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The class Mammalia is divided into two subclasses based on reproductive techniques: egg-laying mammals (yinotherians or monotremes - see also Australosphenida), and mammals which give live birth . The latter subclass is divided into two infraclasses: pouched mammals ( metatherians or marsupials ), and placental mammals ( eutherians , for which ...
[25] [26] Much like newborn marsupials (and perhaps all non-placentals [27]), newborn monotremes, called "puggles", [28] are larval- and fetus-like and have relatively well-developed forelimbs that enable them to crawl around. Monotremes lack teats, so puggles crawl about more frequently than marsupial joeys in search of milk. This difference ...
Although some marsupials look very like some placentals (the thylacine, "marsupial tiger" or "marsupial wolf" is a good example), marsupial skeletons have some features that distinguish them from placentals: [69] [self-published source?] Some, including the thylacine, have four molars; whereas no known placental has more than three.
Marsupials, along with monotremes (platypuses and echidnas), typically have lower body temperatures than similarly sized placentals , [12] with the averages being 35 °C (95 °F) for marsupials and 37 °C (99 °F) for placentals.
Marsupials' reproductive systems differ markedly from those of placentals, [10] [11] though it is probably the plesiomorphic condition found in viviparous mammals, including non-placental eutherians. [12] During embryonic development, a choriovitelline placenta forms in all marsupials.
The class Mammalia is divided into two subclasses based on reproductive techniques: monotremes, which lay eggs, and therians, mammals which give live birth, which has two infraclasses: marsupials/metatherians and placentals/eutherians. See List of monotremes and marsupials, and for the clades and families, see Mammal classification ...
The monotremes have a sex-determination system different from that of most other mammals. [131] In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal. [132] Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses.
Placental mammals (infraclass ... like monotremes, marsupials and most other vertebrates, the urogenital ducts exit through the vulva or penis and the rectum opens as ...