In your opinion, what were the most significant developments from the Mesozoic and Cenozoic Eras? If those dev?
In your opinion, what were the most significant developments from the Mesozoic and Cenozoic Eras? If those developments had not happened, what would do you think the world would be like today?
1) Permo-Triassic extinction
2) Cretaceous-Tertiary extinction
3) Formation of the Ithsmus of Panama because it accelerated the ice ages and the eventual evolution of Homo Sapiens
With 1&2 you can check out numerous web sources for the explanation
Other possibilites:
Triassic-Jurassic mass extinction
Eocene temperature spike