During Mesozoic era, the age of reptiles (dinosaurs), mammals were small, generalised and rare. By the end of Mesozoic or beginning of Coenozoic, the dinosaurs vanished and mammals suddenly expanded into varied evolutionary patterns. Early in Cretaceous period, placental mammals became distinct from marsupials.

During Eocene and Oligocene, most of the orders of mammals originated moving into habitats and ecological niches vacated by the extinct dinosaurs. This evolution from a single ancestral species to a variety of forms which occupy different habitats is called adaptive radiation or divergent evolution.

The concept of adaptive radiation in evolution was developed by H.F. Osborn in 1898. Examples often given as evidence include Darwin’s finches of the Galapagos Islands, varied limb structure of mammals, Australian Marsupials, etc.

Figure 33.10 shows adaptive radiation in mammals. It is based on limb structure.

A. Radiation in Limb Structure of Mammals:


Mammalian limbs are the modifications of the pentadactyl limb. Primitive, ancestral mammals are believed to have been short legged five fingered creatures living on the ground. Their limbs were not modified for any particular type of locomotion. These animals were terrestrial. These terrestrial ancestors formed the ancestors of modern mammals.

These mammals having primitive limb structure are placed in the centre of the figure 33.10. Therefore, adaptive radiation occurred in five different lines or habitats with modifications in their limb structure.

From these terrestrial mammals the different lines radiated in the following manner:

1. One evolutionary line radiates to form arboreal forms which have adapted limbs for life in trees (e.g., squirrels, sloths, monkeys, etc.).


2. Another line leads to aerial representing mammals adapted for flight (e.g., bats) Only bats occupy the position at the terminus of this line, since they are the only truely flying mammals Somewhere along this line we can place for gliding mammals such as “flying squirrel. ”

The arboreal and aerial forms not arose independent