sea reptile egg laying
© Marshall UniversityFossilised remains: A pregnant sea monster that died about 78 million years ago carrying a large foetus - the first expectant plesiosaur mother to be found since the species was discovered almost two centuries ago
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 fossilised remains of a plesiosaur were unearthed in Kentucky carrying a large foetus.

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 foetus, 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 15.4ft long and was carried a 5ft foetus.

Mr O'Keefe told 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 foetal skeleton hadn't fully turned into bone, including the skull, which suggests that the unborn plesiosaur wasn't done gestating.

The foetus 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 journal Science - suggests that this species at least gave birth to live young.

plesiosaurs live birth
© Press AssociationNurturing mother: The discovery suggests plesiosaurs lived in social groups and engaged in extended parenting similar to whales and dolphins
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.

Birthing just one big baby is rare for this period. It suggests this mother reptile would be investing lots of resources into caring for her 'only child.'

This high maternal investment strategy - known as 'K-selected life history' - is at one end of the life-history continuum; the other extreme is the 'r-selected' variety, in which many offspring receive little or no parental investment after birth.

Environment is one factor that influences maternal investment. If the environment is stable then a higher investment in a lower number of offspring pays off because they are less likely to be lost.

If the environment harsh or undergoing changes, making lots of babies is a better strategy. That way if one dies, there are others to pass on one's genes.

Xiao-chun Wu, a researcher at the Canadian Museum of Nature told that: 'Compared with the Triassic, the Jurassic and the Cretaceous had relatively stable environments. It is very possible that some plesiosaurs if not all were K-selected, although more solid evidences are needed.'

The K-selected life history suggests that the plesiosaurs may have lived in familial social groups and engaged in the kind of extended parenting found in whales and dolphins.

The fossil is currently on display at the Natural History Museum of Los Angeles County.