QNAS QNAS QnAs with Neil Shubin

Prashant Nair “ ” Science Writer with a four wheel drive, such as animals that walk on land using limbs. PNAS: Your 2006 description of Tiktaalik furnished a view of the animal’sfrontend. From the rust-brown sediments of long-dried their saga of evolution in his PNAS Inau- Why did it take so long to disinter and de- streams that once rumbled through the rug- gural Article (1). scribe the pelvic appendage? ’ PNAS: Your discovery of Tiktaalik’spelvic ged terrain of Canada s Ellesmere Island, a Shubin: We had collected two blocks of appendage addresses a longstanding conun- team of paleontologists unearthed in 2004 specimens in 2004, one of which contained fi drum in the evolution of locomotion in the fossils of a 375 million-year-old species of sh the front end of the Tiktaalik reference spec- animal kingdom: the contrast between move- that may have nearly crossed an evolutionary imen described in the 2006 papers. We did Tiktaalik roseae ment using fins in the front of the body and Rubicon. Named ,thenow- not look at the second block right away, movement using appendages in the front and extinct animal has come to represent an in- mainly because it didn’tseemtocontain back. Can you explain the evolutionary sig- termediate link between fish and amphibians, much bone and because we had collected nificance of the distinction between these its features likely enabling a leap from water it for the sake of completeness. It took us forms of locomotion? to land. Led by National Academy of Sciences a number of years to prepare, study, and Shubin: Limbed vertebrates, or terapods, member Neil Shubin, a paleontologist at the draw conclusions about this pelvic structure, have four appendages, two in the front and University of Chicago, the team described the found in the second block, partly because the two in the back. The hind appendages are fossils—dated to the late Devonian period— material is delicate and the block was frag- fairly robust and supported by the pelvis. in a pair of 2006 Nature reports, noting how mented.Wehavesincefoundfive pelvic The tetrapod pelvis has a number of modifi- fi ’ fi bones from different individuals, one of the sh s pectoral, or forward, ns showed cations to help support the body under a which is associated with a partial hind fin signs of budding limbs, including a primitive gravitational load: it is the size of a shoulder from the 2006 reference specimen. When shoulder, wrist, and digits. Together, the and attached by a rib to the vertebral column. fi we returned to the site last summer, we found long-snouted skull, limb-like pectoral ns, If you look at the closest finned relatives of fi another pelvis, and we have now put all of and scale-covered body tell a story of a sh the tetrapods, they had large pectoral appen- fi this together in the Inaugural Article. that paved the way for the rst four-limbed dages but tiny pelvic appendages. That gave PNAS: What does the pelvic appendage vertebrates that ventured onto land. But rise to the hypothesis that a major evolution- reveal? the hind parts of Tiktaalik have since ary change associated with the origin of tet- Shubin: The big surprise is the sheer size remained a puzzle. Nearly a decade after rapods was an emphasis on the pelvic ap- of Tiktaalik’s pelvic girdle and hind fin rel- the initial discovery, Shubin and his col- pendage, which meant that creatures with ative to its pectoral girdle; in that respect, leagues describe Tiktaalik’spelvicappend- a “front wheel drive,” such as fish moving it is very much like a tetrapod, suggesting age, presenting a long-awaited sequel to in water with their fins, gave rise to creatures that hind fin-driven locomotion probably began before the tetrapods. That notion is further supported by our 2011 PNAS re- port of an African lungfish, a living cousin of Tiktaalik, that uses its hind finto“walk” under water. PNAS: And these strands of evidence re- inforce the view that Tiktaalik straddled the evolutionary fence. Shubin: The new evidence indeed shows that Tiktaalik had a mix of fish-like and tetrapod-like characteristics, suggesting that hind fin-driven locomotion probably began in fish but didn’t fully develop until after the tetrapods came onto the scene. PNAS: What are the particular features of the pelvic appendage that support that idea? Shubin: Look closely at Tiktaalik’ship joint and you will notice it has a deep socket, similar to the corresponding human socket, which allows us to move our legs in many directions. The socket suggests that Tiktaalik’s

Tiktaalik’ This is a QnAs with a recently elected member of the National Neil Shubin regards the socket in the hip joint of s pelvic appendage. Image courtesy Academy of Sciences to accompany the member’s Inaugural Article of John Westlund, University of Chicago. on page 893.

www.pnas.org/cgi/doi/10.1073/pnas.1321499110 PNAS | January 21, 2014 | vol. 111 | no. 3 | 881–882 Downloaded by guest on September 30, 2021 pelvic appendage was able to support a load, control of locomotion, which is greatly en- the structural changes in the pelvic girdle so the animal may have walked under water abled by hind fins. is another important mystery. Also mys- with its hind fins. Hind limb walking gives an PNAS: Your discovery raises several terious is the timing of onset of the attach- Tiktaalik animal—especially a creature with heavy, air- unanswered questions about .What ment of the pelvic girdle to the vertebral filled lungs in the front of the body—incredi- other secrets might those Devonian stream- column: Did that occur in finned or limbed beds hold? ble ability to maneuver in complex aquatic creatures? Answers to these questions Shubin: Nowthatweknowthatthesize environments, such as swamps, streams, can only come from the fossils yet to be and robustness of the pelvic appendage are discovered. and estuaries. Unlike swimming in a still primitive to tetrapods and arose in fish, the pool, navigating these waters often requires exciting question is how the architecture—in fl fl — fi moving on the oor or on pieces of otsam. particular, the hip joint changed as sh 1 Shubin NH, Daeschler EB, Jenkins FA, Jr (2014) Pelvic girdle and Those kinds of movements call for fine evolved into tetrapods. The sequence of finofTiktaalik roseae. Proc Natl Acad Sci USA 111:893–899.

882 | www.pnas.org/cgi/doi/10.1073/pnas.1321499110 Nair Downloaded by guest on September 30, 2021