In the history of terrestrial animals, few events are more significant than the first time a fish stepped onto land. The creature wouldn't have lasted a day on terra firma, however, had it not evolved a whole new way of eating. Now, thanks to a novel analysis of fossil fish, biologists have a better idea of when this critical adaptation first appeared.
Most fish have teeth, but few use them to catch prey. The majority of fish feed by sucking in a mouthful of water--and their hapless lunch along with it--occasionally wielding their teeth to get a grip on big meals. But on land, thin air makes suction feeding impossible, so vertebrates need to bite to get by. Because fossil fish can hardly demonstrate their feeding style, palaeontologists have looked to changes in the jaw, teeth, and gills for evidence of the sea-land transition. Based on these data, scientists believe that the earliest four-legged vertebrates--or tetrapods--developed the ability to bite some 350 million years ago, in the Carboniferous period. But this timeframe has been hard to verify, says Harvard palaeontologists Molly Markey and Charles Marshall, because jaw shape and tooth type don't always correlate to whether a fish bites or sucks.
The team decided to see whether skull sutures, the line where two bones join, offer a more reliable indicator. A suture deforms according to how its attached muscles pull on the skull: When suction feeders attack, the pressure drop in the mouth pulls apart the plates at the front of the skull. Biters, in contrast, push those same plates together. Over time, the repeated stress leaves its mark on the bones.
Markey and Marshall tested their theory on Acanthostega, a Devonian-era fish whose paddlelike limbs peg it as a transitional form between sea and land vertebrates. The team compared deformation patterns in Acanthostega skulls to three other fish: the aquatic suction feeder Eusthenopteron and the biting, mostly terrestrial Phonerpeton--both extinct--and the extant suction-feeder Polypterus. Although the limbs and gills of Acanthostega suggest the fish spent most of its time in the water, skull stress analysis indicates that it bit, the team reports online this week in the Proceedings of the National Academy of Sciences. As the species is known to have lived 375 million years ago, the findings indicate "more definitively when we first got prey capture by biting," Markey says.
The study is "pretty cool, in the sense it confirms what we think of these animals," says Robert Reisz, a vertebrate palaeontologist at the University of Toronto in Canada. He adds the adaptations make sense for predators who lived in shallow water and were gaining terrestrial traits, even if the animals were not yet capable of living on land.