1. Reproduction:
* Monotremes: Lay eggs. They have a pouch where they incubate their eggs.
* Placental Mammals: Give birth to live young. The young develop inside the mother's uterus and are nourished through a placenta.
* Marsupial Mammals: Give birth to relatively undeveloped young. The young continue to develop in a pouch on the mother's abdomen, where they nurse and grow.
2. Other Key Differences:
* Monotremes: Possess a cloaca, a single opening for the digestive, urinary, and reproductive systems. This is a feature shared with reptiles, birds, and amphibians.
* Placental Mammals: Have separate openings for each system.
* Marsupial Mammals: Also have separate openings for each system.
3. Evolutionary History:
* Monotremes: The most ancient lineage of mammals, dating back to the Mesozoic Era. They represent a unique branch in the mammalian evolutionary tree.
* Placental Mammals: Diverged from a common ancestor with marsupials during the Late Cretaceous period. They are the most diverse group of mammals today.
* Marsupial Mammals: Diverged from a common ancestor with placental mammals during the Late Cretaceous period. They are found primarily in Australia and the Americas.
In Summary:
Monotremes are distinct from other mammals due to their unique reproductive strategy of laying eggs. They also possess a cloaca, a feature absent in other mammals. These characteristics highlight their ancient evolutionary lineage and their unique place in the mammalian world.