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Pelvic girdle and fin of Tiktaalik roseae
Contributed by Neil H. Shubin, December 3, 2013 (sent for review November 12, 2013)
See related content:
- QnAs with Neil Shubin- Jan 15, 2014

Significance
The earliest tetrapods have robust limbs, particularly hind limbs that are enlarged and supported by a number of modifications to the pelvic girdle. In contrast, the closest relatives of tetrapods maintain small and weakly ossified pelvic appendages as compared with the pectorals. This observation has led to the “front wheel drive” hypothesis that held that the closest relatives of tetrapods emphasized pectoral support and locomotion whereas significant pelvic support and locomotion was a tetrapod innovation. The discovery of pelvic girdle and fin material of the tetrapodomorph Tiktaalik roseae reveals a transitional stage in the origin of the pelvic girdle and appendage: although retaining primitive skeletal architecture, these elements are enhanced in size and robusticity much like tetrapods.
Abstract
A major challenge in understanding the origin of terrestrial vertebrates has been knowledge of the pelvis and hind appendage of their closest fish relatives. The pelvic girdle and appendage of tetrapods is dramatically larger and more robust than that of fish and contains a number of structures that provide greater musculoskeletal support for posture and locomotion. The discovery of pelvic material of the finned elpistostegalian, Tiktaalik roseae, bridges some of these differences. Multiple isolated pelves have been recovered, each of which has been prepared in three dimensions. Likewise, a complete pelvis and partial pelvic fin have been recovered in association with the type specimen. The pelves of Tiktaalik are paired and have broad iliac processes, flat and elongate pubes, and acetabulae that form a deep socket rimmed by a robust lip of bone. The pelvis is greatly enlarged relative to other finned tetrapodomorphs. Despite the enlargement and robusticity of the pelvis of Tiktaalik, it retains primitive features such as the lack of both an attachment for the sacral rib and an ischium. The pelvic fin of Tiktaalik (NUFV 108) is represented by fin rays and three endochondral elements: other elements are not preserved. The mosaic of primitive and derived features in Tiktaalik reveals that the enhancement of the pelvic appendage of tetrapods and, indeed, a trend toward hind limb-based propulsion have antecedents in the fins of their closest relatives.
Footnotes
- ↵1To whom correspondence should be addressed. E-mail: nshubin{at}uchicago.edu.
↵2Deceased November 11, 2012.
This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected in 2011.
Author contributions: N.H.S., E.B.D., and F.A.J. designed research; N.H.S., E.B.D., and F.A.J. performed research; N.H.S., E.B.D., and F.A.J. analyzed data; and N.H.S. and E.B.D. wrote the paper.
The authors declare no conflict of interest.
See QnAs on page 881.
Freely available online through the PNAS open access option.
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