The ground shakes as a colony of Triopticpelorovenatorius Baromassobrachiosaurus (T.B.) shamble across the forest floor. A glorious mix of blue, purple, and green colors shift under the treetops as our camera crew provides us with a perfect birds eye-view of the sparse alien canopy.

Isometric View

In this episode of Strolling with Nightmares, we examine the realism behind the musculature and skeletal structure of a bipedal lower body plan and monobrachial (single arm) upper body plan.

Generic Movement

On the two sides of T.B. there are planar joints which allow for the raising, and limited rotation, of both "shoulders" (as depicted in figure 1.0). In addition to the movement provided by the planar joint, there is a ball and socket joint which further increases the flexibility and mobility of the legs.

The leg itself is then further split into a front and back section; thus allowing for the stability of a wide stance quadruped whilst having a bone structure similar to that of a biped. The front section is controlled by a hinge joint, which allows for the front piece to move left and right separately compared to the larger, heavier back "heel" of the leg. Think of it as a single toe.

As the creature moves, the frontal arm provides support for the center of mass as one side of the creature raises, shifts forwards, and lowers; a slow shuffle, with each step requiring a different positioning of the arm for support.

Arm Movement

Due to the relatively low speed of the T.B., the species is unable to chase its prey - instead, it hunts stealthily at range, as if it were a hunter with a rifle.

This creature uses its wide stance in order to stay stable whilst engaging in long range hunting; its incredible accuracy and strength provided to it by the single arm it has grown from the center of its body. The arm folds up, and launches sharp, spear-like projectiles in straight lines, as if it were a spring pushing outwards. (See Figures 1.1 & 1.2 for a depiction of the arm's movement ranges)

Alternative weaponry choices include bladed weapons, as the single arm has enough dexterity to wield the weapon to great effect; at least from the front. It is also able to lob rocks, although the accuracy there is significantly worse than if it were chucking spears.

Bone & Joint Structure of T.B. Figure 1.0 - Bone & Joint Structure of Specimen

Arm Movement Range Figure 1.1 - Arm Movement Range (Side View)

Movement Ranges (Extended) Figure 1.2 - Movement Ranges (Extended)

Is the depicted musculature & skeletal structure realistic? If not, how should it be modified to better address the traits and movement written above?

Tip: If the pictures are too small, open them in a new window. They're actually huge, I promise.

For those interested in different views of the model, here are some additional photo links.

  • 1
    $\begingroup$ A fancy word for "with one single arm" is "monobrachic" (from pseudo-Greek μονοβραχίων / monobrachiôn, single arm); it's so rare that Google find only two results; "monobrachial" is much more popular, with 3900 Google hits. By contrast, "monopodial" ("with one leg") has 180000 Google hits and the variant "monopodal" 51000 Google hits; "monopodic" gets 2900 Google hits. Better Greek would be μονάγκων / monankôn, but that would be awkward in English. $\endgroup$
    – AlexP
    Commented Aug 25, 2017 at 19:40
  • $\begingroup$ @AlexP Hey that's pretty close to what I put in, "monobratso"! I'll change it to monobrachic now. Thanks! $\endgroup$
    – Aify
    Commented Aug 25, 2017 at 19:41
  • $\begingroup$ Another question: what is the function of these large predators living in a colony? It seems like it'd be more likely they'd be lone hunters since they require so much meat to survive. $\endgroup$ Commented Sep 22, 2017 at 13:28
  • $\begingroup$ @A.G.Weyland Safety, perhaps. There are larger creatures on that world than these. Like humans, they grouped together to form societies and large hunting parties. $\endgroup$
    – Aify
    Commented Sep 22, 2017 at 18:26
  • $\begingroup$ @Aify Larger predators makes perfect sense. Humans are rather small, fragile predators which is why we need numbers on our side. If your creatures are small in comparison to other predators, this makes sense. $\endgroup$ Commented Sep 25, 2017 at 12:40

6 Answers 6


I'm sorry to say that this shape seems very unrealistic to function as you describe. I can think of only one halfway plausible scenario.

Think about this from an evolutionary perspective, what and how has this creature evolved? Where is it well adapted to?

It's very wide, so it will be useless in trees or any other overgrown area.

It's very slow which vastly restricts its hunting options.

You've identified that and tried to solve it by saying that it's a ranged hunter, but how would that have come to be? What's the intermediate stage?

The only way I can think of this working is if it was an amphibious ambush predator. Basically think of it as being like a crocodile but it lurks in the river until something gets close by sinking under the water with the legs coiled under it. It then surges out with one long leap and grabs the target.

The wide stance and powerful body is used to launch out of the water and then drag the prey back in and drown it. An extension from this to then using ranged hunting tactics to surge out of the water and then throw a weapon becomes more plausible.

