6
$\begingroup$

Could an animal realistically move around if it lacked knees in some legs?

For specifics, the animal for my world would be a tetrapod-analogue, having 2 large legs (with knees) at the back, and a smaller pair of knee-less legs near the front. The forelegs have a foot at the distal end, with a ball-and-socket joint at the ankle and shoulder. The creature has a complete and rigid endoskeleton, which extends throughout the torso, legs, and feet

$\endgroup$
7
  • $\begingroup$ A better question might be why they would evolve such a form. $\endgroup$
    – John
    Dec 27, 2021 at 1:22
  • $\begingroup$ @John Only a fraction of mutations aren't harmful to an organism and even less are useful. Organisms don't get to pick their mutations but make do with what they got. $\endgroup$ Dec 27, 2021 at 8:54
  • $\begingroup$ No Earthen animal has knees on the front legs. All have elbows, which are the opposite of a knee. $\endgroup$
    – Agent_L
    Dec 27, 2021 at 10:56
  • $\begingroup$ @LiveInAmbeR joints have evolved in every terrestrial animal that has hard parts in its limbs, to not having jointed limbs needs an explanation, its like having a terrestrial animal that can't breath air. how such a creature could even evolve for terrestrial life is in question. $\endgroup$
    – John
    Dec 28, 2021 at 1:00
  • $\begingroup$ do they have an actual shoulder or a fixed ball and socket like a hip joint, shoulders have two joints, humerus to scapula, and scapula to ribcage, both are mobile. $\endgroup$
    – John
    Dec 28, 2021 at 1:14

3 Answers 3

2
$\begingroup$

Sure

You've pretty much reinvented the kangaroo. Your tetrapod has big hind limbs with hip, knee, ankle and toe joint; and smaller forelimbs with some kind of shoulder, wrist and finger joints.

Mostly they hop or leap along without recourse to their forelimbs. Only when moving slowly do they rest their forelimbs on the ground. When they do this, they simply use the forearms as pillars that pivot at the shoulder. Forward motion is accomplished by a short bump of the hind legs, sending the rump airborne, and then quickly curling the spine & hind legs forward to place them on the ground for the next short hop. To hop again, the beastie slightly lifts is front end, swings its forelegs a little bit forward and then rests them again on the ground, ready for their characteristic little rump bump.

enter image description here

$\endgroup$
4
$\begingroup$

The idea perished long ago

Sometimes nature decides to mess around with funny designs. Some 500-600 million years ago during the Cambrian Explosion, we have had Hallucigenia, 8cm, shallow waters

enter image description here

https://www.google.com/search?q=Hallucigenia+species&source=lnms&tbm=isch&sa=X

Failed design, I'm afraid.. it looks funny, but these rigid legs.. not handy. e.g. it won't be easy to free yourself, when these legs gets stuck for some reason. And also notice Halucigenia needed flexible legs (or tentacles) up front, so it could bend over the head to graze for food. With rigid forelegs, it is far more difficult to reach for food.

$\endgroup$
7
  • 2
    $\begingroup$ Hallucigenia's legs are not rigid, they are flexible but lack individual joints, like Mickie Mouse arms, tube feet, or velvet worm legs. $\endgroup$
    – John
    Dec 25, 2021 at 3:49
  • 1
    $\begingroup$ It's a tube like structure, with two toes on it. The animations I've seen these limbs are quite rigid, the creature "dancing" over the sea floor with them. Guess it's quite difficult to reconstruct physiology.. you're looking at a separate phylum that lived 550 million years ago ! $\endgroup$
    – Goodies
    Dec 26, 2021 at 2:13
  • $\begingroup$ if you want to see how they work look at velvet worms, their legs also have no joint but the entire length of the limb can flex, they work through a combination of hydrostatic pressure and muscle contraction. $\endgroup$
    – John
    Dec 26, 2021 at 13:41
  • $\begingroup$ Ok @john there's one on YT showing a slight bend youtube.com/watch?v=9-Z9Ssgb0Kg In a book I have Stephen Gould / The Burgess Shale there's an explanation how Cambrian phylae like Hallucigenia got extinct. Hallucigenia is an example of a dead end, according to Gould, because it had impaired legs: no knees and no means to bend its legs. No idea if newer insights differ about it, but it had no knees.. for sure ! ( :d ) $\endgroup$
    – Goodies
    Dec 26, 2021 at 19:33
  • $\begingroup$ Gould did not have any evidence to back his claim, and as was often hisproblem tried oto talk about something he did not understand. Hallucigenia is likely a velvet worm ancestor, it is not a stretch to say their legs worked in a similar fashion. But more importantly many Hallucigenia and related fossils have bent legs. you would have to ignore quite a lot of evidence to believe their legs were rigid. no knees is not the same thing as rigid, again living velvet worms have no knees but mobile legs. bbc.com/news/science-environment-33262884 $\endgroup$
    – John
    Dec 26, 2021 at 19:47
2
$\begingroup$

They could move, but not very well.

Joints provide improved flexibility in movement. You can test this by tying up your arms and walking on your knees. You can move, but it's a lot harder to navigate around obstacles, or deal with anything complicated.

Creatures with exoskeletons have knees.

enter image description here

It's useful to be able to twist your limbs, so your modification would be weird. It would be possible, but not very fluid.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .