The tiny colonial sea creatures, Rhabdopleurids, have stayed relatively unchanged for over 500m years and outlived some of their newer relatives.

By analysing form and structure, scientists discovered that Rhabdopleurids are the ancestral species of the zooplankton, pelagic graptolite. The zooplankton relatives rapidly diversified, and the newer and more elaborate species lived on the ocean’s surface. They became extinct 350m years ago, while their predecessors, Rhabdopleurids, still survive today, as they have for millions of years, on the ocean’s floor.