Stegosaur expert Susie Maidment of London’s Natural History Museum is studying rock strata in order to pin down the precise dates of dinosaur fossils from the Late Jurassic — the better to understand the biology and evolution of these ancient beasts. (Credit: Emily Osterloff/Natural History Museum)

At the base of a pale hill in the badlands of northeastern Wyoming, Susie Maidment hits her hammer against stone. She breaks off a fist-sized chunk, grabs a loose piece between her fingers and pla