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What has the head of a crocodile and the gills of a fish?
May 2006
Tiktaalik, of course. Pronounced tik-TAA-lik, this 375 million year old fossil splashed across headlines as soon as its discovery was announced in April of 2006. Unearthed in Arctic Canada by a team of researchers led by Neil Shubin, Edward Daeschler, and Farish Jenkins, Tiktaalik is technically a fish, complete with scales and gills but it has the flattened head of a crocodile and unusual fins. Its fins have thin ray bones for paddling like most fishes', but they also have sturdy interior bones that would have allowed Tiktaalik to prop itself up in shallow water and use its limbs for support as most four-legged animals do. Those fins and a suite of other characteristics set Tiktaalik apart as something special; it has a combination of features that show the evolutionary transition between swimming fish and their descendents, the four-legged vertebrates a clade which includes amphibians, dinosaurs, birds, mammals, and of course, humans.
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| A reconstruction of Tiktaalik alongside a cast of its fossil, and a map showing where the fossil was found, on Ellesmere Island, Nunavut, Canada. |
Where's the evolution?
All over the place! Evolution often makes an appearance in the news but is usually buried in an article, underlying a forensic analysis or a new antibiotic, for example. In this case, evolution appeared front and center! Tiktaalik provides clues about a key transition in the history of life. Now extinct, this organism was a close relative of one our own ancestors the first vertebrate to evolve four limbs and crawl out onto dry land. The evolutionary tree below shows the relationship between Tiktaalik, other fish, and four-legged vertebrates.
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In many news articles, Tiktaalik was billed as "the missing link" between fish and land vertebrates but that description is a bit misleading. First, Tiktaalik is more accurately described as a transitional form than a missing link. Transitional forms help show the evolutionary steps leading from one lineage to another by displaying characteristics of both the ancestral and the new lineage. These character suites help us understand the order in which the traits of the new lineage evolved and what functions they served as they evolved. Tiktaalik, for example, had fins with thin ray bones, scales, and gills like most fish. However, it also had the sturdy wrist bones, neck, shoulders, and thick ribs of a four-legged vertebrate. Tiktaalik was specialized for life in shallow water, propping itself up on the bottom and snapping up prey. The adaptations it had for this lifestyle ended up providing the stepping stones for vertebrates to climb onto dry land but of course, Tiktaalik was not "aiming" to evolve features for land-living. Tiktaalik was simply well-adapted for its own lifestyle and later on, many of these features ended up being co-opted for a new terrestrial lifestyle.
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| Tiktaalik had fins with thin ray bones, scales, and gills like most fish; however, it also had the sturdy wrist bones, neck, shoulders, and thick ribs of a four-legged vertebrate.
Image reprinted by permission from Macmillan Publishers Ltd: Nature 440, 757-763 (6 April 2006), copyright 2006. |
Second, a whole series of transitional forms tie fish to four-legged vertebrates not the single, key organism suggested by the phrase "the missing link." Before Tiktaalik was discovered, paleontologists had studied many other extinct transitional organisms, such as Eusthenopteron and Acanthostega, which also provided clues about vertebrates' invasion of land. And as paleontologists continue to explore the fossil record, they are likely to uncover other organisms representing different points in this transition.
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| Many transitional forms help illuminate the origin of land-living vertebrates. As suggested by the dotted line, some have yet to be discovered. |
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Tiktaalik is important, well-preserved, and certainly newsworthy but it was not unforeseen. The paleontologists who found Tiktaalik went looking for it. Previous research suggested that vertebrates' invasion of land took place about 375 million years ago in a river so Shubin and fellow researchers searched for fossils in 375 million year old rocks that had preserved a river delta ecosystem. Having studied other organisms from this water/land transition, the paleontologists knew what sort of animal they were looking for. And when they did discover Tiktaalik (after five separate expeditions to Canada), it wasn't much of a surprise: Tiktaalik had the set of characteristics that they had expected to find in such an organism. In sum, discovering Tiktaalik simply confirmed many of the hypotheses biologists had held for a long time regarding the origin of terrestrial vertebrates. So although Tiktaalik didn't revolutionize anyone's thinking in this area, it does play an important role in moving science forward. Biologists can now capitalize on this knowledge to elaborate their hypotheses (about, for example, why vertebrates moved onto land), to make other predictions, and to discover more transitional forms: huge tracts of rock from this era remain unexplored and ripe for paleontological prospecting.
Read more about it
News and journal articles:
Understanding Evolution resources:
For teachers
Discussion and extension questions:
- What is a transitional form? What makes Tiktaalik an example of a transitional form?
- Which features of Tiktaalik are similar to those of fishes? Which features of Tiktaalik are similar to those of four-legged vertebrates?
- Tiktaalik did not live on land, yet it had many features that terrestrial vertebrates alive today have. How can we explain those features? Were they adaptations? If so what were they adaptations for?
- Watch "Fish with Fingers" on the WGBH Evolution website. Compare Tiktaalik to Acanthostega in terms of anatomy and lifestyle.
- Watch "Fish with Fingers" on the WGBH Evolution website, which was recorded before the discovery of Tiktaalik. In that video, Jenny Clack hypothesizes that feet and legs did not evolve for walking on dry land but for maneuvering in shallow water. Does the discovery of Tiktaalik support or refute that hypothesis? Explain why or why not.
- Imagine that you read a newspaper article that describes Archaeopteryx as "the missing link between dinosaurs and birds." Do some research on this organism and decide whether you think that is an accurate description. Explain why or why not.
Related lessons and teaching resources:
References
- Ahlberg, P. E., and Clack, J. A. (2006). A firm step from water to land. Nature. 440: 747-749.
- Daeschler, E. B., Shubin, N. H., and Jenkins, F. A. (2006). A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature. 440: 757-763.
- Shubin, N. H., Daeschler, E. B., and Jenkins, F. A. (2006). The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature. 440: 764-771.
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