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.
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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