| Limbs
first appear as pectoral and pelvic fins just before the development of
the jaw, but do not become supported by true endochondral bone until the
osteichthyes, or bony fish. Osteichthyes either have ray fins or lobe fins,
and it is the lobe finned fish that came onto land and first tested their
limbs against gravity: all tetrapods including humans are derived from lobe
finned ancestors. As the primary weight bearing and locomotive bones in
the body, the limb bones are mostly dense, compact structures working in
a pyramid-like setup with the number of bones increasing as the limb extends
from the body. The adaptations of this structure are astounding, with a
slight adjustment to the same basic design allowing for flight, speed, swimming,
or even weight bearing. [C] |
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