When Fish First Started Biting (4/18/2007)

Charles Marshall admires the skull of a Phonerpeton, one of the first creatures to evolve teeth, which allowed animals to catch prey on land. The skull is about 275 million years old. - Photo Courtesy: Jon Chase/Harvard News Office

Before fish began to invade land, about 365 million years ago, they had some big problems to solve. They needed to come up with new ways to move, breathe, and eat.

Take the latter, for example. Fish usually pucker up and suck prey into their mouths. But air is 900 times less dense than water, so land-livers must bite into their food to get a meal. Researchers at Harvard University have just completed a study that gives a clear picture of how that change was made.

"Aquatic creatures developed the tools they needed to feed on land before they completely left water," notes Molly Markey, a lecturer on earth and planetary sciences. "Our research suggests that these first tetrapods, four-footed animals, bit on prey in shallow water or on land. Although they may have occasionally captured a meal by suction."

To become biters, the invaders had to change their teeth and skulls, and learn to walk. Along with Charles Marshall, a professor of biology and of geology at Harvard’s Museum of Comparative Zoology, Markey compared the boney remains of a 365-million-year-old fish named Eusthenopteron, two ancient tetrapods called Acanthostega and Phonerpeton, and a modern fish. The salamanderlike Acanthostega spent much of its life in the water, Phonerpeton lived on land. Both Acanthostega and Eusthenopteron possessed lungs and gills, so they could breathe air or water, like today’s lungfishes. All three ancients boasted pointed teeth, indicating that they were meat-eating predators.

Studies done by Jenny Clark at Cambridge University in England show that Acanthostega had short legs that stuck out to its sides, ending in what look like webbed toes. Such limbs would not be very supportive, so it’s likely that the old tetrapod slithered or scooted, rather than walked, when it ventured on land.

Slithering and chewing

One big question is why Acanthrostega and its relatives left their aquatic domain in the first place. Were they trying to get away from bigger predators, or were they looking for new prey to feed on? "It’s likely that both reasons are true," Markey says.

Markey and Marshall compared models of the ancient tetrapods and Eusthenopteron, the fish that stayed at home. They published their findings in the April 16 online edition of the Proceedings of the National Academy of Sciences.

The comparison found that the key to evolving from sucking to biting lay in the tops of the animals’ skulls. These boney skull roofs, rather than being solid, were made up of lots of different pieces. Markey compares them to pieces of a jigsaw puzzle. "Imagine that skull bones are puzzle pieces," she explains. "Places where they touch each other are known as sutures, and the bones can move around them a bit. The sutures get wider or narrower depending on motions such as chewing."

By analyzing sutures in the skulls of the ancient tetrapods and fish, then comparing them with those in a living fish, the researchers could determine how the skull roof deformed under the compression and tension of eating. Such analyses led to the conclusion that Eusthenopteron was a sucker and the awkward-moving Acanthostega was a biter — perhaps the first one in the animal kingdom.

Think of that next time you suck in strands of spaghetti or chew on a piece of chicken.

Note: This story has been adapted from a news release issued by Harvard University

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