something kills off a whole lot of things about every 20 million years
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Notice that the common thread in those exinction events is large animal demise? This might indicate that there is some sort of peridic gravity change. A large dinosaur whole, living, and intact today would be crushed by its own weight - their bones are neither large enough nor dense enough to support their weight.
There was some article published years ago on local gravity fuctuations indicting that gravity is growing weaker in certain regions of the Earth. I don’t remember more than that.
Really, that is a stretch. It’s pretty simple actually. Large animals need a lot of food. If there is a large meteor strike, it blots out sunshine, plants die, herbivores die, and large carniores die.
The othe simple explanation is a blight caused by drought or a virus.
Here is a Wili artical, I spelled Raup wrong, I haven’t read the book in 10 years. It follows the PDF and is a LOT clearer.
http://en.wikipedia.org/wiki/Extinction_event