A pregnant sea monster that died about 78 million years ago may have solved a mystery that scientists have pondered for almost 200 years.

The fossilized remains of a plesiosaur were unearthed in Kentucky carrying a large fetus.

It's the first expectant plesiosaur mother to be found since the species was discovered almost two centuries ago.

Study researcher Frank O'Keefe, of Marshall University in West Virginia, said, “It demonstrates that the plesiosaur gives live birth and did not crawl out on land [to lay eggs]. It puts this 200-year mystery to rest.

“The really interesting thing is how big this bouncing baby is. It's really large by reptilian standards, by human standards, by any standards you use.”

The fact that the expecting mother only carried one offspring, and the sheer size of the fetus, indicate that the marine reptiles gave live birth.

Mr O'Keefe also said that the plesiosaur may have invested much more time and energy into nurturing their offspring than other marine reptiles at the time, similar to how humans invest years raising their kids.

He said, “When the thing is born, you have all your eggs in one basket, so you are going to want to take care of it.”

The plesiosaur mother, of the species Polycotylus latippinus, was about 15.4 feet (4.7 meters) long and was carried a 5-foot (1.5 m) fetus.

Mr O'Keefe told LiveScience.com that: 'This animal is not ready to be born. It's about two-thirds done. It would have been a couple meters [6.5 feet] long by the time it was born.'

Several areas of the fetal skeleton hadn't fully turned into bone, including the skull, which suggests that the unborn plesiosaur wasn't done gestating.

The fetus had disproportionally short flippers and a large head as well, another sign it wasn't fully developed.


'That's what really strikes you about this baby, how not ready for prime time it is. It wouldn't have been able to protect itself or eat,' Mr O'Keefe added.

The finding ― detailed in the Aug. 12 issue of the journal Science ― suggests that this species at least gave birth to live young.

A method of child-bearing called viviparity, live birth has been observed in other marine reptiles from this period, but in past examples, multiple smaller offspring were birthed, often less than 30 percent of the size of the mother.