Just one pangolin remains to emulate the cantilevered bipedal walking of the dinosaurs
Many of the large animals of the Mesozoic (https://en.wikipedia.org/wiki/Mesozoic) walked with cantilevered bipedality. Why is it that the only animal locomoting this way today is one species of pangolin?
Our human bias makes it easy to assume that bipedality is, per se, a meaningful category of posture and locomotion. Actually, bipedality is so heterogeneous that using it as an overall description can be misleading (see https://en.wikipedia.org/wiki/Bipedalism and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1571302/ and https://owlcation.com/stem/Animals-that-are-Bipedal-two-legs).
When the torso is held horizontal, balanced by head and neck on one side and tail on the other, this forms a level beam pivoting on the hind limbs ((https://en.wikipedia.org/wiki/Cantilever). Balance is maintained - despite the instability of just two supporting feet - because the mass of the beam anterior to the hips equals that posterior to the hips.
Of course, this needs a sufficiently long and massive tail. However, the potential adaptive advantage is economical walking, because moving one pair of limbs costs less energy than moving two.
Keeping cantilevered balance while running should be relatively easy for the same reason that riding a bicycle hands-off is easier at some speed than slowly. It is the animals able to keep anterior/posterior balance while walking bipedally that are of particular interest.
Cantilevered bipedality is so different from upright (mainly human) bipedality that the two modes have little in common beyond the weight being placed solely on the hind feet.
In Mesozoic times, cantilevered bipedality was pervasive. It evolved independently in at least two major clades of 'dinosaurs' (https://en.wikipedia.org/wiki/Dinosaur), the saurischians and the ornithischians, that configured the hips in different ways.
It also occurred over a wide range of families and body sizes, and including flying, feathered forms (https://en.wikipedia.org/wiki/Dromaeosauridae and https://www.mcgill.ca/newsroom/channels/news/some-dinosaurs-could-fly-they-were-birds-323548). Even some sauropods (https://en.wikipedia.org/wiki/Sauropoda), although walking on all four limbs, probably held the tail horizontal to balance the long neck (https://itotd.com/articles/4723/the-argentinosaurus/).
Today, various lizards can run bipedally (https://www.youtube.com/watch?v=XAo09yYOpCU), but none walks with cantilevered bipedality.
Birds differ categorically from theropods (https://en.wikipedia.org/wiki/Theropoda) in the bony tail being so short (https://www.fossilhunters.xyz/geography-of-life/into-thin-air-the-origin-of-birds.html), with any long caudal feathers so light (https://es.123rf.com/photo_95366001_peacock-walking-around-on-the-grass.html), that a cantilever is out of the question.
In this sense birds - despite being the descendants of theropods - are posturally in a category of their own. No bird walks with cantilevered bipedality, the emu (Dromaius novaehollandiae) perhaps coming closest (https://www.youtube.com/watch?v=cJPNiXOV2XE) because it is odd in depositing fat on its tail.
Various mammals have independently evolved towards emphasis of the hind limbs, but few of them walk bipedally. Some rodents (e.g. https://en.wikipedia.org/wiki/Pedetes), lagomorphs and macropodid marsupials hop slowly instead of walking, using gaits unknown in the Mesozoic, or 'walk' by moving first the fore and then the hind feet synchronously (https://www.youtube.com/watch?v=8hEKqUG-WVc). Few bipedally-inclined mammals other than kangaroos have tails massive enough to act as cantilevers. And kangaroos - far from walking bipedally - cumbersomely use all fours plus the muscular tail as a fifth limb (https://www.youtube.com/watch?v=Mi53VlMA31I).
Ground sloths (https://en.wikipedia.org/wiki/Ground_sloth) had a bipedal tendency, but it is unlikely that they walked with cantilevered bipedality. Their tails were muscular enough to act as a prop but not long or massive enough to act as a cantilever (e.g. see https://www.gettyimages.com.au/detail/news-photo/mounted-prontoterium-skeleton-of-an-extinct-ground-sloth-news-photo/543654805). The same applies to the extant giant armadillo (Priodontes maximus, https://en.wikipedia.org/wiki/Giant_armadillo and https://www.thedodo.com/in-the-wild/rare-giant-armadillo-rescued-brazil).
The only living mammal known to use cantilevered bipedality as its most frequent mode of walking is the second largest-bodied of the eight species of pangolins: the Cape pangolin (Smutsia temminckii, see https://fascinatingafrica.com/species/ground-pangolin/ and https://sites.psu.edu/shanetheman/files/2020/03/pangolin-2019.jpg and https://www.inaturalist.org/observations/16236845 and https://pangolindiamondscorp.files.wordpress.com/2014/04/cape-pangolin-from-youtube-video.jpg and second photo in https://www.dogcatplace.com/wildanimals/list-of-animals-that-can-walk-move-freely-on-two-legs).
In all species of pangolins the tails are unusually heavy, partly because they are exceptionally armoured and partly because they are, in arboreal species, exceptionally long (and prehensile). Furthermore, the fore claws of pangolins are so prominent that they tend to obstruct walking on the fore feet. Since all pangolins have particularly economical metabolism and walk slowly, all are candidates for cantilevered bipedality.
However, the small species of pangolins walk on all fours, in some cases placing weight on the knuckles instead of the sole or claws. The smallest species of all (see https://www.inaturalist.org/journal/milewski/59232-tamanduas-have-converged-with-african-pangolins-except-in-anti-predator-defences#) is so arboreally specialised that it rarely walks on the ground in the first place. The giant pangolin (Smutsia gigantea) is probably capable of bipedal walking but seems usually to use all fours, possibly because its tail has a different proportional size to that of the Cape pangolin, and its fore limbs are particularly muscular.
This leaves - seemingly almost by accident - the Cape pangolin (https://en.wikipedia.org/wiki/Ground_pangolin) as an afterthought of a mode of locomotion which once ruled the terrestrial world.
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https://www.thethirdpole.net/en/nature/pakistan-pangolin-scales/
https://www.youtube.com/watch?v=EZFSCtmlKCY
Could it be that the highly efficient counter-current exchange system of archosaur lungs, allowed the lungs to be small enough that the upper thorax could be reduced sufficiently to allow a cantilevered bipedal system. Mammals with their large bellow lungs are just too heavy in the thorax to allow it.
@tonyrebelo Come to think of it, what are the actual advantages of bellow lungs, in explanation of why we otherwise efficient mammals have retained such an apparently crude design?
Skinner and Chimimba (2005), on page 356, state: "When they walk the body is balanced on the hind feet with the forefeet and tail held clear of the ground. They may allow the tail to scrape the ground momentarily or take the weight onto the front edge of either set of claws of the front feet in a manner unsynchronised with the hind feet...Pangolins in poor condition drag their very heavy tail, leaving an obvious and unique print...Normally they are slow movers...Under stress they can move at a brisker pace, raised high on the back legs. They try to locate danger by pausing and raising the body into a near vertical position on the back legs, balancing on the broad tail".
I dont think it is so much a matter of retaining a crude design, but of being unable to engineer an alternative.
Bird lungs have air sacs all over the body - the lungs are a filter rather than a sack.
Here is a fairly clear description: https://asknature.org/strategy/respiratory-system-facilitates-efficient-gas-exchange/.
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