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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.

A reconstruction of Tiktaalik alongside a cast of its fossil, and a map showing where the fossil was found, on Ellesmere Island, Nunavut, Canada.

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. 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. Many transitional forms help illuminate the origin of land-living vertebrates. As suggested by the dotted line, some have yet to be discovered.

"Missing" links? "Missing link" is an inappropriate term when referring to a transitional form not yet discovered in the fossil record. "Missing" usually describes something that was supposed to be present but has disappeared for unknown reasons — if, for example, a favorite book suddenly disappeared from your room But you wouldn't describe the book as "missing" if you had lent it to a friend and expected it to be gone. The same arguments apply to so-called "missing links." Biologists know that fossilization is a chancy process — most things that have lived on Earth are eaten or rot away before they can be fossilized. Very few organisms wind up in situations in which fossilization is possible and have body parts that can be preserved in fossils. Thus, biologists expect that most intervening steps of an evolutionary transition (such as vertebrates' invasion of the land) will not be preserved in the fossil record. Occasionally, we get lucky and discover a transitional form that has been preserved in a fossil — but that does not imply that its ancestors, descendents, or other organisms that once lived on Earth are "missing."

Read more about it

News and journal articles:

• A newspaper article covering the discovery from The Los Angeles Times

• The University of Chicago's website describing how Tiktaalik was discovered

• A short summary of the importance of Tiktaalik from the National Science Foundation

• An in-depth review of the transition from water to land from Scientific American

• Background information on lines of evidence relating to evolution, including fossils and transitional forms

• Historical perspectives on fossils and biostratigraphy

• A explanation of the many ways in which new features (like legs and feet) evolve

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.

• Teach about how fossils are formed: In this interactive module for grades 6-8, students are introduced to fossils and the fossilization process by examining how fossils are formed and the factors that promote or prevent fossilization.

• Teach about studying fossils: In this interactive web-based module for grades 6-8, students conduct a simulated field study at a fossil dig in Montana.

• Teach about how scientists learn about the past: This web-based module for grades 6-12 provides students with a basic understanding of how fossils can be used to interpret the past.

• Teach about another major transition in vertebrate history, the evolution of flight in birds: This interactive module for grades 9-12 examines evidence from the fossil record, behavior, biomechanics, and cladistic analysis to interpret the sequence of events that led to flight in the dinosaur lineage.

• 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.