What are the characteristics of monotremes, mammals, and marsupials?
Monotremes and marsupials are mammals, so there’s something wrong with your question. Monotreme mammals reproduce by egg-laying, marsupials do so by given birth to relatively poorly developed young which then, in most but not all cases, develop further in a maternal pouch. (Some species are pouchless.) Placental mammals tend to give birth to better developed young, although that’s not true in all cases. However, for all placentals, a placenta* is involved in the embryonic development of babies. * To complicate matters slightly, some marsupials have also developed a form of placenta, but it’s a simpler and independent development compared to the placental mammal placenta. I could mention lots of other details, but you won’t need them. For example, the cochlear canal of the monotreme ear spirals to no more than 270°, whereas the figure is at least 360° for marsups and placentals. All adult monotremes are toothless, and marsup tooth replacement only occurs with the final premolar on each
