The fact that characteristics of so many different kinds of mammals can be seen in the Tree Shrew, suggests than it might well resemble the ancient creature from which all placentalmammals are descended. Certainly, judging from fossil skeletons such as Megazostrodon, the first mammals to exist in the dinosaur dominated forests must have looked very like it: small, long tailed and pointed nosed, and, by inference, furry, warm blooded, active and insect eating. The reign of the reptiles had been a long one. They had come to dominate about 250 million years ago. They had browsed the forests, munched the lush vegetation of the swamps, and carnivorous forms had evolved which preyed on the plant eaters. Still other species lived by scavengingcarrion. The plesiosaurs and ichthyosaurs were forms that returned to the seas and preyed upon fish; while pterosaurs took to the skies. Then, 65 million years ago, they all disappeared. In the void created by the demise of the dinosaurs a radiation of the placentalmammals began.