1、Prototherian is a group of primitive mammals that includes monotremes like the platypus and echidna.
原兽亚纲是一个包括鸭嘴兽和针鼹等单孔类哺乳动物的早期哺乳动物群体。

2、The prototherian lineage represents one of the earliest branches in the mammalian evolutionary tree.
原兽亚纲代表了哺乳动物演化树中最早的分支之一。