Are the Pectoral Fins of Tiktaalik Really Legs?
The limbs of tetrapods share similar characteristic features which meet the special demands of walking on land. In addition to a distinctive suite of bones in the limbs proper, there are characteristic bones in the ankle (or wrist) and in the digits (fingers and toes). In order to support the weight of the body on land, and permit walking, the most proximal bones of the limbs must be securely attached to the rest of the body. The hind limbs in particular have a robust pelvic girdle securely attached to the vertebral column. This differs radically from that of any fish including Tiktaalik. Essentially all fish (including Tiktaalik) have small pelvic fins relative to their pectoral fins. The legs of tetrapods are just the opposite: the hind limbs attached to the pelvic girdle are almost always more robust than the fore limbs attached to the pectoral girdle. It is significant that the “earliest” true tetrapods recognized by evolutionists (such as Acanthostega and Ichthyostega) have all of
