Walk Like a Sloth–lesson 12: the femur

Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

Getting Oriented


The femur or thigh bone is the largest bone in the sloth’s body and one of the most unusual bones of any animal owing to to its tremendous width and eccentric cross-section.  eccentricThe round head fits into a cup-shaped socket in the pelvis or hip called the acetabulum, while the far or distal end forms the knee joint, articulating with both the tibia or shin bone and patella or knee cap. The head of the femur points upward, about 35° below vertical, and angles forward about 45°, matching the backward-pointing hip sock.  This gives Megalonyx a unique knees-wide-apart stance.  (McDonald, 1977) The distal (down) end of the bone has a shallow wide depression in the center of one side–that’s the trochlea or patellar groove, where the patella articulates.  That’s anterior (forward) obviously, so this femur came from the sloth’s left leg.

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Walk Like a Sloth–lesson 11: the toes

Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

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Like you, and unlike tree sloths and most other ground sloths, Megalonyx had five (5) toes on each foot.  Your toes, like your fingers, have three (3) phalanges (segments) each, except your big toe and thumb which each have just two (2) phalanges. Megalonyx is the same, only the middle toe is the big toe, and has two (2) bones.  In Megalonyx the proximal phalanx (the bone closer to the body) and medial phalanx (middle bone) of the 3rd toe are fused, so completely that all traces that they started out as two separate bones have vanished. What looks like the proximal phalanx is actually the third metatarsal.

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Walk Like a Sloth–more things to do with the scapula

magnifyingglassLook closer

The glenoid cavity is pear-shaped, wider at the bottom and narrowing at the superior end. The oval shape is generally believed to confer a greater range of motion versus the narrower sockets of quadrupeds.  (Aielo and Dean, 1990)  Note the glenoid notch midway up the ventral rim of the socket of the adult’s bone.  About half (55%) of humans have them too. (Prescher and Klümpen, 1997) Scientists disagree about their purpose.  The ventral surface of the scapula or subscapular fossa gives origin to the broad subscapularis muscle, part of the rotator cuff  quartet, which gathers as a wide tendon wrapping around the glenoid cavity to insert on the lesser tubercle of the humerus.  The notch may simply form due to the pressure of a tendon and subsequent atrophy of the bone. (ibid) Alternatively, it may provide extra surface area for anchoring the inferior glenohumeral ligament, one of ligaments surrounding the cavity, important for stabilizing the humerus head (Miles, 1997).

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Walk Like a Sloth–lesson 10: the scapulae

Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

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The scapula or shoulder blade is a large, thin, relatively flat bone. Together with the clavicle (collar bone) and sternebrae (breast bones), the scapula is at the center of the sloth’s so-called shoulder girdle, playing an essential role in controlling movement of the upper arm as well asi_Beam_drawing_large magnifying the arm’s power.   The smoother, flatter side is the ventral or “in” side, configured to fit snugly against the ribs on the back.  The scapular spine marks the dorsal or “out” side, serving to bring  rigidity to this otherwise amazingly thin bone, like the upright part of an “I” beam.

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Walk Like a Sloth–lesson 9: the clavicles

Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

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The clavicle or collar bone connects the scapula or shoulder blade to the sternum or breast bone. Along with the scapula and proximal end of the humerus (upper arm bone) the clavicle makes up the sloth’s so-called shoulder girdle and plays an essential role in helping control the sloth’s arm movements. The larger rounder end of the clavicle connects to the manubrium of the sternum while the smaller thinner end connects to the acromion of the scapula. Both ends are rough and pitted, not smooth as on other joints, serving to better anchor important ligaments within the shoulder girdle. These are synovial joints–in life you would find a cartilage pad between the bones and a layer of fluid to lubricate and cushion the shocks these joints must absorb.

Like your clavicle, the sloth’s collar bone is curved slightly to wrap around the front of the chest (bow side out) and attach to the scapula. Your clavicle bends forward slightly again, making an “S” shape, Megalonyx’s doesn’t, but there is great variability in shape among humans as well as sloths. (Aiello and Dean, 1990)  With the medial or “in” side versus the lateral or “out” side determined, we just need to identify the top or superior side from the inferior side or “bottom.” Look for the cone-shaped  conoid tubercle about 1/3 of the way from the thinner (acromial) end. That goes “down.”  It anchors a ligament that attaches the clavicle to the coracoid process of the scapula.  Note the anterior expansion of the clavicle at the lateral end.  That’s extra surface area for attaching part of the deltoid muscle.

sloth clavicles300

SLOTH CLAVICLES, adult (top), Toddler (bottom)

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Walk Like a Sloth–lesson 8: teeth

Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

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tooth section

SLOTH CANINIFORM, cross section

This caniniform tooth (“in the form or shape of a canine”) is from the sloth’s lower right jaw or mandible.  The sloth had four (4) caniniforms (sometimes called “tusks”)–two (2) on the bottom and two (2) on top.  Slip the tooth out of its alveolus (socket).  The occlusal surface  (top, biting surface) is smooth and slopes toward the tongue or lingual side and away from the outside or labial side. The root is open because this tooth is ever-growing.  We can’t show you with the plastic prototype, but the tooth is composed of two layers of dentine covered with an outer layer of cementum.  The dentine layers are soft and wear away more quickly than the hard cementum layer as the sloth chews, leaving a hard raised edge for cutting.


view from lingual side, root on left

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Walk Like a Sloth–lesson 7: the mandible

Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

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Only part of the mandible is in the traveling trunk–the chin and right lower jaw of the adult. The large alveolus or cavity on the superior (top) surface holds the front tooth or tusk, called the caniniform (“in the form or shape of a canine,” see tooth lesson).  On the other side of the chin there’s part of the corresponding alveolus for the left caniniform.  The narrow inward sloping shelf of bone anterior and between the front teeth is called the mandibular or predental spout.  The teeth behind the caniniforms are called molariforms (“in the form of a molar”).  A gap or diastema, separates the caniniform from the first molariform.   Notice how deep the alveoli are and how close they approach the bottom of the jaw.  These are long, deep teeth–the term for that is hypsodont–more on that in the tooth lesson too.  The small hole at the side of the chin is the mental foramen which provides an exit for the mental nerve and blood vessels that serve the front teeth, chin and lower lip. There’s another exit on the opposite side of the jaw.  The mandible has (4) teeth on each side—one (1) caniniform and three (3) molariforms.


right anterior

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Walk Like a Sloth–lesson 6: sternebrae and sternal ribs

Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

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SLOTH STERNUM,ventral view (Owen, 1842)

SLOTH STERNUM,ventral view (Owen, 1842)

Megalonyx has a sternum or breast bone composed of a stack of eight (8) separate bones called sternebrae  (singular is sternebra).  The sternebrae connect to the ribs through eight (8) pairs of bones called sternal ribs that curve around the front of the chest and connect to the true ribs with a thin layer of cartilage.  The triangle wide, flat, smooth back of the sternebrae is the “in” side or dorsal surface. Notice the protruding opposite side, called the ventral process, is heavily textured.  As in all Megalonyx bones, this increases the surface area for anchoring the sloth’s powerful muscles, in this case, the pectorals.

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Walk Like a Sloth–lesson 1: the atlas

Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

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The longer, deeper-cupped joints oriented up and down are anterior (skull end), the smaller, flatter more oval joints oriented more laterally are the posterior (tail end).  Now you just need to determine the back or dorsal surface from the ventral surface, to see which way the animal is facing.  That’s easily done by looking for the facet inside the vertebral canal on the posterior end.  That’s “down” or ventral and serves to accommodate a bony projection, the odontoid process of the next vertebra, the axis. The flattened wings projecting from the sides of the atlas are called the transverse processes.  Note they are much rougher on their dorsal (top) surface, serving to anchor the muscles used to help raise and turn the head.



SLOTH ATLAS, view from anterior or head end



HUMAN ATLAS, view from anterior or head end (Clipart courtesy FCIT 



SLOTH ATLAS, view from posterior






HUMAN ATLAS, view from posterior or tail end (Clipart courtesy FCIT) 

Key points

#1 The atlas is the first of seven (7) cervical or neck vertebrae. It is the only vertebra that has no centrum, the bony round weight-bearing block that is the largest portion of all other vertebrae.

#2 The atlas is basically just a large ring that attaches the skull to the backbone and gives animals the ability to nod their heads up and down by rolling the skull on the front end (nodding “yes,” hence the “yes joint”). The skull turns right and left by sliding around the second neck vertebra, the axis, on the back end (shaking “no,” hence the “no joint”).   Note the yes sockets are deeply concave (no lateral play in this joint) while the no sockets are relatively flat allowing sloths, like us, to turn their heads from side to side in a wide arc—there’s a much broader side-to-side motion than up and down. . . the better to watch for danger over your shoulder.

#3 There’s no room in the atlas for a centrum for it needs a large central hole to accommodate the spinal cord at its thickest point—where it exits the brain. [Compare this to the size of the spinal cord down at the back end.]  The large holes along each side of the atlas, snaking over and through the transverse processes, are called the transverse foramen—these canals are found only in cervical vertebrae. They protect the vertebral arteries that transport blood to the brain.  Two smaller foramina on each side allow passage of blood vessels into the nerve cord and the bone.greek atlas


 addedinfoAdditional information

The atlas is named for the Greek god Atlas who was tricked by Hercules to hold up the heavens.  He is commonly portrayed supporting the Earth on his shoulders.

