Birth of a Baby

New evidence shows that plesiosaurs gave birth to live young

August 15, 2011

The fossilized bones of a female Polycotylus plesiosaur show that the giant reptile was pregnant when she died more than 70 million years ago.

For a long time researchers wondered whether plesiosaurs—large ocean-going reptiles that lived when dinosaurs existed—laid eggs like other reptiles, or gave birth to live young like whales do today. There had been evidence of live births in an ancestor of plesiosaurs, but the lack of proof for plesiosaur birth was puzzling. Now, new fossil evidence suggests that the whale model is the winner.

Pregnant Plesiosaur

The fossil was uncovered by Marshall University Associate Professor F. Robin O'Keefe and Luis Chiappe of the Natural History Museum of Los Angeles County. It shows a fetus inside of a female plesiosaur called Polycotylus.

Professor R. Ewan Fordyce of the University of Otago, New Zealand, says the fossils inside the female plesiosaur were the right size and in the right place. He claimed that the researchers showed "that this is a fetus and not a young animal that has been eaten."

O'Keefe and Chiappe suggest that plesiosaurs gave birth to live young just like modern whales. Both the ancient plesiosaur and whales of today are large animals that produce large offspring.

University of Calgary professor Anthony Russell says the find is significant. "It would be hard to imagine these animals coming out onto land laying eggs somewhere."

Exciting Evidence

The uncovered fossil is now on display at the Natural History Museum of Los Angeles County. The fossil was originally discovered in 1987 in Logan County in Kansas. It had been stored in the basement of the museum until resources were available to separate the bones for display.

Unlike the typical plesiosaur image of a long neck poking out of the sea, Polycotylus had a short neck with a big head. The fossil is more than 15 feet long, and was dated to between 72 million and 78 million years ago.

Though O'Keefe had seen photos of the fossil before he started working on it, he was still surprised when he first saw it. "I wasn't prepared for the emotional response I had," O'Keefe said. "You don't very often walk up to one and say: 'That is a really cool fossil.'"

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