约3亿年以前，在恐龙出现之前，地球上都是下孔类动物：即一种外表与蜥蜴相似的典型哺乳动物。
Around 300 million years ago , before even dinosaurs showed up , Earth teemed with synapsids : prototypical mammals that bore a superficial resemblance to lizards .
上孔类