 Both modern crocodilians and birds, the closest living relatives of dinosaurs, have four-chambered hearts (albeit modified in crocodilians), so dinosaurs probably had them as well; the structure is not necessarily tied to metabolic rate.Fossils of the troodonts "Mei" and "Sinornithoides" demonstrate that the dinosaurs slept like certain modern birds, with their heads tucked under their arms.This behavior, which may have helped to keep the head warm, is also characteristic of modern birds.