Tiktaalik prehistoric fish, illustration. Tiktaalik is a very early fish that lived 375 million years ago during the Late Devonian Period. Although classed as a fish, Tiktaalik is unusual in having a flat head like a crocodilian, and odd-shaped fins or flippers. Scientists believe the animal, which measured up to 2.5 metres long, could prop itself up on its front limbs in shallow water and move around on land. It is considered a link in the evolutionary chain from fish to four-legged land vertebrates.