No one truly knows how the dinosaurs became extinct, but the fact is they disappeared and a whole host of ecological niches were made available to other organisms, who could harness the resources of these niches due to the absence of competition (and predation) by dinosaurs.
The dinosaurs disappeared around 65 million years ago, with many other land dwelling organisms also dying out around this time. Regardless of what killed off the dinosaurs, it was comprehensive. The general consensus is that a major geological event killed off many of the land dwelling organisms, particularly the larger ones.
This would have caused an overall drop of biomass on land, and therefore 'less food to go round' all the organisms that occupied dry land. Also, many food chain relationships would have been disrupted, causing a gradual breakdown of populations in the long term, sometimes leading to extinction, essentially survival of the fittest.
Summary of Life
Insects, due to their size, were adaptable and already diverse, meaning that at least their short term survival and close relationship with plants (at the bottom of any food chain) was secured
Marine life was still plentiful, and diversifying, while mammals were emerging to be the next dominant force on plant Earth.
Birds were also diversifying, and taking advantage of their proportionately larger body in comparison to insects, alongside their ability to fly.
On the other hand mammals were specialising on land, and trees, which we further investigate on the next page of the timeline below...
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The Ancestors of Mammals