Pregnant Plesiosaur Fossil May Shed Light on Ancient Animal’s Behavior
Photo by Natural History Museum of Los Angeles County
Editor’s note added on Aug. 12 at the end of this post
Scientists have pieced together the first-ever fossil of a pregnant plesiosaur, a giant Mesozoic sea reptile from the Cretaceous Era, with an embryo still inside.
The animal, which roamed the seas 78-million years ago, is about 15-feet long — the length of a minibus — and bones indicate that the baby plesiosaur would have been as long as six feet when born.
The finding, published in the online edition of the journal Science on Thursday, is definitive evidence that the giant sea creature gave birth to single, live offspring, rather than laying eggs, like most reptiles, says paleontologist and lead author F. Robin O’Keefe. This reproductive behavior also indicates that the animals were gregarious social creatures that cared for their young, similar to toothed whales or dolphins, according to the paper.
“What is earth-shattering is that plesiosaurs are doing it differently than other reptiles,” O’Keefe said. “Instead of having lots of little babies, they’re having one big baby — a single, very large fetus.”
O’Keefe had heard rumors about a pregnant plesiosaur encased in rock in the basement of the Natural History Museum of Los Angeles County long before he was invited by the curator of the museum’s Dinosaur Institute to lead the study. “It was scuttlebutt for years,” he said.
The fossils were discovered in 1987 by the Bonner family, a family of fossil collectors in West Kansas. The adult animal was mostly complete, but without head or neck. The embryo consisted of “a mass of poorly ossified and largely disarticulated bones spilled from the body cavity of the adult,” according to the study.
Several pieces of evidence pointed to these bones belonging to an embryo. They adhered to the bones of the adult, were of the same species, suggesting they weren’t food, and were less than fully formed.
Another important indicator was that most of the bones hadn’t ossified, meaning they hadn’t yet developed from cartilage to bone. “Most of the bones in the body are laid down as cartilage first, and then they ossify later,” O’Keefe said. “One way we can tell the state of an embryo is how well ossified it is.”
Each of the fossilized bones were painstakingly cleaned, prepared and preserved. Then O’Keefe and team reconstructed the adult mother and embryo using measurements of the vertebrae of both, along with data from other plesiosaur fossils and sea reptiles from the era. The whole process took more than two years.
“I’ve seen a lot of fossils in my career, and this was a fossil that gave me the chills,” O’Keefe said. “A big, mostly complete fossil with a baby inside of it — that’s awesome.”
They estimate that the embryo was about two-thirds mature — at the end of the second trimester in human terms. The paper also suggests that the animal was social and cared for its young.
“The fact that the animal gave birth to single large babies indicate that the mother also offered parental care after the baby was born,” said Luis Chiappe, curator at the Dinosaur Institute, where the fossil is now on display, and an author of the study. “Whales and dolphins hang out next to the mother for a long time, years sometimes, and are protected by the mother. And living whales and dolphins are very social animals.”
As to how a paleontologist can make a leap from these aquatic mammals to plesiosaurs: “You crawl out on a limb with a saw in your hand,” O’Keefe joked.
Since toothed whales are mammals, not reptiles, he said, a more helpful comparison to the plesiosaur may be the monkey skink or the shingleback lizard in the Egernia group. These green, scaly lizards give birth to live offspring, one or two at a time, and are among the few known reptiles to function within a social group and care for their young.
Anthony Russell, professor of biological sciences at the University of Calgary, said he felt that another appropriate comparative model was missing: sharks.
Sharks, like plesiosaurs, are top predators with single live births, and while they are social creatures, the young are not nourished by the parents in the same way as mammals. “Why wouldn’t they live like sharks?,” Russell said. “We always have that problem in the fossil record when we’re trying to reconstruct life history patterns of an organism that we don’t have a good model for today. It’s possible that [plesiosaurs] could have done something very different, and that then the behavior died out with the group.”
It’s an absolutely valid and excellent point, O’Keefe said in response, but added that in the paper, they chose to focus on examples that they felt provided the best ecological comparison – toothed whales – and examples that were the most closely related on the evolutionary tree: scincid lizards. “Sharks are very far away phylogenetically,” he said. “In fact they’re as far away as you can get.”
The reconstructed fossil with embryo is now on display at the Dinosaur Institute at the Natural History Museum of Los Angeles County.
Illustration by Stephanie Abramowicz, Natural History Museum of Los Angeles County
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