self tapping screws: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 origin for various organisms. The debate centers on two different approaches to finding the time when one particular group split from another.

Molecular biologists use various molecules such as DNA collected from living animals. They compare the same molecules from two different organisms looking for similarities and differences in structure. Differences between molecules measures how long the two groups have been evolving independently of each other. This technique assumes that the rate of molecular change is constant and measurable, the so-called "molecular clock".

Palaeontologists use fossils to track back through time to find when new groups of organisms first appeared. This technique assumes that the fossil record provides an accurate account of the evolutionary history of organisms.

Conflicts between divergence dates derived from the two different techniques often occur, particularly for organisms that are poorly represented in the fossil record. Until now it has been assumed that these discrepancies are the result of either an inaccurate molecular clock or an inadequate fossil record.

Foote's team chose to analyse placental mammals (the group of mammals that produce live young nurtured through a placenta in the mothers womb - including humans, bats, whales, elephants and mice). Fossil evidence suggests placental mammals first appeared on Earth some 65 million years ago while the biochemical data suggests a date of 130 million years. The team asked the question: "Is the fossil record so poor that the group could have been undetected for 65 million years?"

To address this question the team compiled all the known data for fossils of all mammals living in the Cretaceous Period (135-65 million years ago). These fossils were from groups of non-placental mammals most of which are now extinct. They counted between 225 and 460 species allowing for types that were difficult to identify and different methods for counting species. This suggests that the fossil record for mammals during this period is much better than has previously been suggested.

It is exceedingly unlikely that placental mammals could have been present during this period without leaving fossils while similar, closely related animals have such a good fossil record for the same period. The analysis supports the 65 million year date for the origin of placental mammals and brings into question the accuracy of the molecular clock technique.

"We're just trying to make the whole question more explicit and more testable," said Foote. "We thought the right approach was to ask, what would the fossil data have to say in order to favour one hypothesis or the other, and what do the data actually say?"

哺乳最早出现在三叠纪末到侏罗纪初，最早的哺乳动物是属于原兽亚目（过去单列为始兽亚目）的卵生食虫动物，如英国和我国的摩根锥齿兽Morganucodon，我国的中国锥齿兽Sinoconodon等，新近有人将这些最早的哺乳动物从哺乳纲排除出去，作为进化上的旁支。最早的哺乳动物出现后不久，兽亚目的早期成员和植食的原兽类（过去单列为异兽亚目）也开始出现。中生代的哺乳动物虽然分化成很多不同的类群，但所有这些哺乳动物都是些体型非常小的动物，在整个恐龙统治大地的一亿多年时间内，哺乳动物一直是不很起眼的小型动物，直到中生代结束时也没有一种体型超过兔子的大小。植食的原兽类即多瘤齿兽类Multituberculata(左图)是最成功的早期哺乳动物，特征和习性类似啮齿类，可能多数穴居，而一些种类可能树栖。在白垩纪的蒙古，80%的哺乳动物都属于多瘤齿兽类，而在中生代结束的时候，多瘤齿兽类依然残存下来，直到渐新世才由于啮齿类竞争等原因而完全灭绝，其生存的时间达到1.3亿年之久。

参考资料：http://cheni.0086e.net/qiu/class_mammal/prohis.htm

有没有人知道地球上最早出现哺乳动物.

谁知道哺乳动物第一次出现在地球上?