Tiktaalik

Tiktaalik roseae is an extinct species of sarcopterygian fish that represents a transitional form between fish and early tetrapods. Living about 375 million years ago in the late Devonian, its fossils were found on Ellesmere Island in Nunavut, Canada, in sediments of the Fram Formation. The discovery, announced in 2006 by Neil Shubin, Ted Daeschler, and Farish Jenkins, provided a key link in the evolution of vertebrates moving from water to land.

The morphology of Tiktaalik shows a mosaic of features. It retained fish-like traits such as gills, scales, and a finned tail, but possessed tetrapod-like elements as well. The skull is broad and flat with eyes positioned on top, and the neck is capable of independent movement between the head and body. The pectoral fin contains bones homologous to the limb bones of tetrapods, including a humerus, radius, and ulna, and a primitive wrist with bones akin to rudimentary digits, indicating the beginning of limb evolution. A robust rib cage and a shoulder girdle further support its aquatic lifestyle with some capacity for support on land or in shallow water.

Tiktaalik is considered a stem tetrapod within the larger Tetrapodomorpha, more closely related to later four-limbed vertebrates than to other lobe-finned fishes. Its combination of fish and tetrapod characteristics makes it a landmark in understanding the transition from water to land.