[LiteKeeper]

Their pulmonary system is a flow-through, rather than in-and-out as we do. This ensures a constant supply of oxygen during flight. For an "in-and-out" breather to become a "flow-through" breather would require some kind of a hole to open up in the system. This is a virtual impossibility.

Dinos breathed like birds?

[RightWhale]

This is a virtual impossibility.

I don't remember if it was Asimov or Clarke: If a noted scientist says somethign is possible, he is probably correct. If a noted scientist says something is impossible he is probably wrong.