Plesiosaurs were marine reptiles with long necks and large flippers (related news: "Long-Necked Sea Reptiles Had Unexpected Diet, Fossils Show" [October 2005]).
Pliosaurs are believed to be closely related to plesiosaurs. But pliosaurs had short necks and massive jaws that would have been capable of lifting a car and biting it in half.
Such strength likely helped the pliosaurs hunt prey such as plesiosaurs and ichthyosaurs, another group of marine reptiles.
Ichthyosaurs resembled dolphins but used an upright tail fin, as opposed to dolphins' sideways tail fins, to propel themselves through the water.
Most ichthyosaurs averaged 13 to 20 feet (4 to 6 meters) in length, but some reached 75 feet (23 meters).
The new find follows up on fieldwork on Spitsbergen in 2004, when University of Oslo researchers excavated parts of the skulls of an ichthyosaur and two plesiosaurs.
The fossils from both digs were found in a layer of black shale—a type of sedimentary rock—that is between 65 to 100 feet (20 to 30 meters) thick.
"There's something special about the chemistry of the shale which has preserved all the bones in this layer," Hurum said.
According to Hurum, when the reptiles died they probably sank to the ocean floor, where conditions were right for preserving their bodies.
"It's deep water and black mud that they fell into. There were no animals living close to the bottom that could eat these big things that were decaying," he said.
"One specimen [we found] was probably scavenged before it fell to the bottom, but the others look quite complete, and that's amazing," Hurum said.
This summer marine reptile expert Pat Druckenmiller, of Montana State University in Bozeman, accompanied the team to Svalbard.
Druckenmiller, who identified the large pliosaur, said that "it is certainly likely that some of these animals represent new types of plesiosaurs and ichthyosaurs that are unknown to science."
The paleontologist says that many of the fossil reptiles are well articulated.
"This means that if you find the tail sticking out of the hill and you follow it in, you have a good chance of finding the rest of the skeleton—including the all-important skull—also attached," he said.
"This type of preservation, in black shale, is one of the settings considered likely to preserve traces of the animals' soft tissues, such as skin impressions," he added.
Dreams of Eggs
Druckenmiller says that the find shows that the Svalbard island chain is "significant at a global level," in terms of the quality of fossil preservation and the number and different types of marine reptiles found.
The site also provides "a relatively complete chapter from the late Jurassic, about 150 million years ago," he added.
"This age is considered one of the heydays of marine reptile life, but what we know of life from this specific window of time only comes from scattered sites around the globe."
The Oslo team is hopeful that the fossil graveyard will yield important new clues to the history and biology of these ocean predators.
"The dream is that one of the plesiosaurs or the pliosaur was pregnant" when it died, the University of Oslo's Hurum said. "A pregnant one, or one with eggs inside, is not known anywhere in the world."
Researchers still aren't sure if these creatures gave birth to live young underwater or if they hauled themselves on land to lay eggs like modern sea turtles.
Also, having the remains of both young and adult plesiosaurs could shed light on how the reptiles grew and matured, Hurum adds.
The researchers will return to the Arctic island next summer, when they plan to finish excavating the pliosaur and several other specimens.
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