Diversification Of Mammals

Dinosaurs dominated most terrestrial habitats while populations of small mammals continued to evolve. By the middle of the Cretaceous period, about 100 million years ago, three different kinds of mammals had appeared. Modern mammals belong to one of these three groups. The first group is made up of monotremes. They are oviparous, meaning that they lay eggs. The second group is made up of marsupials. They are viviparous, which means that they give birth to live young. In marsupials, the young develop within a pouch on the mother's body for some time after birth. The third group is made up of placental mammals. They are also viviparous, but in this group, the fetus typically develops within the mother's reproductive system for a longer time than it does in marsupials. Also, in this group, the developing fetus receives nourishment through a blood-rich structure called the placenta.

Many scientists think that some sort of natural disaster changed Earth's climate and forced the dinosaurs into extinction about 65 million years ago, at the end of the Cretaceous period. This change opened up many new habitats and resources to mammals. So, mammals took over many of the ecological roles that dinosaurs previously had. Today, nearly all large terrestrial animals are mammals.

figure 43-4

The earliest mammals, such as this species of the genus Eozostrodon, had to compete with the larger, dominant dinosaurs. These early mammals were active at night, when dinosaurs were probably less active.

□ lntenn*tcavttd

www.scilinks.org Topic: Mammals Keyword: HM60906

HlNKS.

Maintained by the National Science • Teachers Association

Was this article helpful?

0 0
Pregnancy And Childbirth

Pregnancy And Childbirth

If Pregnancy Is Something That Frightens You, It's Time To Convert Your Fear Into Joy. Ready To Give Birth To A Child? Is The New Status Hitting Your State Of Mind? Are You Still Scared To Undergo All The Pain That Your Best Friend Underwent Just A Few Days Back? Not Convinced With The Answers Given By The Experts?

Get My Free Ebook


Post a comment