By Zoë Corbyn of Nature magazine

A fossil of a plesiosaur, an extinct marine reptile, has revealed that not only did these animals give birth to live young, they may also have cared for their offspring in a manner similar to today's whales and dolphins.

The 78-million-year-old fossil, of a four-flippered giant belonging to the species Polycotylus latippinus, had lain unexamined in a museum basement for nearly 25 years.

Other extinct marine reptiles, such as ichthyosaurs, mosasaurs and choristoderans, were known to give birth to live young, a strategy called viviparity. This is the first evidence that plesiosaurs did the same, rather than hatching their offspring from eggs on land.

"We have known about plesiosaurs for almost 200 years, but despite an excellent fossil record we have never found a pregnant plesiosaur before," says palaeontologist Robin O'Keefe at Marshall University in Huntington, West Virginia, who analysed the fossil with Luis Chiappe at the Natural History Museum of Los Angeles County in Los Angeles, California.

Unlike other ancient marine reptiles, which gave birth to many small babies, thePolycotylus fossil shows just one big fetus, 150 cm long, inside a mother 470 cm long. The fetus, which has 20 vertebrae, shoulders, hips and paddle bones visible, is thought to be about two-thirds grown. The finding is published today in Science.

Modern animal mothers that have a few, large offspring, such as humans, elephants, and whales and dolphins, invest a lot in parental care. "If you are going to put all your egg in one basket by having a single large baby, it makes a lot of sense that you would want to take care of that baby," says O'Keefe.

Plesiosaurs probably did the same, he speculates, and may have lived in groups for protection, making their social lives more similar to those of modern marine mammals than to other extinct marine reptiles. The alternative, that the baby would have been born ready to take care of itself, doesn't make sense, he says, because the fetus's unfused bones suggest that it would not have been physically independent at birth.

A few living reptiles, such as some skinks, also give live birth to a small number of large young. They too exhibit mammal-like social behaviour, notes O'Keefe.

On display

The specimen is "spectacular", says Michael Everhart, an expert in prehistoric marine reptiles at the Sternberg Museum of Natural History in Hays, Kansas. "It has answered a question we have had for years," he says.

But Everhart also believes it is premature to conclude that plesiosaurs' social and reproductive lives were dramatically different to those of other extinct marine reptiles. "I would like see a half dozen more specimens first," he says.

A private fossil hunter found the plesiosaur in Kansas in 1987. It was donated to the Natural History Museum of Los Angeles County, where it lay unprepared. "There has always been scuttlebutt about the pregnant plesiosaur fossil in the basement," says O'Keefe.

Last year, it was decided that the fossil should be displayed and the necessary funding for its preparation and display was obtained. The plesiosaur has just gone on view in the museum's new dinosaur hall.

This article is reproduced with permission from the magazine Nature. The article was first published on August 11, 2011.