FIGURE 1. Tiktaalik in context.

The lineage leading to modern tetrapods includes several fossil animals that form a morphological bridge between fishes and tetrapods. Five of the most completely known are the osteolepiform Eusthenopteron¹⁶; the transitional forms Panderichthys¹⁷ and Tiktaalik¹; and the primitive tetrapods Acanthostega and Ichthyostega. The vertebral column of Panderichthys is poorly known and not shown. The skull roofs (left) show the loss of the gill cover (blue), reduction in size of the postparietal bones (green) and gradual reshaping of the skull. The transitional zone (red) bounded by Panderichthys and Tiktaalik can now be characterized in detail. These drawings are not to scale, but all animals are between 75 cm and 1.5 m in length. They are all Middle–Late Devonian in age, ranging from 385 million years (Panderichthys) to 365 million years (Acanthostega, Ichthyostega). The Devonian–Carboniferous boundary is dated to 359 million years ago¹⁸.