A quest to complete 'missing link' fossil

Tiktaalik is one of the most spectacular fossils in the entire evolutionary record. If only it were complete. True, this remarkable fish-with-limbs specimen, which made worldwide headlines when unveiled in 2006, reveals with stunning clarity how life crawled from water onto land. Its neck and forelimbs became features of four-legged animals, or tetrapods. The intricate bone structure in Tiktaalik's fins even resembles the human arm.

Yet just three of the roughly two dozen Tiktaalik fossil specimens have well-preserved heads and fins, and none represents the whole skeleton. (A cast of one is in the Harvard Museum of Natural History.) A more complete fossil and those of species that succeeded it could reveal deep evolutionary secrets about how our ancestors became land-dwellers.

"The book is not closed on Tiktaalik," said Farish Jenkins, the Alexander Agassiz Professor of Zoology at Harvard, who was part of the expedition which found it in 2004. This summer the same team - headed by Neil Shubin of the University of Chicago and Ted Daeschler of the Academy of Natural Sciences in Philadelphia - is back at its discovery site, on Ellesmere Island in the Canadian Arctic, aiming to find more fossils of Tiktaalik, which are roughly 375 million years old, and the species that followed it.

"We want to know Tiktaalik very well," said Jenkins. "And if we pull off a hat-trick and get another thing that's more advanced - I mean, that's our secret paleontological lusting."

Scientists are acutely aware of the missing pieces of fossils, so before he left for the Arctic, the Globe asked Jenkins what he was hoping to find on this year's Tiktaalik treasure hunt.

PETER DIZIKES