It was an atypically peaceful scene for those dino-eat-dino days. Amid the shallow streams of a broad floodplain, scores of huge, grazing female dinosaurs were making their nests and hovering near their eggs, as their predecessors had doubtless done for ages untold. But their tranquillity was suddenly disturbed. Out of nowhere came a flood of mud and silt, scattering the lumbering beasts and burying their progeny. The lively dinosaur nursery was lost forever.
Or nearly so. Some 80 million years after that late Cretaceous calamity--give or take 10 million years--its telltale remains have poignantly resurfaced. At a news conference in New...