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Unlike mammals, reptiles lack gluteal muscles and instead have enlarged caudofemoralis muscles to extend the hindlegs. The humanoid legs, on the other hand, rely on the mammalian gluteals in order to function

Is there an alternative muscular structure that allows for humanoid legs that are moved by the musculature of a reptilian tail?

The system can't have muscles that aren't found in either reptiles or mammals, and each muscle must have a realistic amount of strength. The legs should be as strong as human legs. The tail must appear roughly contiguous with the body from the external perspective, and hence there shouldn't be any visible buttocks.

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    $\begingroup$ The presence of a tail is believable (see human vestigial tails). However, when you ask, "is there an alternative muscular structure?" there obviously isn't one in Real Life. That means developing a fictional skeletal structure and developing ligaments and musculature. Are you asking us to do that for you, or are you stopped in your worldbuilding efforts with a specific problem? This Q might be too broad (and the title Q is misleading, it should be "How to design human musculature to allow a reptilian tail"). $\endgroup$
    – JBH
    Feb 20, 2023 at 17:09
  • $\begingroup$ It doesn't matter. You're building your world, you get to make the rules. That means if you want humanoids with lizard tails, go ahead and make them. The physics of how this works is detail irrelevant to your world. $\endgroup$
    – Ian Kemp
    Feb 21, 2023 at 10:39
  • $\begingroup$ If you're fussy about the biology, upright creatures don't have tails. But readers don't usually care. There's about a dozen questions in the form "Can my biped have a tail?" For this specific version, the lizard pelvis is categorically different than the mammal pelvis. That's probably why there's no upright lizards. $\endgroup$
    – user458
    Feb 21, 2023 at 16:05
  • $\begingroup$ Do they really need to be exactly humanoid legs? Therapod dinosaurs like Tyrannosaurus Rex and Velociraptor were bipedal and had (presumably reptilian) tails. $\endgroup$
    – David
    Feb 21, 2023 at 20:07
  • $\begingroup$ Why might they not be? $\endgroup$ Feb 21, 2023 at 21:43

2 Answers 2

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Like a Frilled Lizard Baby

enter image description here

Frilled lizards have reptilian musculature. I know because they are reptiles. And they can run upright. Check it out.

enter image description here

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This is not exactly what you want. I don't believe the lizard can walk on two legs. Only run. They stay upright when at top speed.

But it proves the concept works. Just make some qualitative adjustments to your lizardman's anatomy so they can stay upright while walking or standing still.

Edit: In the comments @AustinHemmelGarn says the frill is unnecessary to stay upright. Chinese water dragons run like this too.

enter image description here

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    $\begingroup$ +1 for the Whose Line Is It Anyway image. $\endgroup$
    – JBH
    Feb 20, 2023 at 17:37
  • $\begingroup$ Does the balance depend on air friction? Where the lizard can adjust the area of the umbrella in order to prevent falling forwards or backwards? $\endgroup$ Feb 21, 2023 at 8:11
  • $\begingroup$ @KevinKostlan I suspect the frill acts as a sail to push the upper body off the ground. The lizards people will need to be modified to have some weight shifted behind the COG to exert a comparable force. $\endgroup$
    – Daron
    Feb 21, 2023 at 12:32
  • $\begingroup$ @KevinKostlan AIUI, balance relies more on the lizard continuing to move (that is, it’s essentially constantly falling and catching itself mid-fall). Some other members of the family Agamidae also exhibit bipedalism but lack the distinctive frill (such as Physignathus cocincinus, commonly known as the Chinese water dragon), and there are other genera outside of that family that do as well (possibly most famously the members of the genus Basiliscus, commonly known as basilisks). $\endgroup$ Feb 21, 2023 at 12:55
  • $\begingroup$ @AustinHemmelgarn The lizardman will need to have their posture modified so they can stay upright without moving forward. $\endgroup$
    – Daron
    Feb 21, 2023 at 13:37
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There is nothing that exists, but there is no reason it can't exist.

Upright bipedalism is unbelievably rare, even more so with downward facing femurs, so no actual examples exist.

the closest match would be therizinosaurus. Which puts right angle turn in the hips to have an upright spine and tail. But dinosaurs do not have ball and socket hip joints to their range of motion is limited. but there is no reason you could not add more musculature and have ball and socket hip and a angled pelvis with tail. Developing new muscles is very easy and happens all the time in evolution, so that should not be an issue you should worry about.

as a side note mammals have a caudofemoralis and archosaurs have glutes, they are called ichiofemoral muscles, they are just small.

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Keep in mind having a tail means balance requires the body to be slung forward. so you will never have completely upright and a large muscular tail. there are several questions about bipedalism and tails, I suggest you look at them for details on how to make it work.

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    $\begingroup$ Please no pictures. You are making me hungry. $\endgroup$
    – Daron
    Feb 21, 2023 at 16:32

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