For 25 years, a fossil of a plesiosaur, a gift from a private dinosaur hunter in Kansas, collected dust in the basement of the Natural History Museum of Los Angeles. When the fossil was finally pulled out of the basement and prepared for display, scientists noticed that the plesiosaur had a passenger— the fossil of a fetal plesiosaur inside her, making this the first evidence that plesiosaurs gave birth to live young.
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, the Polycotylus 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 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.
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