Motherly love in the Cretaceous ocean?
Since they were discovered 200 years ago, the plesiosaurs have posed a riddle. Long, brawny, toothy, their skeletal architecture was unsuited to laying eggs and sitting on a nest — and yet, there was no evidence that these reptiles gave birth to live young.
In the journal Science tomorrow, a pair of paleontologists will describe a stunning fossil that shows a maturing plesiosaur inside its mother’s abdomen.
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The fossil, dating to 78 million years ago, had lain in a museum basement for many years, says first author F. Robin O’Keefe, professor of biological science at Marshall University in Huntington, West Virginia. “Finding a pregnant animal fossil is always really rare, for any group of aquatic reptiles, and finding an undisturbed specimen is very unusual.”
O’Keefe collaborated with Luis Chiappe, director of the Dinosaur Institute at the Natural History Museum of Los Angeles, in studying the fossil, which is part of a new dinosaur-era exhibit at the museum.
Common, but curious
The plesiosaurs were monsters of the deep, says O’Keefe, having lived from roughly 200 million years ago until they went extinct along with the dinosaurs 65 million years ago. “These were apex predators, killer-whale size, and there is a very long, diverse fossil record.”
The specimen in question was complete, except for the head and some neck vertebrae, and was about 4.65 meters long.
Although the fetus was no midget – at 1.5 meters long — it was not ready to be born, O’Keefe says. “It’s really a guesstimate, but we think it is maybe two-thirds developed, definitely not ready for prime time. We have a bit of the back of its skull, and it’s poorly ossified [hardened]. If it was born like that, it would be like having your head made of Play Doh. It had no teeth, tiny flippers, and could not move around.”
The Cretaceous ocean, with its range of giant predators, was “not a place to be that helpless.”
Pride of the plains?
The pregnant plesiosaur was excavated in 1987 by Kansas landowner Charles Bonner. “He knew at the time that this was something interesting, but when it comes out of the ground, it’s inside a plaster jacket,” O’Keefe says. Removing a 15-foot specimen embedded in rock “is a time-intensive and expensive operation.”
But when the natural history museum decided to mount a new paleontology exhibit, the plesiosaur seemed a logical display, and director Luis Chiappe contacted O’Keefe. “He knew it was something interesting, thought it’s maybe a baby,” says O’Keefe. “Would I be interested in working on the specimen?”
A question answers itself
Would any paleontologist not be? And so O’Keefe found himself handling ancient evidence of reptilian motherhood. “My first thought was not some great scientific thought: ‘It’s really cool, you don’t often see fossils that neat.'”
When the rocks spoke, they revealed that this well-known reptile was, finally, in a maternal mood. “So here we have a pregnant plesiosaur, after 200 years of mystery, we have the smoking gun, we now know they gave live birth.”
To prove that, however, the scientists had to discount alternative explanations for finding an embryo inside an adult of the same species:
Chance: “The mother would have to die, drop to the bottom, her ribs would have to be opened up, and the fetus would have to be expelled from an animal of the same species and fall down into the correct part in the mother, and then be buried,” O’Keefe says. “That’s not impossible, but we think it’s pretty unlikely.”
Dinner: Was the embedded fossil the larger animal’s last meal? “That’s a good alternative hypothesis, and lot of reptiles are cannibalistic on their young,” says O’Keefe. But if the smaller animal had been dinner, stomach acids would have eroded the cartilage in its skeleton.
Does momma care?
We tend to think of reptiles as egg layers, but various flavors of birthing live young have evolved at least 80 times among marine reptiles. “The mother could retain the egg and have it develop inside her, or it could go all the way to a full-blown mammalian pattern, using a placenta to connect to the uterine wall,” O’Keefe says. “Given how big this fetus was, there probably had to be some pretty significant communication between mother and young.”
Paleontology shows structure, not behavior. O’Keefe says the new find suggests that mother plesiosaurs probably cared for their young, a rarity among modern reptiles. If one offspring “has absorbed all your reproductive energy, it makes a lot of sense to take care of it,” he points out.
In raising this possibility, he says, “We climbed out as far out on a limb as we thought we could get.”
And although there is no suggestion that the plesiosaurs nursed their young, live birth would distinguish them from many reptilian relatives and move them closer to modern, maternal marine mammals like whales and dolphins.
– David J. Tenenbaum