LOS ANGELES — Plesiosaurs — giant marine reptiles that ruled the oceans 75 million years ago — gave birth to single large babies and may even have nurtured their young, according to a new study.

F. Robin O’Keefe, a paleontologist at Marshall University in Huntington, W.Va., and Luis Chiappe, director of the Dinosaur Institute at the Natural History Museum in Los Angeles, teamed up to study the only known fossil of a plesiosaur mother and her unborn baby. The ancient relic is considered the first evidence that these aquatic behemoths gave birth in the water instead of laying eggs on land, the researchers reported online Thursday in the journal Science.

“It’s a really neat specimen,” said Mike Everhart, adjunct curator of vertebrate paleontology at the Sternberg Museum of Natural History in Hays, Kan., who was not involved in the study.

The fossil was discovered by amateur paleontologists Marion and Charles Bonner while hiking in northwest Kansas in 1987. They noticed flat bones sticking out of the shale; these turned out to be the mother plesiosaur’s pelvis. They continued digging and found the creature’s four flippers, ribs, hips, spinal column and part of its neck.

The Bonners had found many fossils before, but “I had an inkling that this was different,” Charles Bonner said. He and his father wanted scientists to study it carefully, so they sent the specimen to the Natural History Museum, where it sat in storage until recently.

In 2008, O’Keefe and Chiappe decided to take a closer look before showcasing it as part of the Los Angeles museum’s new Dinosaur Hall.

The scientists noticed a constellation of small bones spilling over from the larger fossil’s abdomen that appeared to be miniature versions of the adult ones. The similarities suggested that both sets of remains were from the same species.

A flat seashell-shaped bone — part of the fetus’s pelvis — rests on the inside face of the mother’s shoulder bone, indicating that the baby was growing inside its mother when she died.

The edges of the miniature bones don’t appear rounded or corroded, which would be characteristic signs of damage caused by stomach acid, suggesting the small fossil hadn’t been the larger one’s last meal. The most likely interpretation, the scientists concluded, was that the fossil was that of a pregnant plesiosaur with one large baby growing within it.

Based on the size of its bones, O’Keefe and Chiappe concluded that the infant wasn’t fully developed. At birth it would have been about 5 feet long, about one-third the length of its 15 1/2-foot mother.

The baby’s size suggests that plesiosaurs invested a lot of energy in bearing young and didn’t have many offspring at once, unlike turtles and mice. Other species that birth single babies — such as humans, whales, dolphins and certain Australian lizards — form social groups to help protect their young against predators.

“So we made the leap that plesiosaurs were very social, although we have no direct evidence,” Chiappe said.