Two tiny trilobites in a vast Cambrian ocean drift past sea cucumber parasols and a shaggy, tree-like sponge. Snail tracks loop enigmatically against brushed-gray Silurian slate, and ghostly white crinoids feather a Devonian seascape. A delicate pterosaur flies bravely into the Jurassic gloom, while a Tyrannosaurus rex so big that its teeth fill our field of vision stalks the deep orange sands that mark the end of the Cretaceous period. These...