Ability to Bite Evolved in Fishy Ancestors

In a prior study, the scientists used implanted devices to measure strain along the skull sutures in a primitive freshwater fish (Polypterus). CREDIT: Molly Markey

The ability of ferocious land animals to bite prey evolved in ancient fish, a new study finds.

Fish predominantly capture prey with suction, which can be seen by watching a goldfish constantly puckering its mouth. But land animals can’t use this technique and instead use jaws that clamp together to catch and grasp a meal.

This feeding adaptation is another piece of evidence scientists use to illuminate the evolutionary transition from fish to land vertebrates.

To learn more about this, Molly Markey and her colleague Charles Marshall, both of the Museum of Comparative Zoology at Harvard University, analyzed fossil skulls from an extinct amphibian (Phonerpeton) that lived mostly on land, an earlier extinct amphibian (Acanthostega) that lived mostly in the water and a fishy predecessor, Eusthenopteron.

In particular, they measured the contours of the seams between adjacent skull bones at the roofs of these skulls. Called sutures, these junctures are lined with stretchy collagen, and the bony plates slide miniscule amounts relative to one another when an organism eats.

The scientists then compared the sutures on the extinct creatures to those found on the skull of a living freshwater fish, Polypterus, which they had measured in a previous study. Polypterus uses suction to capture prey, thus its skull sutures gave the researchers a baseline for what a suction-organism’s skull should look like.

"A biting or chewing motion would result in a faint pushing together of the frontal bones in the skull, while a sucking motion would pull those bones ever so slightly apart,” Markey said.

“By comparing the skull roofs of living fish to those of early amphibians and their fishy ancestors, we were able to determine whether the fossil species fed by suction or by biting,” she added.

Biters v. suckers

The suture patterns from Eusthenopteron, a species of lobe-finned fish that lived about 380 million years ago, matched those of suction feeders.

But analysis of the early amphibian Acanthostega showed that, while it had many fish features , it was more likely a biter than a sucker.

“Even though they spent a lot of time in the water, [the earliest amphibian ancestors] were biting on their prey, which is a prerequisite to capturing prey on land,” Markey told LiveScience.

Aquatic biting jaws

This is interesting, Markey said, because it suggests early amphibians inherited their biting jaws from ancestors who lived solely in the water.

The findings, detailed this week in the online edition of the journal Proceedings of the National Academy of Sciences, might help answer an old scientific question: Did fish make the move to land to escape from predators or to exploit new food sources?

“Our findings do support the idea that they came on land to exploit new food sources, but we’re not sure,” Markey said.

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