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These flightless birds are under strong selective pressure to be able to feed on high up conifer needles, and avoid predation from T-Rex sized predatory flightless birds. The forests the birds are evolving in are generally more open than those of our own Earth (due to being dominated by megafauna), but not as open as ones dominated by sauropods.

Importantly while these birds have many similar adaptions to sauropods, I've heard height is a much more significant limitation than total size, and that sauropods held their heads much lower most of the time than is often depicted. So with only tail feathers and not an actual tail it seems like these birds will be limited in how much their neck can extend horizontally compared to a sauropod.

Your answer should include some calculations and/or citations about the relevant physiological constraints on tail-less avian biped height. Particularly when it comes to getting enough oxygen to their proportionally tiny brains.

This is relevant as it seems like how high an animal can pump blood above its heart is actually fairly controversial: https://en.m.wikipedia.org/wiki/Sauropod_neck_posture

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    $\begingroup$ No calculations or anything but the only thing that my brain would allow in my head as I read this is [Therizinosaurus][1] :) [1]: en.wikipedia.org/wiki/Therizinosaurus $\endgroup$
    – Gillgamesh
    Jun 3, 2022 at 18:37
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    $\begingroup$ Three meter (10 ft) tall birds have lived in the recent past. And giraffes reach 5.7 meters (19 ft) tall, and they normally keep their heads up, so that is obviously possible. $\endgroup$
    – AlexP
    Jun 3, 2022 at 18:48
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    $\begingroup$ If you need a fleshy tail to balance, just re-evolve a tail; possibly a detachable one to sacrifice when bitten by a tyrranoraptor. Even feathers are alive, can mutate, and regrow if lost. $\endgroup$
    – DWKraus
    Jun 3, 2022 at 19:17
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    $\begingroup$ It sort of is. Blood is mostly water, and water is heavy. To pump blood five meters up, the heart must develop 0.5 atmospheres of pressure. I wouldn't push that mush further -- the heart is a biological pump, after all, and half an atmosphere of pressure pushes the walls of the heart chambers with a lot of force. $\endgroup$
    – AlexP
    Jun 3, 2022 at 19:18
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    $\begingroup$ @AlexP I mean many animals have been taller than giraffes, so the limit is probably above that. Though there's enough differences that I can't just use other theropod dinosaurs as reference. $\endgroup$ Jun 3, 2022 at 19:29

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10 metres

enter image description here

Last I saw Walking with Dinosaurs on the BBC the diplodocus were grazers and held their necks horizontal as you say.

enter image description here

But the Brachiosaurus was a browser and held its neck vertical.

enter image description here

Likely the diplodocus could not raise its neck this high and the brachiosaurus could not lower its neck.

The upright posture makes the brachiosaurus a 10 metre tall bird. Have the dinosaur people changed their minds in the last 20 years?

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    $\begingroup$ While it is true that all birds are dinosaurs, it is also true that not all dinosaurs are birds. In particular, birds are maniraptoran theropods, whereas Diplodocus and Brachiosaurus were sauropods. The sauropod and the theropod saurischian dinosaurs diverged in the late Triassic, more than 110 million years before the emergence of the birds (here considered the same thing as avialan eumaniraptorans). To put this in perspective, the big dinosaurs went extinct only 66 million years ago... $\endgroup$
    – AlexP
    Jun 4, 2022 at 13:26
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    $\begingroup$ Everything about Brachiosaurus anatomy says it could absolutely lower its neck, and there is no reason to believe it could not. $\endgroup$
    – John
    Jun 14, 2022 at 22:19
  • $\begingroup$ @John Really? Do you know why it is portrayed so differently from the diplodocus in Walking with Dinosaurs? Is it the teeth? $\endgroup$
    – Daron
    Jun 15, 2022 at 9:42
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    $\begingroup$ @Daron because they are two very different sauropods with very different anatomy, diplodocids rear or feed so raising the neck involves raising the hole body, brachiosaurs keep all 4 feet on the ground and raise the head. walking with dinosaurs was not that scientifically accurate. $\endgroup$
    – John
    Jun 15, 2022 at 19:42
  • $\begingroup$ @AlexP it worth mentioning that all the things we think allowed sauropods to get so large are shared with birds, that is hollow bones and a one way air sac respiratory system. $\endgroup$
    – John
    Jul 17, 2022 at 1:45

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