Possibly every palaeontologist, before and after Charles Darwin, has been well aware that the fossil record is very incomplete. Only a tiny percentage of the plants and animals alive at any one time in the past get preserved as fossils, both in terms of numbers of individuals and in terms of numbers of species. The palaeontologist attempting to reconstruct ancient ecosystems is therefore, in effect, trying to complete a jigsaw puzzle without the picture on the box lid and for which the majority of pieces are missing. Under normal preservational conditions probably only around 15 per cent of the species composing an ecosystem are preserved. Moreover, the fossil record is biased in favour of those animals and plants with hard, mineralized shells, skeletons or cuticles, and towards those living in marine environments. Thus, the preservational potential of a particular organism depends on two main factors: its constitution (better if it contains hard parts), and its habitat (better if it lives in an environment where sedimentary deposition occurs).