Others have blamed extinction on competition from the mammals, which allegedly ate all the dinosaur eggs-except that mammals and dinosaurs appeared at the same time in the Late Triassic, about 190 million years ago, and there is no reason to believe that mammals suddenly acquired a taste for dinosaur eggs after 120 million years of coexistence. 

对应答案是mammals and dinosaurs lived together for roughly 120 million years before the extinction

1.我的疑问是for+时间段=after+时间段吗

2.except这里是除了，还是只是。。的意思 两者截然相反。about 190 million指的是前句的

 Late Triassic,吗