How do actinopterygian fishes and sarcopterygian fishes use their pectoral fins? Evaluate the structure of the pectoral fins in those two lineages in the context of the differences in how they are used

Question

What will be an ideal response?

Answer 1

The pectoral fins of actinopterygians are used for steering and braking while a fish is swimming and, in some lineages, the pectoral fins are also used for propulsion. The fan-shaped structure of the fin provides a large surface area, and the flexible fin rays allow the shape of the fin to be adjusted (see Figure 6-4b).
The pectoral fins of the extant sarcopterygians, the lungfishes and coelacanths, play little role in locomotion, although some of them do move the limbs in a tetrapodal walking sequence as they swim. In the extinct tetrapodomorph sarcopterygians, the skeleton of the pectoral fin consisted of a sturdy metapterygial axis with radials distributed along its length (see Figure 6-4d). This lobe-shaped structure would have allowed the sarcopterygians to use their pectoral fins to push against the substrate and to prop up the anterior part of the body in shallow water. Tiktaalik represents a culmination of sorts in this support role and an anticipation of the fully tetrapodal mode of limb deployment as it was able to flex at its "elbow" and "wrist" joints, presumably to facilitate elevation of its head above the water.