Geoscience Reference
In-Depth Information
a hip socket, or acetabulum, that had a hole in the middle of it and a
strongly developed ridge of bone along the top of the socket. As a
result, dinosaurs inherited a vastly different posture and gait from their
common ancestor.
To illustrate this, think of the way a lizard stands. Its hind limbs
extend horizontally out from its hips before the lower hind limb
bones reach vertically down to the ground. The limbs support the rest
of the body in a sprawling posture, and when the lizard moves, its body
basically makes an S-shaped motion. The acetabulum in a lizard is
solid, with no hole in the middle, which makes sense structurally.
Where the lizard's thighbone, or femur, meets the hip, a lot of force
is generated by the muscles that pull the bone horizontally into the
hip socket. The acetabulum is solid to help resist those horizontally
directed forces, and no extra bone is needed along the upper margin
to help provide support.
In contrast, dinosaurs have hind limbs that extend vertically down
from the hips to the ground, resulting in a more upright or erect pos-
ture. Clear evidence for this is found in sequences of fossil foot-
Figure contrasting sprawling posture in a turtle (left) with erect posture in a
dinosaur (right).
