Tiktaalik

Tiktaalik is a genus of extinct fish. This sarcopterygian (lobe-finned) fish from the later Devonian has many features similar to those of tetrapods (four-legged animals).
"Tiktaalik" lived about 375 million years ago. It is part of the transition between fish such as "Panderichthys", known from fossils 380 million years old, and early tetrapods such as "Acanthostega" and "Ichthyostega", known from fossils about 365 million years old. Its mixture of basal fish and derived tetrapod characteristics led one of its discoverers, Neil Shubin, to call "Tiktaalik" a 'fishapod'.
This, and other species like it, prove that legs started to develop before these animals were land-based. They were shallow-water carnivorous fish, or fishapods. "Tiktaalik" was therefore a transitional fossil, and an example of mosaic evolution.