Theropod dinosaur evolution was centred in the head and the hind limbs; these parts of the body became larger and larger, while the rest progressively atrophied (the torso remaining constant). Now, you could say that the reason for this was that long strong legs were needed so the beast could catch its prey, and a huge head with powerful jaws so it could kill it. And on the physical level this is no doubt correct. That is the "how" of evolution. But ”why did meat-eating dinosaurs evolve into two-legged hunters and not, as with mammalian carnivores, four-legged ones?
The answer is that - zoosophically speaking - the Archosaur-bird evolution is centred around the formative forces of the head, whereas the Mammalian evolution is centred around the torso and limbs.
The Anthroposophical writer John Davy puts it this way:
"Dr Steiner described birds as animals which are "all head". The legs are insignificant twigs, while the digestive system - compared with that of a cow - is little more than an afterthought....The bird is a metamorphosis of an animal form whose great achievement was the development of limbs (the reptile) into an animal which is "all head"..."
[Hope, Evolution, and Change, (1985, Hawthorn Press) p.106]
Ideally, the head is in the form of a sphere. And the sphere - the egg - is the beginning of life. Hence baby animals have more rounded heads and smaller snouts than adults; a baby chimpanzee head looks uncannily like a human's in profile. In this respect at least, man, with his globular head, is much closer to the archetypal ideal than other animals.
So the evolution of reptiles to dinosaurs to birds is one of progression from the low, crawling, weak-limbed lizard-like reptile, through the strong limbed but still long-tailed thecodonts and theropods, to the large-headed and short-tailed bird.
The huge head, stock body, small forelimbs and the relatively short tail of the Tyrannosaurus indicates an approach to the more spherical condition. The long tail is a lizard-like feature; the elongate, worm-like or snake-like reptilian form (significantly, in Medieval Britain, dragons were sometimes referred to as "Worms"), whereas the bird- like form is short-tailed and short-bodied, approaching a more spherical condition (celestial bodies being spherical - e.g. sun and moon), as befits an aerial creature.
A series of skeletons from primitive reptile to thecodont to dinosaur to bird. Note the increasingly long and strong fore- and hind-limbs (the hind-limbs developing first), the decreasing length and heaviness of the tail (in the modern bird, the tail is a mere stump), and the shortening of the torso. Indeed, the bird is a creature that has totally given itself up to the head forces, that in a sense has become a head. The food is chewed in the stomach (gizzard stones), not the mouth. The body is filled with air sacs connected to the lungs, and there is also a direct connection between the lungs and the hollow bones, so a bird could theoretically breathe through the end of its arms, if a hole was made into the bone. The wing thus becomes a nostril.
And not only is the whole body short and rather spherical in shape, like a head, but, like the head poised on top of the body of an erect animal, the flying bird relates to the world from a vantage point well above the ground.