In other words it's a cross between a frog and a crocodile :)

  • 2
    $\begingroup$ He could name it a "frogodile". $\endgroup$
    – user9981
    Commented Aug 25, 2017 at 11:54
  • 1
    $\begingroup$ I like the idea of it being an ambush predator. Why does the "wideness" of the creature affect its ability to be useful in a sparse forest? I'm thinking a forest with large areas between trees, like this one, but with more space still, and alien trees. But you're definitely right with regard to not having a intermediate stage for ranged hunting, I had forgotten to take that into account. My final question with your answer (1/2) $\endgroup$
    – Aify
    Commented Aug 25, 2017 at 19:43
  • $\begingroup$ ...has to do with how the creature would leap - I like your idea for using the body to leap, but as the creature lacks "thigh muscles" (this is by design) for jump purposes, I don't understand how it would jump. Actually, there's a lack of almost any muscle that allows it to jump now that I look at it again. You mentioned coiling the legs under, but I don't understand how that would work - could you perhaps provide a diagram? (2/2) $\endgroup$
    – Aify
    Commented Aug 25, 2017 at 19:46
  • $\begingroup$ The legs would act like springs. Muscles would wind them down then it would launch with the stored energy. There are a number of animals and insects that do similar on earth. $\endgroup$
    – Tim B
    Commented Aug 25, 2017 at 20:23
  • $\begingroup$ All trees slow it a lot as it has to go around them. The more the worse of course $\endgroup$
    – Tim B
    Commented Aug 25, 2017 at 20:24

Looking at the diagrams, the joints and musculature seem very unwieldy, and unlikely to arise from natural evolution.

A possible work around is to recast the creature as being "radially symmetrical", in other words, when you look at it from the top, the shape is a tripod, with the legs splayed at rest like a triangle. Since you have it evolving to live in a forest, the shape will be rather tall and narrow, similar to a giraffe if it lives by eating the leaves off trees (you speak of it as being large, so small tripodal animals evolved to live and forage on the forest floor should also exist).

Since the head isn't going to be able to successfully rotate 360 degrees, the "front" arm is going to become more specialized to be able to manipulate objects, bring food to the mouth etc. One can picture the creature either balancing on two legs while the "arm" brings food to the mouth, or it squats or kneels in order to engage the arm.

This would imply that the "hip" structure is quite flexible (perhaps a girdle of bones and cartilage rather than a unitary pelvis), and the elbow and "wrist" joints will also be very flexible. Evolving an elongated "ankle" like many running or jumping creatures have done on Earth would depend on things like the need for energy recovery (creatures like Rabbits often use the ligaments like giant rubber bands to store energy and release it on the step off portion of the movement). In any event, the articulation will be quite different from Earthly creatures.

If the creature is a apex predator (like T-Rex), many of the same considerations apply. It lurks in the forest waiting for the herbivores to come close, then springs into action. The front "arm" might be equipped with a raking or stabbing claw or set of claws, and perhaps the head might be outsized not only to mount a large set of teeth but also to use as a weapon (swinging the head and neck to knock down the prey, for example). If it exists on the plains, then the legs (for both predators and prey) are modified to allow for long, ground covering strides and bursts of speed to run down prey or evade predators).

Perhaps the easiest way to consider how to "design" creatures is to carefully examine the environment they are supposed to live in, and see how they would need to adapt to maximize their survivability in such an environment.

  • $\begingroup$ Ironically, I had purposely strayed away from radial symmetry (and also tried to find a design that strayed from all symmetry) in order to make the figure of the xeno distinctly alien.... Does the shape still have to be long and narrow if it's not in dense forest? The opening segment of my question states a sparse canopy - eg: sparse forest. $\endgroup$
    – Aify
    Commented Aug 25, 2017 at 19:38
  • $\begingroup$ I think you meant apex predator, not climax predator. That said, I do think that the final paragraph is really the most important one in this answer, as well as in general if you want believable creatures. $\endgroup$
    – user
    Commented Aug 25, 2017 at 19:50
  • $\begingroup$ In a sparse forest the creature could be wider, but now you have to consider what it is supposed to eat. What ecological niche does it fill? $\endgroup$
    – Thucydides
    Commented Aug 26, 2017 at 1:05
  • 2
    $\begingroup$ I understand wanting it to look distinctly alien, but if it lives on an Earth-like planet with Earth-like forests, it will resemble the evolutionary model of creatures who evolved on Earth. There have been some odd creatures throughout the history of life on Earth, but it still follows logical leaps forward. I would comb through the fossil record and find an animal you can amend a little to make it "alien." $\endgroup$ Commented Sep 20, 2017 at 19:51

In figure 1.1, that throwing motion would probably not evolve. The creature would probably have a stronger throw if it started from the down position and rotated up, releasing when it is at the correct angle. If the arm uncurls as it is rising, it can get more force out of it since it will be easier to accelerate the arm in the curled position.

Also, from 1.0, I would assume that the creature is either relatively small or is from a low G world since those leg joints don't support the weight very well. As the creature gets larger, its weight will go up faster than the strength of the joints will.


So, the creature is effectively a living catapult. A slow moving distance hunter makes no sense under most circumstances. Wouldn't its kills always be stolen by scavengers before it even got to the kill?

The two legs and one arm configuration could be because of an unfavorable ancestry. Its stuck with modified versions of what its ancestors had.

A catapult for an arm might work well as a defensive weapon. This is especially true if there are a group of them using their catapults together. Groups of humans throwing rocks can fend off most large predators. Of course, they should be in more open territory than a forest for this to be effective.

