Mama Mia. Like many modern marine mammals, plesiosaurs likely gave live birth to only one or a few babies at a time and may have cared for the youngsters like orcas or seals too.

Illustration by S. Abramowicz; both images © Natural History Museum of Los Angeles County

Sea Monster Had a Bun in the Oven

When paleontologists first dug up the fossilized remains of a plesiosaur nicknamed Poly on a Kansas ranch in 1987, they knew they'd discovered something odd. This relic from the age of dinosaurs looked like two animals: one big plesiosaur—a fierce reptile a bit like a cross between a long-necked dinosaur and a seal—and another, much smaller jumble of bones. Now, researchers have identified the mystery creature: Poly the plesiosaur was pregnant.

Pregnant marine reptiles aren't too unusual, says Xiao-Chun Wu, a paleontologist at the Canadian Museum of Nature in Ottawa, who was not involved in this study. Dinosaurs living on land may have preferred laying eggs, but their aquatic kin, more closely related to modern lizards than to Tyrannosaurus rex, had given up the terrestrial life. The flippered plesiosaurs, in fact, seemed incapable of hauling themselves out of the water to find a spot on land, where it's much safer for an egg. Researchers had discovered a gaggle of other pregnant marine reptiles, including tunalike ichthyosaurs lugging around over a dozen fetuses. But they had yet to find a single mama plesiosaur and could only speculate on how these predators that dominated the ocean from more than 200 million years ago reproduced.

That fossil had been a long time coming. More than 2 decades after Poly's discovery, curators pieced together her skeleton, which belonged to a species called Polycotylus latippinus, for an exhibit that opened last month at the Natural History Museum of Los Angeles County in California. Then it became clear that this 5-meter-long dino mom had a sharp-toothed bun, not a meal, in the oven, says F. Robin O'Keefe, a paleontologist at Marshall University in Huntington, West Virginia.

For starters, the smaller plesiosaur with its stubby limbs and big head just looked like a fetus, the team reports online today in Science. The creature's backbones, which in a mature juvenile would likely be solid chunks, were still separated into tiny pieces, he says. The little reptile had also been inside its mother when they met their end from an unknown fate—a thin layer of rock had fused the baby's pelvis to the inside of its mother's shoulder blade, something that wouldn't be possible if the two had died side by side. O'Keefe estimates that the young water dino was only about two-thirds developed. "It's not ready for prime time," he says.

Still, it was hardly a small fry. When the baby plesiosaur came to term, it could easily have hit 1.5 meters in length, or 35% of its mother's total reach, O'Keefe suspects. One big offspring at a time isn't very reptile, he adds. Most known lizards, snakes, and even ichthyosaurs pump out many, even dozens of babies all at once. Marine mammals such as orcas, in contrast, have fewer young but also tend to be more diligent parents, suggesting that the fierce plesiosaur may have been a nurturer.

Plesiosaurs aren't orcas, but a few living reptiles also partake in mammalian-style baby making and rearing, O'Keefe says. Some species of Australian skinks, for instance, live in warrens with as many as 17 relatives. Their stable family home makes child care convenient. Poly's briny lagoon, which extended along what's now the Mississippi River into Kansas, may have been equally calm, he says: "It is plausible that plesiosaurs lived in some fairly stable environments in which they didn't move around too much."

Kenneth Carpenter, a paleontologist at Utah State University in Price, however, doesn't think Poly should get the mom of the year award just yet. The young reptile in her abdomen is missing a few bones, and he suspects that Poly lopped them off when she started to chow down. Many modern-day reptiles similarly feast on juveniles, even those from the same species: "This is a stronger case for cannibalism than it is for live birth."

Digesting or gestating, Poly is now on display for all to admire, or judge, in Los Angeles.