Tag Archives: bipedal

Walk Like a Sloth–lesson 12: the femur

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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

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

compassGetting Oriented

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

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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 4: the claw

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

compassGetting Oriented

The claw is from the middle toe of the left foot.  It’s the largest of all of the claws.  In life the bony core would have been covered with a toenail growing out of the pocket or sheath at the wide end. The nail probably didn’t extend far beyond the tip.  Sloth claws didn’t retract like a cat’s so a long nail would get in the way of planting the foot down flat and would have abraded quickly. Half of the nail sheath is missing from the specimen (broken off after burial) but the accident reveals the impressions left by the major blood vessels feeding the nail and penetrating into the interior through foramina (holes).  The ventral tuberosity would have supported a tough pad of tissue on which the sloths walked, lifting the tip of the claw slightly off the ground. Continue reading →

Walk Like a Sloth–lesson 3: the radius

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

compassGetting Oriented

 The lower arms of mammals have two bones–the radius on the thumb side and the ulna on the little finger side. The narrow end of the radius (also called the head) is the proximal or  near end.  If it looks like a small wheel, it is! The wheel turns in the radial notch of the ulna. Note that the articulating surface on the head of the radius covers about 75° of a circle; this wheel covers about 180° on the human radius, indicating more flexibility in humans than in Megalonyx.  The large end is the wrist end; it has a small rough bump where a tendon anchors it to a corresponding spot on the end of the ulna.  The shaft of the radius is notably triangular with a prominent interosseous crest (literally “between bones”) anchoring the muscles used to turn the paws up and down. 

SLOTH RADIUS

SLOTH RADIUS, left, view from front

SLOTH FOREARM, RADIUS (top) ULNA (bottom)

SLOTH FOREARM, RADIUS (top) ULNA (bottom)

HUMAN FOREARM (left), view from back

HUMAN FOREARM (left), view from back (Clipart courtesy FCIT)

SLOTH FOREARM (LEFT), VIEW FROM FRONT (Clipart courtesy FCIT)

HUMAN FOREARM (left), view from front (Clipart courtesy FCIT)

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