Now is the world's most primitive mammals.

是现在世界上最原始的哺乳动物。

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Many primitive lineages went extinct as sharks and bony fish became masters of the waters and tetrapods, the four-legged animals that eventually evolved into dinosaurs and mammals, conquered the land.

许多原始生物灭绝，鲨鱼和硬骨鱼成为水域的主人，四足动物最终进化成了恐龙和哺乳动物，征服了陆地。

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Mammals arose from a fairly primitive reptile in the Mesozoic era.

哺乳类起源于中生代宛若原始的爬虫类。

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Two long-beaked echidnas, primitive egg-laying mammals, even allowed scientists to pick them up and bring them back to their camp to be studied, he added.

BBC: NEWS | Science/Nature | Science team finds 'lost world'