New details on how fish got tough enough for land
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
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
“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.
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
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
Downs, Jason P. et al. 2008. The cranial endoskeleton of Tiktaalik roseae. Nature vol. 455 (October 16), 925 -929.
Tiktaalik pictures, videos of talks and interviews, including segments from the Colbert Report:
Perkins, Sid. 2006. Amphibious Ancestors. Science News
Vol.169 #24 (June 17th), p. 379.
Perkins, Sid. 2008. Fossil helps document shift from sea to land Web edition : Wednesday, June 25th, 2008 [Go to]
Daeschler, Edward B., Neil H. Shubin and Farish A. Jenkins, Jr. 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature. 440(6 April) : 757–763. doi:10.1038/nature04639.