Estimating the diversity of dinosaurs

Abstract

Despite current interest in estimating the diversity of fossil and extant groups, little effort has been devoted to estimating the diversity of dinosaurs. Here we estimate the diversity of nonavian dinosaurs at approximately 1,850 genera, including those that remain to be discovered. With 527 genera currently described, at least 71% of dinosaur genera thus remain unknown. Although known diversity declined in the last stage of the Cretaceous, estimated diversity was steady, suggesting that dinosaurs as a whole were not in decline in the 10 million years before their ultimate extinction. We also show that known diversity is biased by the availability of fossiliferous rock outcrop. Finally, by using a logistic model, we predict that 75% of discoverable genera will be known within 60-100 years and 90% within 100-140 years. Because of nonrandom factors affecting the process of fossil discovery (which preclude the possibility of computing realistic confidence bounds), our estimate of diversity is likely to be a lower bound.

Fig. 1.

Known and estimated diversity in the last two Late Cretaceous stages (Cam, Campanian; Maa, Maastrichtian). Results from 100 sets of randomizations to assess whether dinosaur diversity was declining near the end of the Cretaceous. (a) Number of genera associated with each stage, comprising well-resolved genera and the poorly resolved genera randomly assigned to that stage. The number of well-resolved genera known from each stage is marked by a gray diamond. (b) Estimated diversity in each stage. Box plots show middle 50% of values (represented by box) with the median shown as a horizontal line within the box.

Fig. 2.

Relationship of known and estimated diversity with rock availability (Tr, Triassic; J, Jurassic; K, Cretaceous). (a) Known diversity (solid black line) and number of terrestrial U.S. Geological Survey formations (gray line), a proxy for rock availability (11). (b) Estimated diversity using the ACE (dashed black line) and number of formations (gray line). Known diversity is strongly correlated with rock availability (R² = 60.3% for first differences), but estimated diversity is only weakly correlated (R² = 12.8% for first differences).

Fig. 3.

Predicted rate of future dinosaur discoveries. Logistic accumulation curves fitted to known cumulative genus counts and constrained with a horizontal asymptote at 1,844 genera. Curves have the form y = 1844/(1 + exp{−[β₀ + β₁(x − β₂)]}). Gray lines indicate predicted dates at which 50%, 75%, and 90% of discoverable generic diversity will be known. (a) Curve fitted using unweighted least-squares. Estimated parameter values are β₀ = 0.23, β₁ = 0.024, β₂ = 2066. (b) Curve fitted using weighted least-squares, with recent observations given more weight so that the curve better fits recent rates of discovery. Estimated parameter values are β₀ = −1.05, β₁ = 0.034, and β₂ = 2006.