Wouldn’t species always be evolving? Example: Why do we find a couple of dozen T-Rex skeletons and not all the evolving stages that lead to the T. Rex over millions of years? And are those specimens classified as different species?
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The T-rex evolved from older and smaller theropods. We do have fossils of them, but with just a few examples it can sometimes be hard to tell what’s a different species and what’s just a juvenile or regional variation.
More generally, evolution happens in quick bursts and slow crawls. When the environment is stable for long periods you often don’t see dramatic changes to the animals occupying various ecological roles. You get smaller tweaks that are hard to parse from fossils.
You get dramatic changes only when the environment itself changes. Mammals stuck to the same set of squirrely body plans for a hundred million years to stay out of the dinosaurs’ way.
Then one day the dinosaurs were gone, and mammal evolution exploded to fill all the vacancies.
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