Mammals

Mammals, often characterized by the presence pf hair and mammary glands, can be divided into three sub groups; Holotherians, Metatherians, and Eutherians. Holotherians encapsulate the monotremes which are the egg laying mammals characterized by a single opening for both fecal removal and urogenital products. Examples of monotremes are platypuses and echidnas.  Metatherians are the marsupials like kangaroos, wombats, opossums, etc. They are characterized by a pouch where the fetuses attach to nipples and finish developing into adolescence. Metatherians also have an inwardly turned angular process on their lower jaws which result in limited replacement of their teeth. Lastly, Eutherians are the remaining placental organisms. While marsupials are placentals as well, eutherians have a longer gestation period and thus produce able bodied young who are at a much more advanced stage in development. Eutherians are also characterized by their tribosphenic molars, molars where the protocone is on the lingual side of the mouth (tongue) and the anterior paracone along with the posterior metacone are on buccal side of the jaw (cheek). In conclusion, while mammals are united by their hairs and mammary glands, they can be further divided using distinct characteristics that allow us the create the three groups Holotherians, Metatherians, and Eutherians.