updated 8/11/2011 6:19:10 PM ET 2011-08-11T22:19:10

Scientists recently got a two-for-one when they discovered the fossils of a huge fetus within the remains of a 78-million-year-old plesiosaur.   

The discovery, described in the latest issue of the journal Science, represents the first known plesiosaur embryo. Plesiosaurs -- now-extinct marine reptiles -- were top sea predators while dinosaurs dominated Earth's land.

The 15.4-foot-long adult female is now the first plesiosaur for which the sex is known with certainty. The findings also provide evidence that these animals gave birth to single, live individuals instead of hatching offspring from eggs on land. Giving birth in this way indicates life was pretty good for the big, predatory reptiles.

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"We speculate that this reproductive mode indicates a sedentary species living in a stable environment," co-author F. Robin O’Keefe, an associate professor in the Biology Department at Marshall University, told Discovery News.

A sometimes-sedentary lifestyle didn't prevent the animals from securing dinner, though.

"Like a lot of large apex predators, these animals would ambush or pursuit predators, basically chasing down large fish, birds, squid, ammonites, or other marine reptiles, then killing and eating them," he said. "Also, like a lot of top carnivores (particularly reptilian ones) they probably ate whatever they could catch, and would not turn up their nose at carrion either."

Other predators within their ecosystem included sharks, mosasaurs, toothed birds and pterodactyls. Plesiosaurs seldom, if ever, interacted with non-avian dinosaurs.

O'Keefe and colleague Luis Chiappe made the embryo discovery after studying the mother plesiosaur, originally unearthed in 1987 by Charles Bonner on the Bonner Ranch in Logan County, Kansas. The researchers believe at least 4 lines of evidence prove the adult plesiosaur fossils include a two-thirds mature fetus.

First, they write that the structure and attachment of the two animal remains establish the "juvenile was both articulated and within the adult body cavity at the time of deposition, before burial." It remains unclear what led to the mother's death.

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Second, both sets of fossils belong to the same species, Polycotylus latippinus. Third, the juvenile skeleton displays embryonic features. And finally, "the juvenile shows no indication of having been consumed by the adult." If that'd been the case, damage caused by stomach acid, gastroliths (stomach stones), and other things would have been evident.

This particular species of plesiosaur had a unique body shape. Its neck "was short by plesiosaur standards," O’Keefe said. "All four limbs are adapted into wing-shaped flippers that the animal used to fly underwater. The tail was short and probably carried a tailfin."

The researchers suspect this plesiosaur lived in social groups and engaged in parental care, which doesn't always happen with marine predators. Great white sharks, for example, cannibalize any competing womb-mates and come into the world toothy and strong with next to zero parental care.

In humans and some other animals, attentive mothering can foster intelligence, since young have a chance to learn while being protected. Plesiosaurs, however, may have been spoiled but still rather stupid.

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O'Keefe explained that "reptiles in general just aren’t intelligence in the mammalian sense. Also, I have seen a lot of plesiosaur braincases, and there is not a lot of room for brain, so the plesiosaurs may have been intelligent by reptilian standards, but I doubt they would have had anything like a mammalian level of intelligence."

The plesiosaur mom and her unborn offspring are now on exhibit in the new dinosaur hall at the Natural History Museum of Los Angeles County.

Chiappe, director of the museum's Dinosaur Institute, said, "Like many specimens on display and in our collection, this extremely important specimen is among the significant fossils that can be admired and studied only here in Los Angeles."

He concluded, "We're very proud that at NHM, these irreplaceable materials are accessible not only to research scientists but to the public, giving people the opportunity to connect the quest for knowledge with the wonder of seeing the remains of these ancient and mysterious animals."

© 2012 Discovery Channel


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