Pregnant Sea Monster in LA

The first and only pregnant plesiosaurus fossil was dusted off and is on display at the Natural History Museum.

A search in the average basement will probably yield old records, a bicycle with deflated tires, maybe even a photo album.

But these timeworn mementos generally don’t date back 78 million years, unless you’re looking in the Natural History Museum’s basement.

That's where the first and only pregnant plesiosaurus fossil was kept for more than two decades, until museum scientists decided to dust it off.

"We knew this was an important specimen that needed to be displayed," said Luis Chiappe, director of the Dinosaur Institute at the Natural History Museum of Los Angeles.

The fossil, nicknamed Poly, depicts a 15-foot long plesiosaurus carrying a two-thirds developed baby in her womb, according to the degree of ossification, the process that turns cartilage into bone.

The first fossil of this giant marine reptile, historically referred to as a sea monster, was discovered 200 years ago.

Until now there had been no evidence of how the four-flippered reptiles from 200- to 65-million years ago advanced their species.

Scientists were previously unsure whether the carnivorous sea animal reproduced by crawling onto land to lay eggs, like other reptiles, or gave birth in the water, like whales.

"This is a confirmation of a suspicion, and a radical rewriting of (scientific knowledge)," said Dr. Frank R. O’Keefe, associate biology professor at Marshall University in West Virginia and lead scientist on the plesiosaurus fossil.

This method of reproducing differs from that of the plesiosaurus' closest relatives, lizards and snakes.

Rather than go the reptilian route, hedging their bets by producing multiple offspring, plesiosaurs focus all of their reproductive energy toward a single large baby.

This kind of reproduction suggests that plesiosauri were social creatures, similar to modern whales and dolphins, O’Keefe said.

Found in Kansas, the fossilized fetus (pictured, right) is about five feet long, and O'Keefe said it would’ve grown to six feet at full term.

"That would be the equivalent of a human giving birth to a 6-year-old," he said.

Scientists at the Dinosaur Institute knew it was a unique fossil when they acquired it in 1987, however, it wasn't until the creation of the Dinosaur Hall that they really had a reason to reexamine the fossil, Chiappe said.

Scientists wanted to make sure what looked like a baby plesiosaur was not a meal. Because there were no marks that would suggest chewing and no evidence of digestion, the remains found in the specimen were definitely those of an embryo, Chiappe said.

Follow NBCLA for the latest LA news, events and entertainment: Twitter: @NBCLA // Facebook: NBCLA

Contact Us