Every last bird on earth come from a single common ancestor, anchored far into "advanced" bird evolution. Ostriches, for example, are not closer to Ostrich-like dinosaurs, and chickens do not represent any closer relationship than say ducks or crows.
The fixation on T-rex in dino-to-bird-transition is also entirely misguided, and based on pop science short-cuts, as Tyrannosaurids did not at all evolve flight, and only a *single* branch of Theropod went the bird-route. We are always talking about single points of evolution branching out.
This means, not even Velociraptor *led to* birds (especially since Velociraptor lived 70 million years ago, and first flight evolved about 150 million years ago, more than double the age!)
Instead, birds evolve from something *very similar* to Velociraptor, from the mid-late Jurassic. All birds! And no T-rex.
I'll probably bring it up again in the future :v It's a bit of a pet peeve, and one paleo-peeps on twitter were discussing in a thread, pop-science has made the overly specific 'T-rex-to-chicken-analogy' into a commonly held belief among the public.
WELL, Tyrannosaurid family is bracketed well within feathered Theropods, and early smaller forms were fluffy, up untill the 6-7 metre size range, where later giant forms appear to be naked/scaly.
(In another random fun fact - despite reputations, the Tyrannosaurid family mostly consists of small to medium lanky-legged, long-snouted oddballs, sometimes with crests and stuff. T. rex belongs to a later advancement of bulky, stubby-legged, tiny-armed giants)
The visible jaw-muscle is based on crocodile/alligator anatomy. It has indeed been popularized mostly by paleo-artists of the 80s and 90s, like Greg Paul who also hugely influenced the movie. Theropods could very well have had protruding jaw-muscles similar to a crocodile, but most dinos would have well developed cheeks and skin hiding all that stuff.
A big debate lately, that has artists involved as well, is wether or not Theropod teeth would be visible, with the mouth closed. Pop-culture has popularized the crocodilian teeth-showing, while most scientific consensus is entirely obscured teeth, as with varanuses. Some dinosaurs, such as small raptors, would possibly not expose their teeth even with their jaws agape.
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