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Comparative Leg Anatomy
Different animals have vastly different leg arrangements. Animals lower on the tree of
life have legs with an overall form which is analogous to higher animals, but the final
shapes, leg attachment schemes, and means of locomotion are very different.
<| Rotated Leg Attachments
Near the bottom end of the vertebrate evolutionary tree is the salamander, whose legs
project nearly straight out from the torso, as shown on the left.
Salamanders are amphibians, which are born in the water and go through a larval stage with
gills and no legs. On transforming into adults, they lose the gills, develop legs, and
live on the land.
When the salamander walks, its vertebral column bends back and forth, and leg movement
is closely linked to this bending. This spinal action is very similar to that of a fish
swimming in water, which suggests a strong tie to the salamander's early swimming instincts
while in larval form.
Higher vertebrates, in contrast, show little "sideways" bending of the spine during walking.
The legs act much more independently than in the salamander, as a result of a wholesale
re-orientation of the leg attachments to the torso, as shown on the right.
Some of the major adaptations are as follows:
the legs are rotated towards each other at shoulder and hip, and relocated closely under
the body, which allows them to more easily hold the body in an upright posture.
the knees/elbows and ankle/wrists are bent, so as to form more of a vertical support
column; limb movements are now more "accordion-like" than "lever-like", which gives greater
power, balance, and spring to the step.
the lower part of the front legs are twisted so that the foot pads [palms] face towards
The last point is quite interesting.
The lower leg twist is necessary because the front leg overall rotates so as to point
"backwards", in which case an "untwisted" leg would have the foot pads pointing to the sky
- clearly not very convenient.
So why did the front legs not rotate to point forwards, like the back legs?
As described in more detail elsewhere, the overall scheme is that the re-aligned
legs front-back operate in a mirror-image fashion - allowing large leg extensions and
retractions during walking and running, while at the same time keeping the COG of the body
relatively unchanged within the torso.
This is obviously more advantageous than an arrangement
employing non-mirror-image leg movements, which would force the animal to compensate for
large COG shifts during each and every movement.
See the drawings of the horse and cheetah below.
<| Walk Like a Dino
Mammals did not actually invent the rotated-beneath-the-body leg arrangement.
In fact, going back in far time, many species of dinosaur also had their legs located
so. How else to support up to 80 tonnes without having to hold the equivalent of deep
knee bends all day?
For more on dino walking:
The drawing at the left shows three postures employed by both ancient and modern reptiles.
Upper is the sprawling stance, used by early reptiles and today's lizards, turtles,
Middle is termed the self-improved stance, used by early and modern crocodilians.
Bottom is the fully-improved ["upright"] stance, used by many dinosaurs and later
by mammals. The last also led to bipedal dinosaurs, like T-rex.
For more about stances:
For incredible pictures of crocodiles galloping:
250 million years ago, and before the dinosaurs, there was a group of ancient
mammal-like reptiles called therapsids [cf, synapsids], some the size of large dogs.
They were present in great numbers, and their fossilized skeletons are almost indistinquishable
from modern mammals - among other features, they also had a "rotated" leg posture and
More about synapsids:
All in all, these subtle changes are what make locomotion in vertebrates so successful.
|Lycaenops - an ancient mammal-like reptile|
origin of mammals,
and Peter Dilworth's incredible
robotic Troodon 
<| Cantilevered Walkers
Moving further back in evolutionary time from the salamander, we find the
phylum of Arthropods - jointed-legged, segmented animals. These include insects,
bugs, beetles, crabs, spiders, mites, centipedes, etc.
Beetles, as shown on the left, and a huge number of other insects like ants, flies,
mosquitos, bees, roaches, and various other bugs, have 6 legs. Arachnids [spiders]
have 8 legs. Some, like millipedes and centipedes, have many.
A typical arthropod leg form is shown on the right. It has several segments, which are
analogous in form to vetertebrates, including femur, tibia, and multiple [4-5] tarsal
segments. The coxa provides an extra degree-of-freedom to the leg, similar to that
provided by the shoulder in mammals. Some legs have additional segments.
Of special interest is that, like in the salamander, the legs project out from the body,
as opposed to being located underneath the body as in mammals.
