The evolutionary step that freed these animals from the water was the development of the amniote egg.
使
些动物
以脱离水生
的
化步骤是羊膜卵

。
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古生物
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古生物
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古生物
And the amniotes quickly split into two branches around 310 million years ago: one that would give rise to all reptiles and birds, called the Sauropsids, and the other that would eventually give rise to mammals: the Synapsids.
大约3.1亿年前,羊膜动物迅速分化为两个分支:一个分支产生了所有爬行动物和鸟类,称为蜥形纲;另一个分支最终产生了哺乳动物,称为合弓纲。