Cambrian explosion -         rapid appearance of most major groups    of complex animals around 530 million years    ago, as evidenced by the fossil record.            The basic body plan of these animals has not changed significantly since the Cambrian Explosion:    tripoblastic = 3 layers of embryo - inner layer forms    digestive tract; mid layer forms muscles and other    internal organs; outer layer forms skin.    bilateral =  animals with left & right sides; and    coelomates = having a body cavity (coelom) which    holds internal organs.   Age of Mammals - controversial        fossil record places it at cenozoic (about 65 million years ago, while Molecular Clock Dating that compares DNA sequence differences when species diverge places the ascent of mammals during the cretaceous period (circa between 145 and 65 mya) at about 100 million years ago.