The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb.

In 2004 Shubin, Daeschler and Jenkins discovered the first well-preserved Tiktaalik fossils in on Ellesmere Island in Nunavut, Canada. Tiktaalik is a non-tetrapod member of Osteichthyes (bony fish) from the late Devonian period about 375 million years before present. It is complete with scales and gills, but has a triangular, flattened head and unusual, cleaver-shaped fins. Its fins have thin ray bones for paddling like most fish, but they also have sturdy interior bones that would have allowed Tiktaalik to prop itself up in shallow water and use its limbs for support as most four-legged animals do. The fins and other mixed characteristics mark Tiktaalik as a crucial transition fossil, a link in evolution from swimming fish to four-legged vertebrates. 

Order of authorship in the original publication: Shubin, Daescher, Jenkins.

(Thanks to Juan Weiss for this reference and its interpretation.)