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, 371 (1691), 20150219

How Many Dinosaur Species Were There? Fossil Bias and True Richness Estimated Using a Poisson Sampling Model

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How Many Dinosaur Species Were There? Fossil Bias and True Richness Estimated Using a Poisson Sampling Model

Jostein Starrfelt et al. Philos Trans R Soc Lond B Biol Sci.

Abstract

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

Keywords: Dinosauria; Poisson; diversity curves; fossil record; sampling bias; taxonomic richness.

Figures

Figure 1.
Figure 1.
Species level sampling estimates from TRiPS for all dinosaurs, ornithischians, sauropodomorphs and theropods. (a) Estimated sampling rates (λt) in sampling events per species per million years. (b) Estimated binomial sampling probabilities (pbinom,t) of species within the plotted time interval. In both panels, estimates are in red (squares and full line) for all dinosaurs, blue for Ornithischia (circle and dashed line), green for Sauropodomorpha (triangles with dotted line) and purple for Theropoda (diamonds with dash-dotted line). Confidence intervals on all rates and probabilities and estimates from 100 replicated observation count matrices (see main text) are reported in the electronic supplementary material. Lad, Ladinian; Car, Carnian; Nor, Norian; Rha, Rhaetian; Het, Hettangian; Sin, Sinemurian; Pli, Pliensbachian; Toa, Toarcian; Aal, Aalenian; Baj, Bajacian; Bat, Bathonian; Cal, Callovian; Oxf, Oxfordian; Kim, Kimmeridgian; Tit, Tithonian; Ber, Berriasian; Val, Valanginian; Hau, Hauterivian; Bar, Barremian; Apt, Aptian; Alb, Albian; Cen, Cenomanian; Tur, Turonian; Con, Coniacian; San, Santonian; Cam, Campanian; Maa, Maastrichtian.
Figure 2.
Figure 2.
Species richness estimates from TRiPS. Black circles connected by full line indicate observed species counts, triangles connected by dotted line indicate range-through species counts while coloured line and shading indicate maximum-likelihood estimate and 95% CIs for true species richness estimated using TRiPS. Corresponding sampling estimates can be seen in figure 1. All estimates with confidence intervals are also in the electronic supplementary material.
Figure 3.
Figure 3.
Simulation and estimation results. The grey scale indicates TRiPS's success rate (the fraction of simulations that had confidence intervals that span the true value) in the simulations, lighter areas indicate higher success rate. For each combination of duration and sampling rate in the figure, the full range of all other parameters are represented (i.e. the success rate is the mean value over sampled speciation and extinction rates, as well as degree of individual lineage variability in sampling rate and number of initial lineages; see main text). Estimated sampling rates for Dinosauria (red squares), Ornithischia (blue circles), Sauropodomorpha (green triangles) and Theropoda (purple diamonds) are plotted against their corresponding stage durations.

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