Dinosaur diversity and the rock record

Abstract

Palaeobiodiversity analysis underpins macroevolutionary investigations, allowing identification of mass extinctions and adaptive radiations. However, recent large-scale studies on marine invertebrates indicate that geological factors play a central role in moulding the shape of diversity curves and imply that many features of such curves represent sampling artefacts, rather than genuine evolutionary events. In order to test whether similar biases affect diversity estimates for terrestrial taxa, we compiled genus-richness estimates for three Mesozoic dinosaur clades (Ornithischia, Sauropodomorpha and Theropoda). Linear models of expected genus richness were constructed for each clade, using the number of dinosaur-bearing formations available through time as a proxy for the amount of fossiliferous rock outcrop. Modelled diversity estimates were then compared with observed patterns. Strong statistically robust correlations demonstrate that almost all aspects of ornithischian and theropod diversity curves can be explained by geological megabiases, whereas the sauropodomorph record diverges from modelled predictions and may be a stronger contender for identifying evolutionary signals. In contrast to other recent studies, we identify a marked decline in dinosaur genus richness during the closing stages of the Cretaceous Period, indicating that the clade decreased in diversity for several million years prior to the final extinction of non-avian dinosaurs at the Cretaceous-Palaeocene boundary.

Figure 1

Time-series plots of rock-record quality and diversity for each of the three major dinosaur clades. Alternating grey/white bins mark durations of standard international stages (see Gradstein et al. 2004) starting with the Anisian stage of the Middle Triassic. Individual substage data points are marked on the curve. (a) Number of DBF. (b) Ornithischia TDE (unfilled circles) and PDE (unfilled triangles). (c) Theropoda TDE (unfilled circles) and PDE (unfilled triangles). (d) Sauropodomorpha TDE (unfilled circles) and PDE (unfilled triangles). The y-axis in figure 1a represents the number of DBF; in figure 1b–d, it represents the number of valid dinosaur genera (inclusive of Mesozoic birds).

Figure 2

Time series of residuals of observed TDE from predicted MDE based on power-law models. Time-scale divisions as in figure 1. Dashed lines mark 95% confidence limits. (a) Ornithischia, (b) Theropoda and (c) Sauropodomorpha. See text for further details.