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These were large creatures - they were allowed to grow that large on a planet that sustained them.
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One was caused by a handy little killer called a nemotode which, when entering the body through water, is microscopic but grows in the gut to lengths as huge as the animal is.
One found in an Asian elephant grew to over 25' long. Imagine how big they became in dinos. They were painful and fatal.
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Earth was a very hostile volcanic world for the giants, too. They inhaled toxic fumes from constant eruptions, another culprit in reducing the populace.
So:
1. Dinosaurs were in a weakened state due to viral infection and parasites such as leish-maniasis and nemotodes long before the asteroid hit.
2. They were also victimized by disease-carrying insects, mostly sandflies, which were rampant in that era and impossible to stop.
3. When the KT asteroid hit, it most likely found a planet of a small population of giant reptiles already dying out. This was just the coup-de-gras.
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If all this is true, and empirical forensic science gives much proof of it, then it shows us early fair warning of our own early demise. We know the sun will die, it's too small to go nova but will become a red giant and...you know the rest. All life will already be extinct by then.
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All you need do is visit Indo-China or areas in Malasia, to see how quickly disease becomes epidemic. The next step is pandemic and North America is not immune. We can learn from what happened 65 billion years ago by preparing for what we ourselves have put into play: don't forget, we built a national tourist attraction over a super volcano, which is overdue.
Yellowstone