Among the most significant events in evolution was the invasion of the land by terrestrial vertebrates. Paleontologist Neil Shubin used the theory of evolution to correctly predict the location of fossil remains of a transitional species in Canada's High Arctic. The species, Tiktaalik roseae, has many features of both fish and four-limbed land vertebrates. Shubin and his colleagues published their dramatic findings in the prestigious journal Nature.
Introduction
Fossils of the now famous Tiktaalik roseae were first discovered on Ellesmere Island in the Canadian Arctic in 2004 (Figure 1). Tiktaalik has an odd mix of features. It has fins and scales, a neck and wrist bones, and an unusual flattened head. As a result it was nicknamed "fishapod" The discovery not only produced a very important transitional fossil, but also served as a perfect example of how scientists use and test theories.
Figure 1 (a) Fossils of Tiktaalik were discovered (b) within 1000 km of the North Pole on Ellesmere Island in the Canadian Arctic.
The Power to Predict
Neil Shubin is a palaeontologist and professor at the University of Chicago. He is keenly interested in major evolutionary steps, including the evolution of the first land vertebrates. According to the theory of evolution, the first land vertebrates evolved from fish that made the transition onto land. Therefore, the theory predicts that in the past there must have been species that were transitional between lobe-finned fish and the first simple land vertebrates. Lobe-finned fish have bones extending part way into their fins and were therefore considered the most likely ancestors of the first animals to walk on all fours on land. When he began his search in the 1990s, Shubin knew that no transitional fossil species between land vertebrates and lobe-finned fish had been found. He hoped that by using the theory of evolution he could predict where to find such fossils.
Shubin began his research by gathering information on all the fossil finds of both primitive land vertebrates and lobe-finned fish. He noted several key points:
- The first fossils of lobe-finned fish appeared 390 million years ago.
- The earliest fossils of land vertebrates appeared 360 million years ago.
- The earliest land vertebrate fossils were found in freshwater sedimentary deposits.
- Fossils of lobe-finned fish were also associated with freshwater sedimentary deposits.
Shubin used these facts to make three inferences:
1. The transition from sea to land occurred some time between 390 million and 360 million years ago (Figure 2).
2. The transition occurred in freshwater ecosystems.
3. Fossils of transitional species would be found in freshwater sedimentary deposits from about 375 million years ago.
Figure 2 Theory predicted that transitional species would have lived between 390 million and 360 million years ago.
Image: An illustration showing how 360 million years ago the earliest know tetrapods, then after that transitional species, 390 to 380 million years ago lobe-finned fish.
Shubin now knew what he was looking for and set out to find it in the map room of the local library. There, Shubin examined geological maps of the world looking for sedimentary rocks that had been deposited in freshwater and were approximately 375 million years old. They also had to be at Earth's surface. Rocks buried a half kilometre underground would be inaccessible. Of the three suitable deposits in North America that Shubin found, only one had not been explored in the past.
Print Page 357
Field Work Begins
In 1999, Shubin and his team organized their first expedition to Ellesmere Island. At first their explorations were unsuccessful. Their first year's dig site turned out to be of marine deposits from an ancient ocean. In 2000, they moved their exploration site to the east and began again. That year they unearthed a rich fossil deposit containing many freshwater fish species. In the following years they returned to the same site to keep digging. In 2004, they unearthed Tiktaalik and knew immediately that their predictions had been confirmed.
The "Fishapod"
The 10 fossil specimens that Shubin and his team found are extremely well preserved. They range in size from just under 1 m in length to almost 3 m. Tiktaalik really does look like a cross between a fish and a four-legged land animal. Like fish, Tiktaalik had webbed fins supported with thin bones, gills, and scales. However, like four-limbed vertebrates, Tiktaalik had a neck and shoulders, thick ribs, and sturdy wrist bones (Figure 3). The skull of Tiktaalik was flattened, with eyes on top and two notches that are closer in size to those of land vertebrates than of fish. In early land vertebrates the notches function as primitive ears.
