Cobaltduckworlda has had somewhat of a similar biological past as earth, but not exactly. As examples, sentience has happened multiple times, and not just in primate-like species. Beasts that didn't survive our ice-age continue to roam the plains of cobaltduckworld. Mammals arose and eventually led to humans and dolphins and horses, but also elves and brocc. And oh, yeah, there are pterosaurs.
Once, there were velociraptors and iguanadons and many others as well. They went extinct some 10s of millions of years ago, but not the pterosaurs. Pterosaurs survive in the modern era, with dozens of species ranging in size from about equal to a duck to twice that of a condor.b
Maybe it was a meteor in a different form, or a continent-splitting geological upheaval, or a radical shift in climate. Everything that happened to the "dinosaurs" on earth happened on cobaltduckworld, except for the pterosaurs.
The challenge is to explain what this event was, and how its aftermath played out. Magic is allowed, but not encouraged.
a I really need to think of a proper name.
b I know I keep saying that modern literature needs more quetzalcoatlus, but I don't want my humans to get eaten.