Those were hard-headed fish that stuck their necks out to move from water onto land. And now fossils of a creature with fins in both worlds show how its hard head and neck evolved.
That first neck fossil belongs to Tiktaalik roseae, a scaly, fishy, shallow-water predator that grew up to nine feet long, says codiscoverer Neil Shubin of the University of Chicago and the FieldMuseum. Roughly 375 million years old, Tiktaalik fits into a gap in the fossil record of evolutionary changes between ancient fish and four-legged creatures on land.
Shubin and his colleagues first unearthed Tiktaalik specimens in 2004 but have only now finished cleaning rock out of the skulls and piecing together details of structures inside the head. The switch from fish head to the tough, look-around head of a tetrapod on land was gradual, the team reports in the Oct. 16 Nature.
“It wasn’t quite as sudden a change as originally thought,” says Jenny Clack of the University Museum of Zoology in Cambridge, England, after looking at the new paper.
“Head skeletons are among the first changes you can see in the direction of tetrapods,” she says. Legs get a lot of attention in the popular press as the big innovation for colonizing land. Yet heads and necks make a big difference to animals poking their heads out of oxygen-poor water or lugging their bodies across land.
“Think about doing a push-up,” Shubin says. That’s what early fish did when rising out of water, and they gained an advantage over less-flexible species if they could move their heads without reorienting the whole fronts of their bodies. Earlier work on Tiktaalik showed that its head bones weren’t fused to its shoulders — it had the first known neck.
Another trend to look for in the shift to land is a tougher skull with fewer moving parts, according to Shubin. “When you feed in water, you gulp,” he says. In contrast, land animals need a skull able to deliver a powerful bite for grabbing food.
He evokes the image of a crocodile at lunch. “It’s chewing prey that’s writhing,” Shubin says. Fish have gulpers’ skulls with lots of parts that can shift around, but writhing prey would twist such skulls to pieces.
Painstaking removal of the rock around a Tiktaalik fossil skull now reveals the inner details of a lineage evolving less fishy heads. The big boomerang-shaped hyomandibula bone in fish connects a lot of moving parts of the skull. Tiktaalik’s hyomandibula, however, has lost some of its connections and shrunk to one-third scale (compared to fish). In modern land animals, the equivalent bone appears as just a little bone in the ear, the stapes.
Tiktaalik had gills and lungs, Shubin says. Removing rock has revealed fossils of the gill skeleton. That view ranks as a highlight of the new work for Per Ahlberg of Uppsala University in Sweden, who studies a different species from the transition between water and land.
The new view shows gills in transition too. They’ve lost the fishlike bone flap, the operculum, which enhances the flow of water over gill tissues. “Tiktaalik was already specializing in breathing air,” Shubin says. And that shift to air was playing an important role in gradually changing the old fish head.
The changes were taking place in fresh water not far from the equator, Shubin says. Since then, land masses have migrated, and Shubin and his colleagues discovered their specimens on Canada’s Ellesmere Island, some 600 miles above the Arctic Circle.