Alternatively, trebuchets (similar to catapults) are a good way of smashing hard walls. Maybe the prey is small but hide in termite like mounds, which need to be smashed opened. It is a good strategy to find food. Gorillas and even humans eat termites.

http://www.wikihow.com/Build-a-Trebuchet Gorillas breaking termite mounds. https://www.youtube.com/watch?v=l-sd4FsfZQU

Catapult like limbs have significant advantages over other limbs in that they use less muscle to exert a high force over a short amount of time. For instance, fleas and locusts have a version of a catapult in their legs.

Biological catapults work by having a locking mechanism. The flea leg is locked in a flexed position. For your catapult arm, you would need to study how levers work in catapults. After locking the limb, the muscles would contract slowly, storing energy in a block of protein similar to the Resilin in insects. Then, the leg (or arm) is unlocked. Elastic recoil makes the limb move very quickly back into the relaxed position. http://jeb.biologists.org/content/212/18/2881

A wide stance where each leg acts as two legs would probably be because of stability. All four "feet" should be lightweight. Legs are Pendulums. The feet need to accelerate with every stride. Extra mass in the leg means that more energy is wasted moving the feet relative to the body. The mass that costs the most is the mass at the end of the limb. I say this because your alien is packing giant bone extensions on the back half of its legs that are without obvious function.

So, looking at the arrangement of the joints, the main body of the alien is like the upper part of a human torso if you cut off the human head and stuck a small alien arm in its place. The alien's head is sitting between the shoulder blades.

See the "top view." http://www.pasttime.org/wp-content/uploads/2013/07/Shoulder-evolution.jpg

What you should really have is less bone and give it a clavicle in shape and location. The poor thing doesn't have any space for organs!

Overall, if you give the creature a catapult in its arm and if it moves slowly, its musculature should be small and maximize efficiency over speed and strength. This is the strategy used by tortoises.


Some thoughts ...

First, I'm a little confused by the gigantic bone spurs on the back half of the rear legs. Not really sure how those help. If you got rid of them, you could have the creature be essentially a "reverse kangaroo", with the arm taking the place of the tail.

Generic movement would be either short hops (powered entirely by back legs) or a "knuckle-dragger" kind of walk, looking a lot like a guy on crutches. At rest, it will tend to lean forward a lot and rest on all 3 legs. Assuming that the big spurs are gone.

Hunting would be akin to male lion (when they bother to) hunting... Trioptic is big enough that it's neither faster than the prey nor has more endurance. What it has is terrific acceleration from jumps. If it can get with in 2-3 jumps of the prey it can catch it before Lunch manages to get up to a speed Trioptic can't match. It's big enough that this stealth part may be hard, so it'll have to hunt in (at least) pairs. The beater goes upwind and chases the prey toward the hidden catcher, who is downwind and breaking its outline with trees.

The second thing I'm worried about is the projectiles. It seems like a bit of a stretch to evolve those, as it's expensive to throw away body mass. Speaking of stretch, one approach might be to give the creature an extra joint (hence extra length) to it's front arm, which it usually keeps tucked up, but can extend out to a pretty good reach for an attack. Think chameleon or mantis or squid. It's strategy might be, using above info:

  • Wait semi-concealed until beater chases prey toward catcher Trioptic.

  • Trioptic then uses explosive leaps to get close to prey

  • Uses long-reach arm to trip the Lunch. See cheetahs hunting for examples.

  • Once Lunch is down, use big claws to cripple or slay it

Even with all this, it'll be hard to hunt enough provender to satisfy this beast. So some other thoughts:

  • It'll help if there are other, smaller predators around. Prey "wants" to grow big enough to disqualify them as predators, which (ha ha!) makes them big and ungainly enough for Trioptic to target.

  • Another great Trioptic tactic would be to sniff out these smaller predators' kills, and bully them away; Trioptic will steal their lunch.

  • You could have Trioptic be omnivorous, kind of like bears. If it's too frustrated hunting, it can sulk away and go eat some grass and berries.

TL;DR -- I think you can make this guy work!


Unfortunately this creature cannot be changed to be realistic without massive changes. The design does not offer any evolutionary benefit, and it has quite a few disadvantages:

Wide body doesn't help it move faster or more efficiently, doesn't offer much to stability, and prevents the creature from fitting in between natural obstacles.

The legs are actually the largest impediment to its movement. With lighter legs it would weigh less, making massive bones no longer necessary. In this creature the legs are bigger in order to support bigger legs. This simply isn't helpful, and is counterproductive too its movement.

It almost certainly cannot swim.

It certainly cannot climb, or possibly even go up a steep incline.

Its arm is in a location that offers poor leverage for throwing things.

So what should be changed? For starters I assume the heavier build is linked to a defensive need? If this were the case the width should be decreased and almost certainly the height. Animals with more armored builds always tend to be tighter and more compact to maximize protection.

The arm should also probably be moved to the back like a scorpion tail and be long enough to reach all the way around in front of it. The extra length would help it work as a catapult.

And the legs, they basically are simply not evolutionarily justifiable.


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