In a message dated 2/9/01 3:04:20 AM EST, k.clements@auckland.ac.nz writes: << What I had not heard before was his claim that this competition was size-related. David felt that birds and mammals replaced dinosaurs from the bottom up. >> You can read more about this on pp 85-88 of my Mesozoic Meanderings #2 (second printing), published in 1992. (Still sold out, alas.)