研究人员现在知道，这些化石属于生活在约两亿五千万年前的古老哺乳动物。
Researchers now know they belong to an ancient relative of mammals that lived around 250 million years ago.
哺乳动物与恐龙同时出现于三叠纪末期，即约两亿两千万年前。
Both mammals and dinosaurs appeared near the end of the Triassic period, some 220 million years ago.
两亿五千两百万年前的二叠纪末期，发生了地球上迄今为止所知的最大一次生物灭绝。
The end of the Permian period, 252m years ago, was marked by the biggest extinctionof life known to have happened on Earth.
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