In outward appearance many of the mammals have changed little since they evolved from the non-mammalian synapsids in Late Triassic times. Throughout the Jurassic, Cretaceous, Paleogene and Neogene periods they have remained small, compact creatures, not adapting into any of the wide ranges of lifestyles occupied by the great reptiles and dinosaurs. However, some of them have a number of interesting specializations. The surviving groups in the world are monotremes, marsupials and placentals (multituberculates died out back in the Paleogene).