The two major groups of mammals, marsupials and placentals, first developed in the Cretaceous Period. Both these groups give birth to live young. Marsupial young are born in an underdeveloped state. The young crawl into a special pouch on the mother's belly, where they continue to develop and grow. Most early marsupials resembled modern opossums.
Placentals give birth to fairly well-developed young.
These young grow inside the mother's body, where they receive nourishment from a special organ called the placenta. Many early placentals resembled shrews. For more information on the evolution of animals whose ancestors lived during prehistoric times, click on the links below. We'd like to hear from you. Click here to submit your comments.