The oldest known fish. Research published in Nature gives the first clear picture of exactly what these earliest fishes were like.
The 518 million-year-old fish Metaspriggina walcotti was about 6cm long, bore a pair of large protruding eyes and small paired nasal capsules. It breathed through seven pairs of external gills. It had a stout rod supporting its spine (notochord) enabling strong W-shaped muscle bands to develop along its entire body.