does anyone know when mammals first appeared on earth ?

哺乳动物是新生代的统治者，但是哺乳动物的历史远不局限在新生代，在中生代早期就已经出现，而其起源更可追溯到古生代后期爬行动物刚出现的时期。
Mammals were the rulers during the period of Cenozoic,however,the history of mammals are not just restricted in Cenozoic and they started to appear in the planet early in Mesozoic.Their origin can date back to late Paleozoic when the creeping animals came to being.

在如下文章中你可以找到答案，建议有时间耐心读下去
Fossils cast doubt on Molecular Clock

Fossil mammals tell their own story
A new analysis of fossils has fuelled the debate on the real age of placental mammals.

A team of palaeontologists, lead by Mike Foote from the University of Chicago, has attempted to measure how accurate the fossil record really is. Their results, published in the February 26 issue of Science, throws doubt on the accuracy of the "molecular clock" method of measuring when groups of organisms first appeared.

For the last 20 years two camps of scientists have been arguing over the dates of orig