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Missing Links and Found Links

In and out of the water, transitional forms from the fossil record illuminate the nuts and bolts of evolution

Pat Shipman

Enter the Fishapod

Tiktaalik’s broad, flat head...Click to Enlarge Image

I am equally enamored of another found link, the fossil skeleton of Tiktaalik roseae, described on April 6, 2006, in the journal Nature. Tiktaalikis a name suggested by the elders of the Nunavut people, who live where the fossils were found on Ellesmere Island in the Canadian Arctic; it means "large, shallow-water fish." This 375-million-year-old fish shows a delicious combination of unexpected features, some inherited from its fishy ancestors and some typical of later land-dwelling tetrapods (four-footed animals). Neil Shubin of the University of Chicago, co-leader of the discovery team, jokingly calls the newly discovered species a "fishapod."

Tiktaalik's fins, gills, scales and primitive jaw show it was a fish. Unlike fish and like tetrapods, it had a distinct neck, so its head moved independently of its body. Its flattened head and broad body make Tiktaalik look somewhat like a weird, scaly crocodile, an impression enhanced by its four-to-nine-foot length. Its skeleton differs markedly from those of crocodiles or alligators, though, despite the overall resemblance in body shape. Tiktaalik's front fins hold the biggest surprise. Each was a sort of half-fin, half-leg containing the bony elements found in a limb—with a functional wrist, elbow and shoulder—and yet retaining the bony "rays" of a fish fin. According to team member Farish Jenkins, Jr., of Harvard University, the front fins were sturdy enough to support the creature in very shallow water or on land for brief trips.

Its broad and robust ribs were imbricated, like tiles on a roof. They helped to support the body on land and probably housed lungs to supplement the gills. The presence of lungs is expected because many of the primitive fish in Tiktaalik's ancestry had lungs for gulping air at the water's surface as well as gills. Soft tissues are rarely preserved in fossils, so the lack of fossilized lungs is unremarkable. With or without lungs, Tiktaalik was uniquely adapted to moving between land and water.

"We were absolutely surprised at the features of the specimens," Ted Daeschler of the Academy of Natural Sciences, co-leader of the team, told me. "That is one of the beauties of this material. We knew the end points—fish at the beginning and tetrapods at the end—but we could not have predicted the sequence in which those anatomical changes occurred." Discovering the unexpected is one of the joys of paleontology.

Changes in the forelimbClick to Enlarge Image

A dramatic change in habitat—becoming a land animal when your ancestors lived in water—required many anatomical changes. Sturdy limbs replaced flexible fins. New foods had to be found, and new means of getting them had to be developed. In this case, when Tiktaalik crawled up on land it probably preyed upon insects. Predatory fish in the past and present often suck aquatic food into their mouths using the same mechanism that passes water across the gills. But Tiktaalik does not have a bony gill cover, which means there was less water flow over the gills and a less effective sucking mechanism. Too, its snout is longer than in its predatory ancestors. Both of these changes suggest that Tiktaalik was snapping up prey, perhaps from the air, rather than gulping down prey along with water.

Eventually tetrapods left the water and relied solely on lungs for respiration, abandoning their gills. By simply being, Tiktaalik not only proves that such major adaptive changes occurred but also reveals how this specific transition from water to land occurred.

This remarkable fossil shows us something else: that the transition was not an all-or-nothing affair. "Land" or "water" is too simple a dichotomy for the realities of ecosystems. There are many habitats—swamps, or shallow, plant-choked streams, or ponds that shrink seasonally and occasionally dry up—that require a range of adaptations to both land and water. Tiktaalik may have been at home in such places.

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