Tiktaalik roseae is an extinct sarcopterygian ‘fishapod’ which evolved during the late Devonian period (~375 mya) It is regarded as an important example of evolutionary transitionalism - displaying both fish and tetrapod characteristics.
Discovered in Arctic Canada in 2004, Tiktaalik has the scales and gills of a fish but also powerful lobed fins and a flattened crocodile-like head. (Daeschler, Shubin, Jenkins, 2006) These features illustrate how vertebrates may have moved from water onto land. There's some evidence that tetrapoda may have already moved onto land prior to the Late Devonian period (Ahlberg, 2010) but Tiktaalik is still thought to have been among the first vertebrates to cross this margin.
The morphology of interest includes a dorso-ventrally flattened head with raised dorso-anterior eyes - meaning that Tiktaalik would have been able to move just below the water surface, in much the same way as modern crocodiles. A muscular pectoral girdle and freely moving neck suggest that Tiktaalik would have been able to prop itself up on forelimbs and to turn its head - enabling it to scan surface margins. Tiktaalik also had a well-developed pelvic girdle indicating that this trait may have evolved prior to vertebrates moving on to land. (Shubin, 2013).
Most distinctively, Tiktaalik has lobed limbs that end in squat fins. These lobe-fins have internal bone structures that correlate to the arm and leg bones of all land vertebrates.
Tiktaalik also has spiracles on top of its head which is suggestive of lung development as well as gill breathing ability. This would have been useful in the warming shallow waters of the time that may have been low in oxygen content.
All of the above anatomical features combine to indicate that Tiktaalik would have been a very effective tidal margin predator.
It is unlikely that we are the direct descendants of Tiktaalik but rather a convergent evolutionary branch - meaning that different lineages of vertebrates probably made independent transitions onto land - possibly at even earlier times. Tiktaalik is just a perfect example of such an evolutionary step and speculation on its existence led to it being discovered on Ellesmere Island in 2004.
With thanks to Prof. Neil Shubin and Dr Jean-Bernard Caron for feedback on anatomical detail.