Figure 23.53: Mammalian Diversity (A) The duck-billed platypus is one of the five surviving species of prototherians—mammals that lay eggs. (B) Female marsupial mammals have a ventral pouch in which they nurture their offspring, which are extremely small at birth. The young of this opossum have grown large enough to leave her pouch. (C–F) Eutherian mammals. (C) Almost half of all eutherians are rodents, such as this North American porcupine. (D) Flight evolved in the common ancestor of bats. Virtually all bat species are nocturnal. (E) Many large mammals are important herbivores in terrestrial environments. Nowhere are these assemblages more spectacular than on the grass and brushlands of the African continent. (F) Bottlenose dolphins are cetaceans, a cetartiodactyl group that returned to the marine environment.