This broad range of useful features has made mammals highly adaptive and successful. They first appeared about 200,000,000 years ago in the age of dinosaurs and replaced reptiles as the dominant class of land animals after 65,000,000 years ago. I mportant to mammalian success is their reproductive system. Their bodies took the amniote egg revolution of reptiles one step further. In effect, the uterus functions as the protective egg shell.
Young mammals spend a long period of their early development within their mother's uterus.
Pregnancy and milk production require mothers to significantly increase their calorie consumption in order to provide nutrients for their infant. All rights reserved.