Plesiosaurs didn't lay eggs

The plesiosaurs - giant marine reptiles which were contemporaries of the dinosaurs but were not technically dinos themselves - didn't lay eggs like most reptiles. A fossil of a plesiosaur carrying a single, large youngster showed they'd evolved for live births, like whales. (Just Showing Off Department: evolving toward fewer, larger young is known as "K selection.") Paleontologists had debated this a long time, because there was no evidence that plesiosaurs were really built to haul out on land like crocodiles to lay eggs, but there was no evidence they didn't behave this way, either. The decisive fossil of Polycotylus latippinus sat in a museum for 20 years before someone looked closely at it. This is not unusual: most large museums have huge back catalogs they just don't have the personnel to examine thoroughly.