Fossil bias and true richness estimated using a Poisson sampling model ( TRiPS )

14 The fossil record is a rich source of information about biological diversity in the past. However, the fossil 15 record is not only incomplete but has inherent biases due to geological, physical, chemical and biological 16 factors. Our knowledge of past life is also biased because of differences in academic and amateur interests 17 and sampling efforts. As a result, not all individuals or species that lived in the past are equally likely to 18 be discovered at any point in time or space. To reconstruct temporal dynamics of diversity using the fossil 19 record, biased sampling must be explicitly taken into account. Here, we introduce an approach that 20 utilizes the variation in the number of times each species is observed in the fossil record to estimate both 21 sampling bias and true richness. We term our technique TRiPS (True Richness estimated using a Poisson 22 Sampling model) and explore its robustness to violation of its assumptions via simulations. We then 23 venture to estimate sampling bias and absolute species richness of dinosaurs in the geological stages of 24 the Mesozoic. Using TRiPS, we estimate that 1936 (1543-2468) species of dinosaurs roamed the Earth 25 during the Mesozoic. We also present improved estimates of species richness trajectories of the three 26 major dinosaur clades; the sauropodomorphs, ornithischians and theropods, casting doubt on the Jurassic27 Cretaceous extinction event and demonstrating that all dinosaur groups are subject to considerable 28 sampling bias throughout the Mesozoic. 29