The other half of Tiktaalik
When the discovery of Tiktaalik was revealed in 2006, you might have noticed none of the reconstructions included its tail end. Why didn't we include Tiktaalik's tail!? Did we forget? No! In fact, we didn't include any of that information because we didn't have the fossil evidence to back it up. But when we published the first papers describing Tiktaalik, we still had many, many pounds of rock that had not been examined, including a huge slab that included the other half of the famous type specimen, NUFV 108. In the subsequent years, preparator Fred Mullison chipped away at that block and found a pelvis and partially preserved hind fin.
The technical description and more images can be found in the January 13, 2014 early online edition of PNAS in a paper titled "The Pelvic Girdle and Fin of Tiktaalik roseae" by Neil Shubin, Ted Daeschler, and Farish A. Jenkins, Jr.
What do the new bones look like?
Recall that Tiktaalik is special because it has a mix of both fish and tetrapod characteristics. The pelvis and hind fin only add to this story. Compared to its fishy contemporaries, Tiktaalik's pelvis and hind fin are much larger relative to the shoulder and front fin. You can see in the image below that the relative size of the pelvis as compared to the shoulder is much more similar to the tetrapod than the fish.
Although no femur was found, Tiktaalik's fin rays and several other bones suggest the hind fin was comparable in size and complexity to the front fin. The shape and size of the hip socket reveal that the fin was capable of a wide range of movements, from swimming to supporting weight and rotating more like a tetrapod limb. But the overall structure of the pelvis is still more fish-like. Whereas tetrapods have a pelvis made of three parts, Tiktaalik's pelvis is still made of one, like fish. (View the pelvis in 3D)
What do the new fossils reveal about Tiktaalik?
"It's reasonable to suppose with those big fin rays that Tiktaalik used its hind fins to swim like a paddle," Shubin said. "But it's possible it could walk with them as well. African lungfish living today have similarly large pelves, and we showed in 2011 that they walk underwater on the bottom."
Overall, the mix of fish and tetrapod characteristics show us that the structures and mechanisms necessary for the invasion of vertebrate life on land evolved in the water first. Not only that, but before this discovery, we thought the front fins held the key to how vertebrates began to walk on land. The "front wheel drive" theory that fish dragged themselves out of the water with strong front fins and puny hind fins no longer holds. It appears that an "all-wheel" or even a "rear-wheel drive" system is a more appropriate analogy as the hind fins were just as important and may have even been involved in a walking behavior first.