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Comparative
anatomy. Origins of the tetrapod limbs
The
invasion of the land led to a complete change in emphasis
in the propulsive elements of themuscular system. In fish
the axial musculature is much more important as a mover
of the body than is the appendicular musculature. The evolution
of land vertebrates is characterized by an increasing emphasis
on the limbs for propulsion and by a corresponding de-emphasis
on the axial musculature. The limbs of tetrapods are generally
similar in overall pattern. Primitively, atleast, most major
groups have similar characteristic features: the fore and
hind feet have five digits; there is one bone in the proximal
part of the limb (nearest to the body) and two in the distal
part (away from the body); and there are a wrist or ankle
joint, an elbow or knee joint, and a shoulder or hip joint.
Although most muscles have several roles, the major actions
of tetrapod limb muscles are similar: some primarily resist
the downward force of the body at hip and shoulder; others
press the supporting fore or hind feet down onto the ground
at wrist or ankle or pull back on the supporting limbs (at
all three joints) to create thrust; others primarily pull
the “swing” limbs forward into a new support position.
The
limbs may originally have developed more as supportive struts.
Structurally, the tetrapod limb can be derived from the
pattern found in the paired fins of Sarcopterygii, a class
of lobe-finned fishes. These were once a large radiation
but largely have been replaced by the Actinopterygii, the
class of ray-finned fishes. Today, the lobe-finned fishes
are represented by Latimeria, the coelacanth, and the lungfishes,
or Dipnoi. The lungfishes, denizens of shallow andseasonal
waters, habitually use their fins as supports, but propulsion
is largely achieved by undulations of the body as is the
case with other fish.
