Let me share with you a difference between prototheria and metatheria.
| Aspect | Prototheria | Metatheria |
| Known as | Monotremes | Marsupials |
| Reproduction | These are egg-laying mammals | These are pouched mammals with live birth (viviparity) but a short gestation period |
| Development | Hatch from eggs outside the mother’s body | Birth at a very early stage of development and offspring continue to develop in the mother’s pouch |
| Placenta | Lacks a true placenta | Simple, short-lived placenta or absent |
| Mammary Glands | Mammary glands without nipples; milk is secreted through openings in the skin | Mammary glands with nipples; milk is delivered through nipples to the young |
| Urogenital System | Single urogenital opening (cloaca) | Separate urogenital and digestive openings |
| Geographic Distribution | Found primarily in Australia and New Guinea | Found in various parts of the world, but predominantly in Australia and the Americas |
| Number of Species | Limited number of species | Diverse group with a larger number of species |
| Dentition (Teeth) | Typically have reptilian-like teeth | Dentition often adapted to herbivorous or omnivorous diets |
| Lifespan | Generally longer lifespan | Shorter lifespan |
| Developmental Stages | Egg, hatchling and juvenile stages | Neonate, pouch young and weaned stages |
| Examples
| · Platypus · Echidnas
| · Kangaroos · Wallabies · Koalas · Opossums |