Which fossil fish is a transitional form between fish and tetrapods?

Tiktaalik is the fossil fish regarded as a transitional form between fish and tetrapods.

Tiktaalik lived during the Late Devonian Period, about 375 million years ago, in shallow freshwater environments. Its fossils were discovered in 2004 on Ellesmere Island in Nunavut, Canada, and the discovery was formally published in 2006 by Edward Daeschler, Neil Shubin, and Farish Jenkins.

The animal combined fish features—including scales, gills, and fins—with tetrapod-like traits. Its broad, flattened skull, movable neck, ribs, lungs, and robust fin bones suggest that it could support part of its body in shallow water or possibly briefly on land. Its fins included bones corresponding broadly to an upper arm, forearm, and wrist.

Tiktaalik is not necessarily the direct ancestor of modern tetrapods, so calling it a “missing link” can be misleading. It is better understood as a close example of the evolutionary transition toward four-limbed vertebrates. Ichthyostega was a later early tetrapod, while lungfish and coelacanths are living relatives, not the answer to this question.