(See "Giant 'Sea Monster' Fossil Discovered in Arctic.")

The study focused on a 78-million-year-old, 15.4-foot-long (4.7-meter-long) adult Polycotylus latippinus plesiosaur fossil found in 1987. The fossil's abdominal cavity contains tiny bones—parts of a plesiosaur that hadn't been born by the time its mother died.

(See a prehistoric time line.)

The finding, detailed in this week's issue of the journal Science, is the first proof that plesiosaurs were viviparous—that they gave birth to live young.

"The fetus is too large to make an egg physiologically or mechanically feasible," study co-author Robin O'Keefe told National Geographic News. "And why carry a big egg around?"

The discovery, while not completely unexpected, may fill a frustrating gap in plesiosaur knowledge, he said.

"Scientists have long known