The Pterosaurs begin re-taking the skies. However, due to the increased CO2 levels. The Larger Pterosaurs die because their large lungs are unable to cope with the low amounts of Pure Oxygen. The Smaller pterosaurs survive, but are almost wiped out by larger, more aggressive Bird forms. They eventually die out after completely losing their homes and are unable to survive the changes.
Due to the lowered pure Oxygen, the Reptiles have to surface more often. The smaller ones are killed off by larger Sea-reptiles and Sharks due to their inability to cope with the lowered Oxygen supply.
The larger Sea Reptiles eventually die over Millions of years due to the lowered amount of big enough prey.
After several million years of Shark rule in the Oceans, the Whales (note: Whales are Mammals) come. But the Whales take over and are able to survive due to the large amounts of Sharks.
The dinosaurs survive for several million years extra before being slowly killed off by the vast volcanic actively.
Large Sauropods are killed due to a shortened amount of digestible plants.
Megacarnivores begin to hunt smaller dinosaurs and eventually turn on themselves.
Small, fast Omnivore's become top Dinosaurs. But by the time they evolve better, Mammals have already risen up.
The Omnivores stay in the vast Forests where some shrink to hide from the Birds (at 49MYA some Birds ruled the earth at 3.5 metres!)
Due to the large amounts of vegetation some become herbivorous.
Grass Evolves! This creates a vast ecosystem of different Creatures. Due to the open spaces, and the vast amounts of food, some become Carnivores. But still they are under 1M tall. However, the Herbivores have reached over, but stay in the Forests where there is a wider range of Vegetation.
Ice Ages! Due to the Ice ages, many are wiped out. those that survive are are on lands where they can easily travel from North to South to avoid the harsh Winters.
The only Dinosaurs to fully survive the last Ice age are those stuck in South America, which had formed with North America before the Ice Ages.
South America's Jungles and its rain forest begin to regrow and the Dinosaurs have to devolve into smaller forms to survive the smaller space. Some Herbivores manage to create large open areas where they rise in number.
c.8,000. Alt.Humans (who would obviously look different, survived through evolution in lands where the dinosaurs already died out) who have already settled in NA. move to SA..
Some dinosaurs are killed by the Hunters.
800s-1500s. the Alt. equivalents of the Norse, the Irish Monk and Christopher Columbus all settle in the Americas where they hear (or read though paintings) of stories about large monsters living in the Forests. All but the Norse choose not to go. The Norse are more or less killed by either hostile tribes, or by the carnivores who kill them easily due to their armor slowing them down.