Fish to Tetrapod-Skeletal Changes –Pelvic Girdle, Limbs and Axial Skeleton

Australia: The Land Where Time Began

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.

Sources & Further reading