They were unwieldy pond-dwelling predators with long necks that did not have enough joints and neck ribs that must have kept their heads practically immobile. They had all the grace of Godzilla -- the 1950s version. For decades, scientists wondered how they caught enough prey to survive.

"They are really grotesque creatures," said paleontologist Olivier Rieppel of Chicago's Field Museum, who has studied the ancient reptiles for 25 years. "This neck is not going to be very flexible."

But in research reported yesterday in the journal Science, Rieppel and co-authors Chun Li of China's Institute of Paleontology and Paleoanthropology and Michael C. LaBarbera of the University of Chicago said a newly discovered fossil suggests that these animals, known collectively as protorosaurs, used their long necks not as supple spears but like lethal suction hoses.

"You bend the neck, the ribs splay out and the esophagus widens, creating a vacuum," Rieppel explained in a telephone interview. The protorosaur drifted over an unsuspecting fish, sucked it up and swallowed it whole before the victim realized what was happening.

The protorosaur case is a classic example of one of evolutionary biology's eternal mysteries: Does a particular characteristic develop to serve a specific function, or is it an aberration caused by growth patterns or some other factor? In the protorosaurs' case, what seemed at first like an aberration probably turned out to be a functional trait.

"Darwin himself was a bit complex on this," said evolutionary biologist Lawrence Witmer in a telephone interview from his Ohio University office. "He believed in natural selection, of course, but he also said that if everything was perfectly functional, there would be no evolution. There may be no functionality" to a given characteristic.

And it is hard to be sure. Owls have outsize eyes, an obvious adaptation for an animal that hunts at night, Witmer said. Tyrannosaurus rex, on the other hand, "seems like it had tiny eyes" relative to its huge skull, Witmer said. "Maybe they just grew slower than the skull, or maybe the eyes are functionally right-sized -- we don't know."

In coming up with their new view of the peculiar protorosaur's feeding habits, Rieppel and his colleagues studied a 230 million-year-old fossil skeleton of a species known as Dinocephalosaurus orientalis, discovered recently in a limestone deposit in southern China. The name means "terrible-headed lizard from the Orient."

It had a 51/2-foot neck and a snakelike head with fangs. Its trunk was about three feet long, and it had four paddlelike feet, indicating that it was a fully aquatic animal.

The newfound fossil was similar to Tanystropheus longobardis, a European protorosaur that had puzzled scientists -- including Rieppel -- since its discovery 150 years ago. Its giraffelike, ribbed neck had 12 vertebrae, making it so stiff-necked that its head was virtually locked in place.

"We concluded in the 1980s that the neck had no function," Rieppel said. "It simply grew faster than the body."

But the fossil in China has 25 vertebrae on a shorter neck, providing greater flexibility -- an indication of functional adaptation: "It was clear that [the neck] has to do something," Rieppel said.

To be successful, any aquatic predator must eliminate a "pressure wave" that alerts an intended victim when a snout suddenly thrusts through the water. "Feeding in a dense medium is a tricky thing," Rieppel noted.

Fish overcome this by swallowing water along with their prey, while crocodiles slash horizontally, cutting the water instead of pushing it forward.

D. orientalis was an awkward animal whose ribbed neck would not have allowed it to lift its head from the water to breathe. Instead, Rieppel said, it floated along the water's surface with head and neck extended.

Once it spotted its prey, it would flex its neck as it attacked, enlarging its gullet to create suction, enabling it to vacuum up the target without creating a pressure wave. "Evidently this animal could function," Rieppel said. "Finding him brought the other guy [Tanystropheus] back into focus."

An artist's rendition of Dinocephalosaurus orientalis, whose 25-vertebrae neck is believed to have factored into its ability to prey on fish.