction of lifeforms on the planet, but it is generally conceived that the K-Pg extinction event took place in a geologically short period of time.

3. Alvarez Hypothesis: Nobel prize winning physicist Luis Alvarez and his team came up with possibly the most accepted theory behind the extinction of dinosaurs so far. They came up with the theory that the extinction event was triggered by a meteorite/asteroid impact about 65 million years ago. in 1980 this team of scientists discovered that sedimentary layers of rock worldwide contain concentrations of Iridium many times more than what is usually found on Earth. Iridium is a very dense compound and is rarely found in the Earth's crust. The explanation that Alvarez and his team came up with was a meteor impact occurred towards the end of the Cretaceous. Alvarez and his team estimated the asteroid to be around 10 kms across. This impact would result in throwing a thick cloud of ash and dust up in the atmosphere, which would block sunlight and prevent photosynthesis in plants for atleast a few years. This would explain the extinction of plants and polyplanktons, which would further disintegrate the food chain as dying herbivores would result in dying predators. Eventually the whole ecosystem would more or less collapse. Smaller animals however would survive as they would not necessarily need the same amount of food to sustain themselves as larger animals. Sulfuric compounds would be injected into the stratosphere which would have further reduced the amount of sunlight reaching the Earth's surface.

Furthermore, the incendiary fragments from the impact would have resulted in global firestorms. According to scientists, the oxygen levels during the Mesozoic era (Triassic, Jurassic and Cretaceous periods) were between 40% to 30% higher than it is today (which is also one of the reasons behind the evolution of gigantic plants and creatures). This high level of oxygen would mean the fire from the meteor blast would have burned through lush forests resulting in high carbondioxide levels. Combined with the blocked atmosphere from ashes of the meteor impact, this would have resulted in a temporary greenhouse effect on a global scale. Mass extinction would have occurred in a very short span of time.

This theory although very plausible is still debated between scientists. Although it explains rapid extinction events it does not really explain extinction over a longer period of time. However, more craters have since been discovered, and it is now believed that multiple asteroid impacts took place towards the end of the Cretaceous period. Now it is generally agreed however, that multiple impacts combined with the Deccan volcanic activities and regression of the world's oceans all happening almost around the same time contributed to the extinction of around 50-70% of lifeforms on Earth, including the dinosaurs.

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