Dino World

If it hadn't been for one specific disaster, the alt-world palaeontologists would likely still be separating the Mesozoic and Caenozoic ... but by the evolution of grasses. It was sheer luck - for dinos, misfortune - that, when grasses emerged, which had confused the natural order already, a disaster occurred that shifted the ecology (temporarily) heavily to these grasses - and to bugs, rodents, and small birds.

Absent Chicxulub, dinosaurian herbivores would have evolved digestive-systems to handle those tougher flora. I can easily imagine a feathered horse-analogue. The two-footed raptor family was already around and would have chased those horses. The whole saurian family had efficient respiratory systems and would own the mountains as surely as the eagle owns the sky.

We mammals already had the forest canopy; lemurs and monkeys would stay there. And with our stronger bones, some mammals could enter the rivers, fend off the crocodiles, and evolve into the hippopotamus.

One might wonder if the placenta was the killer app allowing longer gestation, against a dinosaurian's egg. But consider that we humans are effectively gestating our young for months or even years after birth. Our babies are quite helpless. Like baby birds.

It could be that on alien planets sentient velociraptors (mounted on pegasi?) are more common than are sentient mammals.

HERBIVORES 11/21/2020: The hadrosaurs and 'ceratopes were already adjusted to the new flora. Hadrosaurs just scooped up lilypads; the 'ceratopes had the guts for the grasses. Their carnivores by contrast seem less robust to changing conditions. So, the PETM ten million years later looks like the shock to push the latter off.