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I am designing a creature that is quadrupedal but can run after prey on its hind legs. What kind of hip bone would allow for the excess stress to be put on it during bipedal locomotion? The animal is roughly the size of a mountain lion and weighs on average 100 pounds. It is a predatory carnivore. The planet it inhabits in very similar to Earth. It has an identical atmosphere that is 23% oxygen.

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    $\begingroup$ Have you already looked at the structure of real life animals that exhibit a similar mode of locomotion? An example would be hadrosaurs, which usually moved on four legs but ran bipedally. $\endgroup$ – Inquisitive Geek May 3 at 15:32
  • $\begingroup$ I have tried that but I couldn't find a good diagram of the hip. $\endgroup$ – ShimmeringCosmos May 3 at 15:33
  • $\begingroup$ Your oxygen content concern looks like a good separate question. $\endgroup$ – Zeiss Ikon May 3 at 16:36
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    $\begingroup$ Two rules for oxygen. Be somewhere between 0.16 and 0.5 atm for human breathability. But you can never have more than 35 % of the total atmosphere as oxygen, because above that concentration fires will never stop burning. This results in runaway wildfires bringing the oxygen content down to 35% again. Of cause the latter rule only applies if you desire to have a biosphere. $\endgroup$ – TheDyingOfLight May 4 at 14:21
  • $\begingroup$ @TheDyingOfLight Thanks soo much, I'll update the question with this data! $\endgroup$ – ShimmeringCosmos May 4 at 15:10
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For vertical facultative-bipeds (Adapted primarily for walking on four legs, but able to walk on two legs) the only real consideration they have is where the center of gravity.

Bipeds need the center of gravity directly over the feet, otherwise they fall over, that is why in facultative-bipeds like kangaroo and some hadrosaurs have small front limbs and large tails it moves the center of gravity as close to the hips as possible. Basically anything the animal can do to move the center of gravity this way helps.

What exactly will help depends on what their anatomy and pelvic anatomy in particular is like overall. note that although the front limbs are skinny they are still close to the same length as the back limbs otherwise they cannot walk on all fours. If you look at hadrosaurs it looks like their front limbs are very short but they are the only dinosaur with free floating shoulders so the shoulder blades add to the limb length.

the limbs proportions also look different because the hind limb has to be able to support the entire weight of the animal while the front limbs only need to be able to support a a fraction of the weight. that is why dinosaurs also have larger pelvises compared to mammals, they need to be stronger and attack more muscle.

enter image description here

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The dinosaur Iguanodon comes to mind, and looking up iguanodon hip gave me these images

Specific to the hip

and

Full skeleton

I hope this helps.

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  • $\begingroup$ Deinonychus was a raptor genus, pretty quick, and the larger species were right in the size range the querent is after. There is also some evidence they may have been the smartest of all the dinosaurs. $\endgroup$ – Zeiss Ikon May 3 at 16:38
  • $\begingroup$ Yes, but is there a belief that Deinonychus went on all fours @ZeissIkon? That might be good for, say a skull, but maybe not too god for the hip $\endgroup$ – Peregrine Lennert May 3 at 16:58
  • $\begingroup$ Ah, missed the "usually quadripedal" qualification. $\endgroup$ – Zeiss Ikon May 3 at 17:14
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So, what you're asking for is called a facultative biped. There are many, many types of animals which are facultatively bipedal, even more than there are obligately bipedal animals. Some examples include pangolins, agamids, iguanas, sifakas, ornithopod dinosaurs, a lot of basal archosaurs like rauisuchians, prosauropods, ground sloths, chalicotheres, monitor lizards, beavers, raccoons, skunks, gibbons, indris, Eudibamus, baby sauropods, stegosaurs, tree kangaroos, and even cockroaches.

What's important here isn't the structure of the hip, but rather the distribution of mass. The main reason why the average quadruped can't walk on two legs is that too much of its mass needs its forelegs for support. If you have an animal where most of the weight is directly above the hips and hindlimbs, which never leave the ground, then you're good. Also, the hind legs should be longer than the forelegs.

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There are lizard species that do this. For example Chlamydosaurus kingii, the Northern Australian Frilled Dragon that runs two-legged on land

Video: https://youtu.be/XAo09yYOpCU?t=29

enter image description here


and Basiliscus basiliscus, the Jesus Christ Lizard that can run on water!

Video: https://youtu.be/45yabrnryXk?t=40

enter image description here

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