by Greg Mayer
In a paper in press in the Proceedings of the National Academy of Sciences, Neil Shubin, Ted Daeschler, and the late Farish Jenkins describe the pelvis and partial hind limb of Tiktaalik roseae, the lobe-finned fish from the Canadian high arctic that they discovered in 2004 and described in Nature in 2006. Tikataalik is a transitional form from fish to tetrapods, and presents such a suite of advanced (for a fish) features that Neil dubbed it the “fishapod”. The newly reported finds show that Tiktaalik had a very substantial pelvic girdle and limb, which must have had a significant role in locomotion.
Of course, it’s not a surprise that Tiktaalik had hind limbs– most vertebrates do– but the nature of the hind limbs in this, the most tetrapod-like of fish, is of especial interest. It’s been known for a while that Neil et al. had found the hind limb, and their publication on it has been eagerly awaited. The most important find, a pelvis and part of the associated limb, was actually found on the original holotype specimen (the one from which the species was described) found in 2004; four other isolated pelvises were found in later years. Since the first publication, preparator Fred Mullison has been working to free all the bones from the encasing rock.
So, what have we found out? The pelvis is robust, with an ilium and pubis, and a large acetabulum for receiving what must have been a substantial femur. There’s no ischium (the third bone in a typical tetrapod pelvis). The Tiktaalik website has 3D scans of the pelvis which you can rotate to see the full morphology.
Only a portion of the hind limb was preserved: the intermedium, two radials, and several bony fin rays (lepidotrichia). We can tell from the acetabulum though that the femur must have been robust.
Here’s how the team summarized their findings:
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. …
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.
Shubin, N.H., E.B. Daeschler and F.A. Jenkins, Jr. 2014. Pelvic girdle and fin of Tiktaalik roseae. Proceedings of the National Academy of Sciences in press. pdf
First two Images from the Tiktaalik website 2014 New Discovery page.