Atmospheric density fails as an explanation because of the long time period involved in their existence through the Mesozoic and would not have been consistent during their evolution into different ecological niches. They were successful marine predators as were the avians that they coexisted with. The more interesting question to me is that since they occupied similar niches with the birds, why did they fail to survive the end Cretaceous as the birds obviously did.

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(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily available to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

"Quaerendo inventis"