Tiktaalik was not able to walk, but its limb bones would have allowed it to prop itself up in a "push-up" position. Based on the overall shape of its head and body, Tiktaalik likely lived in shallow fresh water.
Scientific Recognition
Shubin and his fellow researchers knew that their findings were of great scientific significance. They reported their initial findings in two scientific papers that they submitted to the prestigious journal Nature in October 2005. After peer review, the articles were accepted for publication in February 2006 and published in April of that year. The Tiktaalik discovery was the cover story.
Figure 3 Tiktaalik had features of both fish and land vertebrates.
Image: An illustration of the Tiktaalik that had features of both fish and land vertebrates how 380 million years ago it was a lobe-finned fish, then developed a fin with wrist a Tiktaalik and at 360 million years ago climbed out of the oceans and became a land-living vertebrate.
Further Reading
Clark, J.A. (2005, December). Getting a leg up on land. Scientific American, 293, 100-107.
Daeschler, E.B., Shubin, N.H., & Jenkins, F.A. (2006). A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature, 440,757-763.
Daeschler, E.B., Shubin, N.H., & Jenkins, F.A. (2006). The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature, 440,764-771.
Shubin, N.H. (2008). Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body. New York: Pantheon Books.
GO TO NELSON SCIENCE
8.6 Questions
1. How did an understanding of the theory of evolution enable Shubin to predict the age and type of sedimentary rock in which he would find Tiktaalik? K/U
2. Why did Shubin look for fossils in Canada's north? K/U
3. It took five years of field work to find Tiktaalik. What does this suggest about the nature of palaeontology? What challenges do you think Shubin's team faced? A
4. Some fish have primitive lungs. Do you think Tiktaalik had lungs in addition to gills? Why or why not? Use the Internet and other sources to check your prediction. [Go To Nelson Science] A
5. Articles submitted to the science journal Nature undergo a rigorous peer review process before being accepted. What is the benefit of such a process? Why would scientists not publish their findings in journals that do not require a review? T/I A
Tiktaalik—Triumph of a Theory
ABSTRACT
Among the most significant events in evolution was the invasion of the land by terrestrial vertebrates. Paleontologist Neil Shubin used the theory of evolution to correctly predict the location of fossil remains of a transitional species in Canada's High Arctic. The species, Tiktaalik roseae, has many features of both fish and four-limbed land vertebrates. Shubin and his colleagues published their dramatic findings in the prestigious journal Nature.
Introduction
Fossils of the now famous Tiktaalik roseae were first discovered on Ellesmere Island in the Canadian Arctic in 2004 (Figure 1). Tiktaalik has an odd mix of features. It has fins and scales, a neck and wrist bones, and an unusual flattened head. As a result it was nicknamed "fishapod" The discovery not only produced a very important transitional fossil, but also served as a perfect example of how scientists use and test theories.
Figure 1 (a) Fossils of Tiktaalik were discovered (b) within 1000 km of the North Pole on Ellesmere Island in the Canadian Arctic.
The Power to Predict
Neil Shubin is a palaeontologist and professor at the University of Chicago. He is keenly interested in major evolutionary steps, including the evolution of the first land vertebrates. According to the theory of evolution, the first land vertebrates evolved from fish that made the transition onto land. Therefore, the theory predicts that in the past there must have been species that were transitional between lobe-finned fish and the first simple land vertebrates. Lobe-finned fish have bones extending part way into their fins and were therefore considered the most likely ancestors of the first animals to walk on all fours on land. When he began his search in the 1990s, Shubin knew that no transitional fossil species between land vertebrates and lobe-finned fish had been found. He hoped that by using the theory of evolution he could predict where to find such fossils.
Shubin began his research by gathering information on all the fossil finds of both primitive land vertebrates and lobe-finned fish. He noted several key points:
- The first fossils of lobe-finned fish appeared 390 million years ago.
- The earliest fossils of land vertebrates appeared 360 million years ago.
- The earliest land vertebrate fossils were found in freshwater sedimentary deposits.