Because of this, the legs are oriented and move in much different ways than in mammals,
as shown by the beetle on the left. The upper leg segments generally point upwards and
the lower segments downwards. The tarsals scrape the ground.
Looking over many pictures of 6-legged arthropods shows that the 4 rear legs typically
point backwards while the 2 front legs point forwards.
An obvious question is, why are there no beetles [or other arthropods] with only 4 legs?
Pure conjecture --> 4 legs are in general too weak and too few to hold up and move the body
efficiently, when they extend outwards away from the torso, as opposed to being oriented
directly beneath the torso. All the 4-legged beetles were squashed by dinosaurs 65,000,000
years ago, because they couldn't get out of the road fast enough.
So much for brilliant theories ... it may simply be that arthropods have 6 [or more] legs
so they can use the middle and rear to stand and move on while the front are used for probing,
grasping, fighting, and eating. In fact, some arthropods are uniquely adapted to this - eg,
the praying mantis has 2 enormous, highly-specialized front legs plus 4 regular-looking ones
under the thorax. Better to have 4 feet for running plus 2 left over for simultaneous fighting,
than to have only 4 total and to have to stop running in order to use 2 for defense.
It was probably not the dinosaurs who eliminated [possible] early classes of 4-legged
beetles, but rather their better-designed 6-legged enemies.
A common 6-legged gait is tripod-to-tripod - front-back legs on one side touching the
ground together with the middle leg on the other side to form one tripod, alternating
with the opposite arrangement. Conceptually, this type of gait is both statically
and dynamically stable. An arthropod resting on one tripod will not fall over.
Compare this to a quadruped standing on one diagonal, or a biped standing on a single
leg - both of these are dynamically, but not statically, stable.
Click here for
tripod gait in action.
And interestingly - someone has come up with "whegged" robots that use 4- or 6-wheels, each with
3-legs, and which do classic tripod and diagonal gaits - see
arthropod leg modeling.
<| Empowered Locomotion
Most arthropods have rather wimpy looking legs, compared to higher vertebrates, and their
walks seem more akin to skittering than to powerful thrusting. When the rubber [foot pad]
meets the road, a crab or beetle is clearly no match for the power and efficiency of a
horse or cheetah.
The re-alignment of the legs to underneath the body, described above, was clearly the key
feature which allowed eventual development of larger, more powerful body types.
In comparison, no insects are very large or overly powerful, although a few, like the
grasshopper, have rear legs especially adapted for powerful jumping.
The largest vertebrates with side-projecting legs are alligators and crocodiles, and
while powerful, these animals are slow and cumbersome when out of the water
- although some may be deadly over short distances; see
On the other hand, roaches are reputed to be the fastest insects around, having been
clocked up to 3.4 MPH [1.5 m/s]. They use 6-legged gaits at low speeds, but are said to
employ both 4-legged and ultimately 2-legged gaits, when running at high speeds. See the
work of R.J. Full.
Although the roach is undoubtedly unaware of it, its ontological adaptations appear
to pre-curse later changes in locomotive phylogeny.
Lower animals, as far back as arthropods, have multi-segmented legs analogous to
mammals, but vastly different attachment geometries and gaits.
Lower in the chain of life, legs project outwards from the body and require more of a
levering action to hold the body up.
Higher up in the chain of life, leg attachments have re-oriented so the legs are located
vertically beneath the body; this allows the animal to stand upright with little expenditure
of energy, and also greatly increases its power and stride.
The legs of higher animals have a bent geometry, with corresponding front-back leg
joints bending in a mirror-image fashion with respect to each other.
Multi-legged animals employ different gaits as a means to co-ordinate the many legs
during movements; the tripod-to-tripod gait is common for 6-legged creatures.
"Vertebrate Adaptations", readings from Scientific American, W.H. Freeman & Co, 1968.
"A Textbook of Entomology", by H.H. Ross et al, pub J.Wiley & Sons, 1982.
"Evolution of the Vertebrates", by E.H. Colbert & M. Morales, pub J. Wiley & Sons, 1991.
© Oricom Technologies, Sept 2001, updated June 2002