The fossil of a pregnant plesiosaur who died 78 million years ago indicates that the ancient sea monsters were surprisingly like today’s marine mammals.
The aquatic, carnivorous reptiles gave birth to live young, scientists report August 12 in Science. And they probably birthed just one plesio-baby at a time – one very big baby, estimated to be more than 40 percent of the mother’s body length at birth. Putting so much effort into a single offspring suggests that like today’s marine mammals, plesiosaurs offered a bit of postnatal maternal care.
“If you’re going to put all your eggs in one basket, then you’re going to want to take care of that egg when it comes out,” says paleobiologist F. Robin O’Keefe of Marshall University in Huntington, W.Va.
Scientists had long suspected that plesiosaurs were viviparous and birthed their young in the sea, since plesiosaurs couldn’t easily haul themselves onto land, lay eggs, and nest. And other Mesozoic aquatic reptiles, such as ichthyosaurs, were viviparous – as are many of today’s reptiles.
But it wasn’t until O’Keefe peered at a rather remarkable fossil that scientists had some tangible support for the water-baby theory.
Last year, Luis M. Chiappe, director of the Dinosaur Institute at the Natural History Museum of Los Angeles County, asked O’Keefe to look at and reconstruct an enormous Polycotylus latippinus fossil. Excavated in Kansas nearly 25 years ago, the fossil was scheduled for display at the museum. Since the late 1980s it had been packed away in a basement – “a buried treasure, in a way,” O’Keefe says.
P. latippinus is the short-necked, large-headed kind of plesiosaur – not the long-necked, Loch Ness monster look-alike. Called “Poly” by the team, the fossil was huge and nearly complete, with bones in excellent condition.
At first, O’Keefe recalls, “all I did was walk around it for about 45 minutes with chills, because it was just so awesome. It’s a totally wicked fossil.”
But 4.7-meter-long Poly wasn’t alone. She was carrying a fetus, now a small pile of underdeveloped bones spilling from her body and mingling with the remains of her right flipper.
O’Keefe found that the fetal bones were the same species as Poly, not the remains of another kind of animal she had ingested. But the most important evidence for internal development came from the fetal pelvis stuck to the inside surface of its mother’s coracoid, a large bone. “That’s really critical,” he says. “Otherwise, it would have been hard to say for sure where the embryo came from.”
Though there is some ambiguity about the original positioning of some of the bones, says paleobiologist Ryosuke Motani of the University of California, Davis, “the authors show an overlap between the adult bone and the juvenile bone in a convincing way. It makes sense for plesiosaurs to have a baby in the body.”
O’Keefe estimates that the 1.5-meter-long fetus was roughly two-thirds of the way through gestating based on developmental progressions that have been observed in a related species, nothosaurs.
“The preservation is outstanding, but tantalizing for what is missing,” says paleobiologist Michael Caldwell of the University of Alberta in Edmonton. “The adult is perfect but the embryo is quite incomplete. And so we are missing the interesting juvenile features that would tell us more about plesiosaur [development].”