Placental Mammals: Study
New research from Stockholm University shows that the typical mammalian heater organ, brown fat, evolved exclusively in modern placental mammals.
- A placental mammal is an animal that has a placenta.
- The placenta is a vascular organ formed during gestation of female mammals (except for monotremes and marsupials), built up of maternal and fetal tissues jointly, and which serves for the transport of nutrient substances from the mother to the fetus and to eliminate fetal waste products.
- Placental mammals carry their fetus in the uterus until they are born at an advanced stage.
- The young get their nourishment through a placenta before birth. The placenta delivers nutrients and oxygen to the fetus in the uterus.
- The placenta permits a long period of fetal growth in the uterus. As a result, the fetus can become large and mature before birth.
- Classified under the subclass Eutheria, placental mammals have 4,000 identified species.
- Fossil evidence shows that the first placental mammals evolved between about 163 million and 157 million years ago during the Jurassic Period (201.3 million to about 145 million years ago).
- The placentals include all living mammals except marsupials and monotremes (egg-laying mammals).
- Marsupials and monotremes have a less-developed, less-efficient type of placenta that limits the gestation period.
