Evolution Diary

When ancient amphibians evolved to live on land, they were already prepared for the challenge of feeding in air, scientists report.

The skulls of early amphibious tetrapods—the four-legged ancestors of all land animals with backbones—show that the creatures were most likely taking a bite out of their prey.

In contrast, fish most often catch their food by sucking it up from afar.

An early amphibian called  Acanthostega  appears to be crawling from the water in this reconstruction of a fossil specimen found by Cambridge University paleontologist Jenny Clack. Acanthostega is a tetrapod, the evolutionary ancestors of all land animals with backbones.

New analysis of tetrapod skulls shows that biting instead of sucking—a key evolutionary step toward life on land—evolved while the creatures were still largely aquatic.

“This is what we see goldfish doing,” said lead author Molly Markey, a paleontologist at Harvard University in Cambridge, Massachusetts.

The ability to bite was a key step in evolving to live on land, because air is 900 times less dense than water, making suction at a distance virtually impossible, Markey explained.

“The early terrestrial tetrapods had to get up close to their food and bite it directly with their jaws,” she said.

(Related photo: “Giant ‘Terrible Fish’ Packed Most Powerful Bite” [November 29, 2006].)

The findings suggest that biting rather than sucking evolved while tetrapods were still largely aquatic.

Sucking It Up

To investigate the transition from sucking to biting, Markey and colleagues looked at the arrangement of skull bones in living and fossil animals.

“You can think of the skull as a jigsaw puzzle,” Markey said. “Individual bones are the puzzle pieces, and the sutures are the edges where they touch each other.

“The shape of sutures can indicate how the skull deforms a little during feeding,” she said.

Reporting in this week’s issue of the Proceedings of the National Academy of Sciences, the team found that during suction feeding, the skull bones of living fish pull apart at the front and push together at the rear.

“The shapes of the skull sutures reflect these motions,” Markey said.

“We saw these same suture patterns, which indicate suction feeding, in a fossil fish but not in the fossil aquatic tetrapod Acanthostega,” Markey said.

Study co-author Charles Marshall said: “This suggested that sucking wasn’t [the aquatic tetrapod's] primary feeding mode.

“Acanthostega was biting on something,” Marshall added, “but we don’t know whether it caught food underwater or by ambushing terrestrial prey at the water’s edge.”

Story reposted from National Geographic