Fossil bones of the huge herbivorous dinosaurs known as sauropods were first discovered and studied between 1840 and 1880, providing evidence for the gargantuan dimensions of the adults. The shape of sauropod teeth suggested what they ate. But aside from trackways, or series of fossilized footprints – which established that sauropods at least occasionally lived in herds – fossils incorporating direct evidence of other behavior, such as reproductive behavior, have been almost nonexistent. Because no modern land animals even approach sauropod size, scientists have also lacked a living analogue to use as a guide to possible sauropod behavior. Until the recent discovery of fossilized sauropod nesting grounds, scientists were thus uncertain whether sauropods laid eggs or gave birth to live young.