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Fish With First Neck Evolved Into Land Animal -- Slowly

James Owen
for National Geographic News
October 15, 2008
 
The skull of a 375-million-year-old walking fish reveals new clues to how our fish ancestors evolved into land dwellers.

The fossil fish—called Tiktaalik roseae—was discovered in the Canadian Arctic in 2004 and provides the 'missing link' between fish and land vertebrates, according to scientists. It's also the proud owner of the world's first known neck.

The new study confirms that the prehistoric fish, which had limblike fins, heralded a momentous departure from water for vertebrates (animals with backbones) but that this evolutionary transition wasn't as sudden as previously thought.

Careful analysis of the fossil skull indicates that Tiktaalik was an intermediate step between fish and land animals, with physical features from both—allowing the fish to live in shallow water.

"We see that cranial features once associated with land-living animals were first adaptations for life in shallow water," said study co-author Jason Downs of the Academy of Natural Sciences in Philadelphia, Pennsylvania.

From Fins to Limbs

Researchers already knew that Tiktaalik was a nine-foot (three-meter) crocodile-style predator with a neck, primitive lungs, and leglike fins adapted for life in the shallows.

The team's latest findings, to be published tomorrow in the journal Nature, reveal the switch from an aquatic to a terrestrial life in vertebrates also involved incremental, complex changes to the skull.

The study indicates Tiktaalik shared many internal skull features with more primitive fishes, but that its head was also amphibianlike, suggesting the animal was possibly capable of breathing air and feeding on land.

"The new study reminds us that the gradual evolutionary transition from fish to tetrapod [four-limbed land animals] and the transition from aquatic to terrestrial lifestyles required much more than the evolution of limbs," said Ted Daeschler, a paleontologist at the Academy of Natural Sciences who co-led the 2004 team.

Air Breather?

"It may have been more capable of air breathing and land walking than more primitive finned animals, but it's still an aquatic animal," said Downs.

Tiktaalik's less fishy skull features suggest its head was increasingly more mobile. Cranial changes included a flattened palate, a solidly constructed head, and a much shortened hyomandibula—a bone that supports the gill cover.

This suggests the animal wasn't particularly good at pumping water into its body, Downs said.

"This sort of pumping function is necessary in fish for breathing and feeding underwater, but it's not important once out on land," he added.

"But that's not to say Tiktaalik was incapable of gill breathing. If you look at the gill skeleton it's quite unchanged from the primitive condition," Downs said.

So if Tiktaalik was also breathing air, "it might have been as simple as reaching its head out of water and taking a gulp," he said.

Scientists had previously argued for the rapid evolution of the first land vertebrates from fish. But prior to the Tiktaalik discovery, the internal head skeletons of such intermediate fish forms were missing from the fossil record, the study team said.

"Tiktaalik continues to demonstrate itself as an animal that is really helping us to inform this fin-to-limb transition," Downs said.

Fossil Evidence

Per Ahlberg, a professor of evolutionary biology at Uppsala University, Sweden, says he agrees with the latest study findings, which were funded in part by the National Geographic Society's Committee for Research and Exploration. (The National Geographic Society owns National Geographic News.)

He said that in many respects Tiktaalik's skull is reminiscent of an earlier amphibianlike fossil fish called Panderichthys.

(See related: "Ancient Fish Had Primitive Fingers, Toes" [September 24, 2008].)

"What's nice with Tiktaalik, however, is that the material is better preserved so it's giving us a clearer picture of how the braincase is being rebuilt during the fish-to-tetrapod transition," Ahlberg said.

"Tiktaalik shows a somewhat more advanced stage of this rebuilding than Panderichthys, so it's another valuable piece in the puzzle," he added.

The appearance of the first four-legged vertebrates was a drawn-out evolutionary process that "didn't happen all in one go," Ahlberg said.

"It's very much a matter of comparing one animal with another and putting changes in context rather than focusing on your individual missing link," he said.

Downs, the study author, said the way Tiktaalik fits so neatly into the fossil record perfectly demonstrates "the predictive power of paleontology."

"What's most surprising about Tiktaalik is that, really, it's not surprising at all," he added.
 

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