A small, 160-million-year-old Chinese fossil has something big to say about the emergence of mammals on Earth. The shrew-like creature is the earliest known example of an animal that used a placenta to provide nourishment to their unborn young. Its features clearly set it apart from marsupial mammals, which adopt a very different reproductive strategy. The discovery pushes back the date the two groups took up their separate lines, according to Nature magazine. To compliment this: Dinosaur demise allowed mammals to 'go nuts'