Rodents possess a single pair of upper and lower chisel-like incisors, unlike the marsupials.Unlike in the rodents, marsupials have their fertilized eggs surrounded by chorion and amnion membranes.Rodents are found worldwide, whereas marsupials are found only in Australia and Americas.
Marsupials include kangaroos, opossums and koalas, whereas rodents include beavers, mice, porcupines, squirrels, flying squirrels, gophers, agoutis, chinchillas, coypu, mole-rats, rats, and capybara. Young rodents undergo a considerable period to develop before they are born, unlike the young marsupials. It is believed that placental mammals evolved after the origin of marsupials. What is the difference between a Marsupial and a Rodent? Most of the rodent species possess smaller bodies, except the species called capybara (Capybara is the largest of all rodents and may weigh up to 50 kg). The rodents are well-adapted to live in a wide range of terrestrial and semi-aquatic habitats all over the world. The most characteristic feature of the rodent is the presence of a single pair of upper and lower chisel-like incisors. This mammalian category includes beavers, mice, porcupines, squirrels, flying squirrels, gophers, agoutis, chinchillas, coypu, mole-rats, rats, and capybara. Order Rodentia has over 2000 species of animals and represents 42% of all the living mammalian species. Rodents are placental mammals that have a placenta to nourish the embryo though the embryonic development, which takes place in the uterus. Virginia opossum is the only marsupial species found in North America. Australia and New Guinea have the largest diversification of marsupials like no other places on Earth. All the living species of marsupials including kangaroos, opossums, and koalas are confined to Australia and Americas. Once the embryonic marsupial is born after about eight days of fertilization, it crawls into marsupial pouch and starts feeding on milk produced by the mother. During the early embryonic developmental stages, marsupial egg has considerably high amount of yolk. However, not all the marsupial possess this feature and hence it is considered as a poor diagnostic characteristic feature. The other specialized feature is the presence of abdominal pouch called the marsupium in female marsupials. Thus, the major difference between the marsupials and the rest of the mammals is the embryonic development pattern. Even through their egg is surrounded, eggshell formation does not occur as it does in the monotremes. Unlike in the other mammals, marsupials have their fertilized eggs surrounded by chorion and amnion membranes. All the rodents are placental mammals and placed in the Order Rodentia. 
There are 17 orders of placental mammals. Placental mammals use a placenta to nourish their embryos throughout their entire development in the uterus. Marsupials are also called pouched mammals. Monotremesare the egg-laying mammals, which includes the duck-billed platypus and two species of echidna.
Modern mammals are categorized into three major groups Monetremes, Marsupials, and Placental mammals.
Class Mammalia is composed of about 4500 living species, but this number is less when compared to the number of living species in other vertebrate groups such as fishes, amphibians, reptiles and birds. The other specialized mammalian characteristic features include the placenta, specialized sensory systems according to their living habitats, endothermy and specialized teeth suited for their dietary habits. The most remarkable features that are confined only to mammals are the presence of hair and mammary glands. At present, mammals are the most advanced and highly adapted animals of all vertebrates and dominate most of the habitats in the world. About 220 million years ago, first mammalian species evolved and they reached their maximum diversity in the Tertiary period, about 15 million years ago. Knowledge about mammals, in general, would be helpful in understanding the difference between these two mammals, marsupials and rodents. The key difference between marsupials and other mammals including rodents is the embryonic development pattern.
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