Diversity partitioning during the Cambrian radiation

Abstract

The fossil record offers unique insights into the environmental and geographic partitioning of biodiversity during global diversifications. We explored biodiversity patterns during the Cambrian radiation, the most dramatic radiation in Earth history. We assessed how the overall increase in global diversity was partitioned between within-community (alpha) and between-community (beta) components and how beta diversity was partitioned among environments and geographic regions. Changes in gamma diversity in the Cambrian were chiefly driven by changes in beta diversity. The combined trajectories of alpha and beta diversity during the initial diversification suggest low competition and high predation within communities. Beta diversity has similar trajectories both among environments and geographic regions, but turnover between adjacent paleocontinents was probably the main driver of diversification. Our study elucidates that global biodiversity during the Cambrian radiation was driven by niche contraction at local scales and vicariance at continental scales. The latter supports previous arguments for the importance of plate tectonics in the Cambrian radiation, namely the breakup of Pannotia.

Keywords: Cambrian radiation; Pannotia; alpha diversity; beta diversity; low competition.

Fig. 1.

Global genus-level diversity of marine animals from the Ediacaran to the earliest Ordovician. (A) Raw counts of the number of genera (sampled-in-bin). Note log scale of y axis. (B) Sampling-standardized genus-level diversity (sampled-in-bin) based on shareholder quorum subsampling with 70% frequency coverage per stage. The brown line refers to all marine genera, and the blue line excludes archaeocyaths. Ma, million years ago. Ava, Avalon assemblage; Dru, Drumian; For, Fortunian; Guz, Guzhangian; Jia, Jiangshanian; Nam, Nama assemblage; Pai, Paibian; St2, Stage 2; St3, Stage 3; St4, Stage 4; St5, Stage 5; St10, Stage 10; Tre, Tremadocian (Ordovician) Whs, White Sea assemblage.

Fig. 2.

Alpha diversity and beta diversity from the Ediacaran to the earliest Ordovician based on unweighted by-list subsampling of 45 collections per stage. Error bars are SDs of 100 subsampling trials.

Fig. 3.

Sampling-standardized extinction and origination rates through the Ediacaran and Cambrian. Rates are per-capita rates of Foote (70) but not standardized by stage durations (71). Error bars are SDs of 100 subsampling trials.

Fig. 4.

Alpha-beta-gamma plot for the first three Cambrian stages, the time of continuous diversification. Note log scale of all axes.

Fig. 5.

Distance-decay curves during the first three Cambrian stages plotted as taxonomic dissimilarity among 5 × 5° paleogeographic grids. Blue circles, Stage 3; green circles, Stage 2; red circles, Fortunian. The black lines denote separate regressions for three distance intervals.