HARVARD GAZETTE ARCHIVES
Missing link crawls out of muck
Newly found species fills evolutionary gap between fish and land animals
By Steve Bradt
Paleontologists have discovered fossils of a species that provides the missing evolutionary link between fish and the first animals that walked out of water onto land about 375 million years ago. The newly found species, Tiktaalik roseae, has a skull, a neck, ribs, and parts of the limbs that are similar to four-legged animals known as tetrapods, as well as fishlike features such as a primitive jaw, fins, and scales.
These fossils, found on Ellesmere Island in Arctic Canada, are the most compelling examples yet of an animal that was at the cusp of the fish-tetrapod transition. The new find is described by scientists at Harvard University, the University of Chicago, and the Academy of Natural Sciences in Philadelphia in two related research articles highlighted on the cover of the April 6 issue of Nature.
"This previously unknown, extinct animal represents the beginning of the emergence of fish onto land, and the evolutionary transformation of fins into limbs," says Farish A. Jenkins Jr., Alexander Agassiz Professor of Zoology at Harvard and curator of mammalogy and vertebrate paleontology at Harvard's Museum of Comparative Zoology. "The skeletal material is three-dimensional and exquisitely preserved; most material this old tends to be flattened or otherwise distorted. The geometry of the limb joints clearly indicates that segments of the fin could move independently. The 'shoulder' and 'elbow' could flex, and the 'wrist' could extend, converting the fin into postures appropriate to support the body from below and propel the animal on land."
"Tiktaalik blurs the boundary between fish and land-living animal both in terms of its anatomy and its way of life," says Neil Shubin, professor and chairman of organismal biology at the University of Chicago.
Tiktaalik was a predator with sharp teeth, a crocodilelike head, and a flattened body. The well-preserved skeletal material from several specimens, ranging from 4 to 9 feet long, enabled the researchers to study the mosaic pattern of evolutionary change in different parts of the skeleton as fish evolved into land animals.
"Human comprehension of the history of life on Earth is taking a major leap forward," says H. Richard Lane, director of sedimentary geology and paleobiology at the National Science Foundation. "These exciting discoveries are providing fossil 'Rosetta Stones' for a deeper understanding of this evolutionary milestone - fish to land-roaming tetrapods."
One of the most important aspects of this discovery is the illumination of the fin-to-limb transition. In a second paper in the journal, the scientists describe in depth how the pectoral fin of the fish serves as the origin of the tetrapod limb.
Embedded in the fin of Tiktaalik are bones that compare to the upper arm, forearm, and primitive parts of the hand of land-living animals.
"Most of the major joints of the fin are functional in this fish," Shubin says. "The shoulder, elbow, and even parts of the wrist are already there and working in ways similar to the earliest land-living animals."
At the time that Tiktaalik lived, what is now the Canadian Arctic region was part of a landmass that straddled the equator. It had a subtropical climate, much like the Amazon basin today. The species lived in the small streams of this delta system. According to Shubin, the ecological setting in which these animals evolved provided an environment conducive to the transition to life on land.
"We knew that the rocks on Ellesmere Island offered a glimpse into the right time period and the right ancient environments to provide the potential for finding fossils documenting this important evolutionary transition," says Ted Daeschler of the Academy of Natural Sciences in Philadelphia. "Finding the fossils within this remote, rugged terrain, however, required a lot of time and effort."
The nature of the deposits where the fossils were found and the skeletal structure of Tiktaalik suggests that the animal lived in shallow water and perhaps even out of the water for short periods.
"Out of water, these fish encountered gravitational forces very different from the relative buoyancy they enjoyed in an aquatic setting," Jenkins says. "Restructuring of the body to withstand these forces is evident in the ribs, which are plate-like and overlap like shingles, forming a rigid supporting mechanism for the trunk."
Jenkins adds: "Fish feeding in water readily orient the mouth toward food by deftly maneuvering the entire body; the head is rigidly attached to the trunk by bones linking the skull and shoulder girdle, and thus fish have no 'neck.' The challenge of whole-body maneuvers on land was met by freeing the skull from its bony connections to the trunk, thus developing a true neck, which in turn allows the head to move independently of the body."
The new fossils were collected as a result of four summers of exploration in Canada's Nunavut Territory, less than 900 miles from the North Pole, by paleontologists from Harvard, the Academy of Natural Sciences, and the University of Chicago. Although the team has amassed a diverse assemblage of fossil fish, Shubin said, the discovery of these transitional fossils in 2004 was a vindication of the team's persistence.
The scientists asked the Nunavut people to propose a formal scientific name for the new species. The Elders Council of Nunavut, the Inuit Qaujimajatuqangit, suggested "Tiktaalik" (tic-TAH-lick) - the word in the Inuktitut language for "a large, shallow-water fish."
The scientists worked through the Department of Culture, Language, Elders and Youth in Nunavut to collaborate with the local Inuit communities. All fossils are the property of the people of Nunavut and will be returned to Canada after they are studied.
The team depended on the maps of the Geological Survey of Canada. The researchers received permits from the Department of Culture, Language, Elders and Youth of the Government of Nunavut, and logistical support in the form of helicopters and bush planes from the Polar Continental Shelf Project of Natural Resources Canada. The National Science Foundation, the National Geographic Society, and the Putnam Expeditionary Fund of Harvard University, along with an anonymous donor, also helped fund the project.