[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Sprawling knees and bipedalism



Ronald Orenstein quotes P. Wellnhofer:

"Three-dimensional reconstruction of the pelvis of Ahanguera revealed that
the hind legs could not be brought into a vertical position under the body,
but were splayed slightly to the side.  Thus bird-like, bipedal locomotion on
the ground was scarcely possible.  Orientation of the hip socketsobliquely
upwards and the slight bend of the articular head of the thigh bone make
quadrupedal locomotion on the ground more probable."

In contrast to most theropods, most birds have a pelvis that is wider
posteriorly than anteriorly.  This widens the acetabula to the  width of the
breast, permitting parasagittal movement of the femora, which are more or
less permanently tucked in near the chest.  However, the rhea, among other
birds, retains, or has reverted to, the primitive narrow sacrum. (In fact the
vertebral column gradually disappears between the ilia, only to reappear near
the distal end of the ischia).  With a narrow inter-ilia the acetabula are
correspondingly not far from the midline.  In order to clear the chest, the
femora are splayed, the angle of the femoral head is obtuse and the distal
articulations are similar to those of pterosaurs in alignments which provide
for a parasagittal movement of the tibia, thus putting the kibosh to
Wellenhofer's speculation.

On another note R.O. asks: "...modern birds the size of Mononykus seem to be
able to mate quite effectively without grappling (though there may be some
biting), so why should Mononykus have had to do otherwise?"

Good question.  But birds have a tiny pygostyle that flips up allowing ready
access to the cloaca, and beating wings which can help these clawless avians
maintain position.  In contrast, _Mononykus_ is like an alligator on stilts.