- Fossils of lobe-finned fish were also associated with freshwater sedimentary deposits.
Shubin used these facts to make three inferences:
1. The transition from sea to land occurred some time between 390 million and 360 million years ago (Figure 2).
2. The transition occurred in freshwater ecosystems.
3. Fossils of transitional species would be found in freshwater sedimentary deposits from about 375 million years ago.
Figure 2 Theory predicted that transitional species would have lived between 390 million and 360 million years ago.
Image: An illustration showing how 360 million years ago the earliest know tetrapods, then after that transitional species, 390 to 380 million years ago lobe-finned fish.
Shubin now knew what he was looking for and set out to find it in the map room of the local library. There, Shubin examined geological maps of the world looking for sedimentary rocks that had been deposited in freshwater and were approximately 375 million years old. They also had to be at Earth's surface. Rocks buried a half kilometre underground would be inaccessible. Of the three suitable deposits in North America that Shubin found, only one had not been explored in the past.
Print Page 357
Field Work Begins
In 1999, Shubin and his team organized their first expedition to Ellesmere Island. At first their explorations were unsuccessful. Their first year's dig site turned out to be of marine deposits from an ancient ocean. In 2000, they moved their exploration site to the east and began again. That year they unearthed a rich fossil deposit containing many freshwater fish species. In the following years they returned to the same site to keep digging. In 2004, they unearthed Tiktaalik and knew immediately that their predictions had been confirmed.
The "Fishapod"
The 10 fossil specimens that Shubin and his team found are extremely well preserved. They range in size from just under 1 m in length to almost 3 m. Tiktaalik really does look like a cross between a fish and a four-legged land animal. Like fish, Tiktaalik had webbed fins supported with thin bones, gills, and scales. However, like four-limbed vertebrates, Tiktaalik had a neck and shoulders, thick ribs, and sturdy wrist bones (Figure 3). The skull of Tiktaalik was flattened, with eyes on top and two notches that are closer in size to those of land vertebrates than of fish. In early land vertebrates the notches function as primitive ears.
Tiktaalik was not able to walk, but its limb bones would have allowed it to prop itself up in a "push-up" position. Based on the overall shape of its head and body, Tiktaalik likely lived in shallow fresh water.
Scientific Recognition
Shubin and his fellow researchers knew that their findings were of great scientific significance. They reported their initial findings in two scientific papers that they submitted to the prestigious journal Nature in October 2005. After peer review, the articles were accepted for publication in February 2006 and published in April of that year. The Tiktaalik discovery was the cover story.
Figure 3 Tiktaalik had features of both fish and land vertebrates.
Image: An illustration of the Tiktaalik that had features of both fish and land vertebrates how 380 million years ago it was a lobe-finned fish, then developed a fin with wrist a Tiktaalik and at 360 million years ago climbed out of the oceans and became a land-living vertebrate.
Further Reading
Clark, J.A. (2005, December). Getting a leg up on land. Scientific American, 293, 100-107.
Daeschler, E.B., Shubin, N.H., & Jenkins, F.A. (2006). A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature, 440,757-763.
Daeschler, E.B., Shubin, N.H., & Jenkins, F.A. (2006). The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature, 440,764-771.
Shubin, N.H. (2008). Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body. New York: Pantheon Books.
GO TO NELSON SCIENCE
8.6 Questions
1. How did an understanding of the theory of evolution enable Shubin to predict the age and type of sedimentary rock in which he would find Tiktaalik? K/U
2. Why did Shubin look for fossils in Canada's north? K/U
3. It took five years of field work to find Tiktaalik. What does this suggest about the nature of palaeontology? What challenges do you think Shubin's team faced? A
4. Some fish have primitive lungs. Do you think Tiktaalik had lungs in addition to gills? Why or why not? Use the Internet and other sources to check your prediction. [Go To Nelson Science] A
5. Articles submitted to the science journal Nature undergo a rigorous peer review process before being accepted. What is the benefit of such a process? Why would scientists not publish their findings in journals that do not require a review? T/I A