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Australia: The Land Where Time Began |
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Fish to Tetrapod-Skeletal Changes –Pelvic
Girdle, Limbs and Axial Skeleton One of the largest structural contrasts between a
fish and a tetrapod is
the size and function of the pelvic girdle. The pelvis is a relatively
small simple ossification in most fishes, as exemplified by
Eusthenopteron, and is
anchored to the body wall and there is no bony connection to the
vertebral column. It consists of a single pair of boomerang-shaped
elements in
Eusthenopteron, that are
oriented with the apex downwards, which don’t meet each other at the
midline, so no symphyseal junction. The articulation for the fin was at
the pelvis, at about the apex of the boomerang. The pelvic girdle had already become a much
different structure from the condition seen in a fish by the time of the
earliest known tetrapods, being a large robust element that had a long
ventral edge along the pelvic symphysis that was formed by the meeting
of the 2 halves of the girdle. The acetabulum, the point of articulation
for the hind limb, was longer than the femur head, which allowed
movements of the hind limb. There were long processes dorsally, most
characteristically, internal to which 1 of the ribs articulated to form
a sacral joint. These processes provided insertion points for the
muscles that moved the legs and tail, and below the acetabulum, the
large platelike region. Various buttresses strengthened the region
surrounding the acetabulum. From the
Carboniferous
onwards each half of the pelvic girdle of tetrapods was composed of 3
separate ossifications: dorsally, the ilium, attachment for the sacral
rib, below it the pubis anteriorly and the ischium posteriorly, in the
earliest tetrapods these regions were not separately distinguishable. The differences in limb construction, and the
emphasis placed on different body parts that are used in locomotion,
between fishes and tetrapods are reflected in the structure of the
girdle in fishes and tetrapods. The body wall muscles, which are
attached to the axial skeleton, are the main muscles used by fish in
propulsion, the axial skeleton being flexible and the musculature
complex. The emphasis shifts to the appendicular skeleton, the limbs and
girdles, in tetrapods for propulsion. There is a corresponding
enlargement of the limbs and girdles of tetrapods, the axial skeleton is
stiffened, their musculature is more differentiated, though
the body wall musculature is relatively reduced. There are
articulations on the neural spines connecting one with another, the
zygapophyses, which provide anchorage for substantial ribs. In fishes
the ribs are usually thin and flexible and true zygapophyses are usually
absent. In fishes the vertebral centra are ossified relatively poorly,
often remaining as hollow cylinders around the notochord, whereas in
tetrapods they are constructed more solidly and often completely
replaced the notochord. This construction allows the vertebral column to
function as a bridge connecting the 2 girdles and keeping the body of
terrestrial tetrapods off the ground. Much of the propulsion in fishes is provided by the
tail and body, the fins being largely used for steering and braking. The
pelvic fins are usually small, and they are usually smaller than the
pectorals in the fossil lobe-finned fishes. In unspecialised tetrapods
from the Palaeozoic
onwards, by contrast, most of the thrust is provided by the hind limbs
which are larger than the forelimbs. In the very earliest tetrapods this
was not the case. The most obvious difference between fishes and
tetrapods is one has fins and the other has limbs with digits, though
there are some technical difficulties associated with this view, But
here Clack1 considers the similarities and differences
between the structure of the paired appendages of fish and tetrapods. Similar bones are present in the fore and hind fin
pairs in a lobe-finned fish such as
Eusthenopteron, that have
a single element in common, the first axial radial, which is attached to
the girdle, identifying the fish as a
sarcopterygian.
These are called the humerus in the fore limbs and the femur in the hind
limbs of tetrapods. Further elements arise at the distal end of this
element and articulate with it to form a chain, the metapterygial axis.
There are 2 elements articulating with this first axial radial in
Eusthenopteron and
tetrapods. In tetrapods these are the radius and ulna in the fore limb,
and in the hind limb they are the tibia and fibula. The more posteriorly
situated of these elements in
Eusthenopteron give rise
to 2 more, with variation of the precise pattern among the lobe-fins. In
various lobe-finned fish groups the fins differ in structure further
along the fin than they do near the base. There are elongated paired
fins that are supported by a long series of segments, each of which has
a branching radial arising from it, in groups such as some of the
lungfishes. These branches occur mainly on the anterior (or leading)
edge of the fin hence their name, the preaxial radials. Preaxial radials
and postaxial radials are both seen in the lungfishes. The fin skeleton
is shorter and more branches more profusely in the early members of the
line that led to tetrapods. There are flanges and foramina for muscle
attachments, and passages for nerves and blood vessels on the humerus,
the first axial radial of the forefin, and several of these features are
common to the humerus of
Eusthenopteron and the
early tetrapods. The ectepicondyle is a ridge that extends along most of
the length of the bone on its uppermost surface, from which muscles pass
to the radius. The entepicondyle is a flange that extends from about
halfway along the length of the bone, arising on the posterior edge,
from which muscles extend to the ulna. The movement of the lower or
distal part of the fin is controlled by the muscles arising from these
processes. There is a ridge on the underside of the bone, the ventral
ridge, which extends diagonally across most of the length of the bone in
fins from the head of the humerus to the entepicondyle, and it is often
pierced by a number of foramina. A fringe of bony elements that are derived from the
skin, and are therefore dermal, the lepidotrichia or fin rays, that
bounds the outer rim of fish fins. The similarities of construction noted between the
limb of tetrapods and certain lobe fins lie in the first 3 skeletal
elements. Common to all lobe-fins is the pattern of a single element
that attaches to the body, though the more distal pattern in which 2 or
more elements arise from the distal end of this element is
characteristic to the line that leads to tetrapods. The resemblance
breaks down further along the fin skeleton. The divergence of skeletal
pattern that is seen further away from the body in lobe-finned fishes is
exaggerated in the contrast between fins and limbs. The ulna and radius or tibia and fibula, present in
the limbs of most tetrapods, articulate with each other distally, and
with a series of blocklike bones that are connected by complex joint
surfaces which allow them much more freedom of movement. The
characteristic elbow and knee or wrist and ankle joints are produced by
these, allowing the limb ends to be contact the ground at appropriate
weight-bearing angles and transmitting thrust. Digits arise by
sequential budding of a number of radial elements.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |