The earliest true mammal
Whatever the exact shape of the genealogical tree, at least one group of the reptiles completed the transition to a mammalian status some 200 million years ago. A fossil from the upper Triassic of a small animal (Megazostrodon) discovered in 1966 in southern Africa, is possibly the earliest true mammal. This creature was only about 100 mm long and resembled in body form a modern day shrew. Details of its jaw and skull link it firmly with true mammals and its teeth were specialised for eating insects. There is little doubt that it must have been both warm-blooded and fury. What we cannot determine is whether it laid eggs like a platypus or gave birth to live young and suckled them by means of a breast.
Even with the advantage of warm bloodedness the first small mammals were quite over shadowed in both numbers and size by the dinosaurs until 65 million years ago. Equipped with warm bloodedness, mammal were able to be active at night when the great reptiles became torpid and therefore survived in the shadow of the dinosaurs.