We don’t know that a T. rex from the start of those 3 million years looked and acted exactly the same as one from the end. It’s also difficult because we have no way of seeing where tyrannosaurs might have been heading. There are some other cases where after a few million years a dinosaur species does seem to gradually blur into the next – not often, but that’s because it is not common to have that continuous a record of them.
I guess the question is more what you expect the pace to be. The interval from Australopithecus to us was a very eventful one, and singling out an extremely odd species as its endpoint. What makes you think Tyrannosaurus could have used changes on that order?
My initial thought is that all of those are shared duties, and being the smart guy or gal does not guarantee that others will mate with you. So the smart guy or gal with the weird head has to still seem attractive enough to screw. And pass the weird head on to those who will continue to use it to make life easier for the family that takes them in, enough to seem like a desirable mate.
That’s assuming traits belonging to the specific individual. All living things are the results of the traits of species, not just of individuals. If Big Brain isn’t passing along their genes directly, but are enabling their family to live (and therefore breed) more successfully, the combination that made those genes is still getting passed along, albeit indirectly.
Folded differently…really, they got chromatin spread out of something? Nope, the surprisingly Open Access OUP article has promoters tagged. And it was like hey, let’s take these organisms that survived the randovirus together and embiggen their grooming/instakill organs so it’s hilarious? Dial the whiskers like way down for no reason. Also give them expressive eyebrows that for some organisms just fall off sometime. And yeah, get Grandma Hidescraper who does grooming where we live to tell the kids when to tuck all the stuff up out of the river valley when big rains (are about to) come. Load-test that alluvial plain.