几乎所有现存哺乳动物都是胎生的。哺乳动物演化中一个关键事件是当今胎生动物祖先和有袋动物祖先之间的分异。
在这种分异的胎生动物一边所发现的一个化石，将这种分异的时间提早了3500万年，提早到距今约1.6亿年前，这个时间已经深入到了“侏罗纪”时期。
怀孕的蛇颈龙化石首次证实蛇颈龙是胎生动物而非卵生动物
来自中国的这一化石表明，包括我们自己和大多数我们所熟悉的哺乳动物在内的这个类群的最早成员，是一种已适应了爬树和在树木之间跳跃（它们之所以这样做，可能是为了远离树下的恐龙）的一种小型动物。该化石的年龄表明，“侏罗纪”哺乳动物演化速度快于人们以前所认为的。
生物探索推荐英文论文摘要：
A Jurassic eutherian mammal and divergence of marsupials and placentals
Placentals are the most abundant mammals that have diversified into every niche for vertebrates and dominated the world’s terrestrial biotas in the Cenozoic. A critical event in mammalian history is the divergence of eutherians, the clade inclusive of all living placentals, from the metatherian–marsupial clade. Here we report the discovery of a new eutherian of 160 Myr from the Jurassic of China, which extends the first appearance of the eutherian–placental clade by about 35 Myr from the previous record, reducing and resolving a discrepancy between the previous fossil record and the molecular estimate for the placental–marsupial divergence. This mammal has scansorial forelimb features, and provides the ancestral condition for dental and other anatomical features of eutherians.
