Dinosaur Extinction: When and how did dinosaurs become extinct

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Dinosaur Jungle   >   Dinosaur Facts   >   Extinction


Dinosaur Extinction

Dinosaurs inhabited the earth for over 165 million years, during the Mesozoic Era.

Most dinosaur species lived and died out during this period, but at the end of the Cretaceous period (65 million years ago), all remaining types of dinosaurs (with the exception of birds which had already evolved, and are widely believed to be descended from dinosaurs) died out. At the same time many other creatures (for example Ammonites, and Belemnites, as well as Pterosaurs, and many large marine reptiles such as Mosasaurs. Plesiosaurs and Pliosaurs) also became extinct. In fact, all animals weighing more than 55 pounds (25 kilograms) seem to have died out!

While are not sure what happened, we do know that something happened at about 65 million years ago. This event shows up as clear boundary in the rocks, known as the "K-T boundary", the extinction event usually being referrred to as the "Cretaceous-Tertiary mass extinction".

K-T boundary in rocks near Drumheller, Alberta, Canada

Scientists have proposed many different possible explanations for what may have happened, and why this mass extinction might have happened:

(Note: Sometimes these suggested explanations are informally called "theories", however in this case "hypotheses" is the correct term, since in science a theory means a well-substantiated explanation of some natural phenomenon or observation, whereas as "hypothesis" means a suggested explanation which needs to be tested against experimental evidence).
    Artist's impression of KT impact event
  • Asteriod Impact Hypothesis

    • Hypothesis: An asteriod hit the earth.

    • Effect: Apart from the immediate impact of fires and flooding (if it hit the sea), this would have caused catastrophic changes in the environment because of the dust thrown up in to the air blocking out sunlight.

    • Our Evaluation: This is currently the most popular hypothesis with scientists, because, as Luis and Walter Alvarez pointed out, iridium (which comes from asteroids) has been found all over the world in rocks at the K-T boundary (which were laid down at the time of the mass extinction). There is also a large crater at Chixulub in southern Mexico, which appears to be of the right age, and could have been made as a result of an asteroid hitting the earth.

    Artist's impression of Deccan traps volcanism
  • Volcanism Hypothesis

    • Hypothesis: Volcanoes (there is known to have been very active volcanic around this time, especially in India in a region that is today known as the Deccan Traps) could have caused catastrophic changes in the environment.

    • Effect: The dust from volcanoes would have affected the climate, sulphur from volcanoes could have caused immensely strong acid rain, and poisonous substances such as selenium could have been released into the atmosphere.

    • Our Evaluation: After the Asteroid Impact Hypothesis, this is probably the second most popular explanation among scientists - partly, because we know that volcanoes were active at about this time.

    Artist's impression of a supernova
  • Supernova Hypothesis

    • Hypothesis: A nearby star exploded as a supernova.

    • Effect: A nearby supernova would have bathed the earth in deadly radiation.

    • Our Evaluation: If this hypothesis is true, rocks at the K-T boundary should contain 244Pu (a long-lived isotope of Plutonium). However, this does not seem to be the case.

  • Climatic Change Hypothesis

    • Hypothesis: Changes in the Earth's climate caused the mass extinction. This could have been caused by gradual changes in the positions of continents effecting ocean currents and winds, or by changes in the Earth's orbit around the sun, or even by a relatively sudden greenhouse effects.

    • Effect: Depends on the particular climate change theory in question.

    • Our Evaluation: This hypothesis can not be discounted, and even if climate change was not the main cause of the extinction, it could have played a part - there is some evidence that the Earth cooled at the end of the Cretaceous period.

  • Flowers Drugged the Dinosaurs Hypothesis

    • Hypothesis: The first flowering plants appeared during the Cretaceous period, and if dinosaurs ate them they could have been drugged by them.

    • Effect: Could perhaps have wiped out some dinosaurs.

    • Our Evaluation: This hypothesis does not explain the extinction of many other species, especially marine species. It also struggles with the fact that dinosaurs and flowering plants were contempories for many millions of years, and only suddenly, at 65 million years ago, the dinosaurs died out.

  • Mammals Ate the Dinosaurs' Eggs Hypothesis

    • Hypothesis: Mammals ate the dinosaurs' eggs.

    • Effect: Could perhaps have wiped out some dinosaurs.

    • Our Evaluation: This used to be a popular hypothesis - but does not explain the extinction of many other species, especially marine species. It also does not explain why dinosaurs were very successful for millions and millions of years, during most of which time, mammals were also around.

  • Caterpillars Ate All the Plants Hypothesis

    • Hypothesis: Caterpillars ate all the plants and there was not enough food for herbivores, and eventually not enough meat for carnivores.

    • Effect: Could perhaps have wiped out some dinosaurs.

    • Our Evaluation: This hypothesis does not explain the extinction of marine species, and some may wonder whether caterpillars could really affect every continent on the Earth at the same time to this extent.

  • Disease Epidemic Hypothesis

    • Hypothesis: The dinosaurs could have been wiped out by a disease epidemic.

    • Effect: Could perhaps have wiped out some dinosaurs.

    • Our Evaluation: The problem with this hypothesis, is could it effect dinosaurs to the extent of wiping them all out, and how could it explain the simultaneous extinction of marine animals?

Although many scientists now favor the asteroid impact hypothesis, it is not yet actually certain that this is the reason why the dinosaurs died out. It is possible that one of the other theories might be the correct one. It is also possible, that there could have been several events which happened together, and between them, killed all the dinosaurs.

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