The Great Depression and World War II combined to sink the study of dinosaurs into a decades-long lull. Scientists considered dinosaurs a group of unrelated animals that left no descendants, and dinosaurs were presented as stupid, slow, stuck in swamps, and doomed to extinction. Scientific dinosaur artwork, primarily from Rudolph F. Zallinger and Zdeněk Burian, reflected and reinforced the conception of dinosaurs as slow and static (one artistic quirk that became commonplace in representations of Mesozoic landscapes, the presence of a volcano, was a hallmark of Zallinger's). From such ideas came the alternate use of "dinosaur" as something out of date.