https://en.wikipedia.org/wiki/Feathered_dinosaur
this article is from The NY Times and I am only linking to it because the first photo in that article is an amazing fossil of a feather. https://www.nytimes.com/2020/09/30/science/dinosaur-feather-...
I know what you're trying to say, but every living thing today, from the mammals to the fungus among us, have spent the same amount of time evolving, dating back to the first life, or at least the first life encoded for reproduction. Except for the birds, the dinosaurs quit early.
100 million years is an extremely long time. A lot happens in that amount of time.
during that period of time, the ancestors of every living thing on earth was also mutating and naturally selecting.
during the entire time the dinosaurs roamed the earth, the mammals and/or their ancestors were also evolving and saving up genetic endowment for being the fittest in every eventuality; sadly, the dinosaurs bet on the wrong ... horses.
[1] One specie or a few species? I'm really curious about the current most popular hypothesis.
This is an ankylosaur which was a heavily armored dinosaur.