The annoying thing about fossils is trying to work out what the heck the creature looked like before it was trapped in a mudslide, lost some limbs, got squashed flat, and was chemically altered by millennia of burial. This is not easy. And as Robert Sansom of the University of Leicester points out, it’s made extra difficult by the fact that some discriminating features used to identify these creatures decay faster than others. That introduces a bias in how organisms are classified, he warns. His group is doing lab tests of decay rates of different bits and pieces of animals to sort these biases out.