Early observers were struck by the great length, width and thickness of the sloth’s  transverse processes–evidence of great muscular forces moving the head. (Owen, 1842)  Also note the roughness of the dorsal anterior surface of the atlas.  The ridges anchor one of the muscles that helps raise the head.  A sloth’s head, like ours, is heavy in proportion to the body so these muscles need to be large and strong.  Most of the head-lifting is done by the larger nuchal muscles running from the other cervical vertebrae, collar bones, ribs and shoulder blades to the back of the skull.
The broad rough surface on the dorsal (top) side of the transverse processes serves to anchor additional muscles for raising the head as well as some used to help turn the head which attach to the axis.   The muscles

image borrowed from Wikipedia

image borrowed from Wikipedia

used to lower or nod the head, the prevertebral muscles, are much smaller. Two of these attach to the underside of the transverse processes.   Most mammals let gravity do most of the work of lowering the head

(which is why our head drops when we nod off) so their prevertebral muscles are relatively small.

The sabre-toothed cat is a notable exception.  Their prevertebral muscles are much more developed to aid in stabbing.



Things to do

Move-your-head-like-a sloth.  Use the prototype to see how the joints of the atlas work to allow sloths toleidy1853 turn and move their heads “yes” and “no.” [Suggestion:  use your knuckles on a couple of fingers to represent the joint with the skull, occipital condyles.]

In life, the vertebral column is wrapped in a tough elastic covering called intervertebral substance which acts like a spring holding the vertebrae together and bringing them back into alignment as you turn and move. Springing saves energy–15%, or more, so animals don’t have to use as much muscle power to hold their spines in position. (Hildebrand, 1985)  But the atlas lacks the intervertebral substance—it is attached to the head and the axis with only ordinary cartilage joints and ligaments, allowing the atlas and skull to turn in any direction without resistance.  Imagine how much more work it would be to turn your head if the atlas was wrapped in the tough elastic that covers the other vertebrae.  [Try turning your head as though it were held by a giant rubber band (with straining sound effects of course!.  Now let it spring back to the center (“boingggg.”)]

atlas flexm
Flex Your Muscles
The human atlas offers little leverage for moving the head, so most of the work is done by larger and more superficial groups of muscles attached to the torso.  However, the deep small muscles serve an essential function (besides ensuring your head doesn’t fall off)—they constantly relay information to the brain about the position of your head and neck.  This so called proprioceptive function may be more important than their contracting ability. (Aiello and Dean, 1990).  Close your eyes.  Move your head.  You don’t need your eyes open to sense when your head is level.  Your inner ears are helping too.


thinkaboutThings to think about

Why are atlases and skulls so often missing at fossil sites? As a consequence of the lack of intervertebral “elastic” covering the atlas, the head is only loosely attached to the rest of the body.  After death, the heavy skull and atlas are often the first bones to become separated from the skeleton, especially in water.  In contrast, the rest of the vertebrae are often the last bones to separate or disarticulateFinding the adult sloth’s skull at the fossil site is strong evidence the animal decayed where it died, and probably lived there–the sloths weren’t swept there by a flood, for example.  Having some confidence the animals lived where they were found means the other fossils (e.g. seeds, pollen, turtle bones, etc.) found with the bones are evidence of the sloths’ habitat and wider ecosystem.  What could these other fossils tells us? [Answer: nearby geography, plants, climate, season, etc.]


futureFuture research

Scientists are using morphometrics, a science measuring subtle differences in the shapes of bones in different species, to study the atlas and help identify different forms of locomotion. In theory, the head of a quadruped hangs from its neck requiring more robust muscular support and atlas bone structure than an animal with an upright posture (e.g. a biped, like humans) where the head is simply balanced on top. (Manfreda et al., 2006)



Atlases are different for every species of mammal, but all have a similar and unmistakable shape because they all serve the same basic purpose—allowing the skull to turn freely.   Whether the similarity between humans and sloths is related to a common posture and mode of locomotion, or an adaptation to something else is not known and the answer awaits further research.  By itself the atlas can’t tell scientists if Megalonyx is bipedal, but the similarities with the human atlas are striking.


To learn more about or to borrow the University of Iowa Museum of Natural History Geo-2-Go Discovery Trunks  call or contact the museum.



Aiello, L. and Dean, C. 1990.  Introduction to Human Evolutionary Anatomy. Academic Press Limited.  San Diego, CA

Educational Technology Clearinghouse, Florida Center for Instructional Technology, College of Education, University of Southern Florida http://etc.usf.edu/clipart/

Hildebrand, M. 1985.  Walking and running.  In Functional Vertebrate Morphology.  M. Hildebrand, D. M. Bramble, K. F. Liem and D. B. Wake (eds.) Harvard University Press,  Cambridge, MA.

Manfreda, E., Mitteroecker, P., Bookstein, F. L., and Schaefer, K. 2006. Functional morphology of the first cervical vertebra in humans and nonhuman primates.  The Anatomical Record Part B:  New Anat.): 2898: 184-194.