Biodiversity dynamics and their driving factors during the Cretaceous diversification of Spatangoida (Echinoidea, Echinodermata)

Abstract

Abstract Variations in recorded diversity over time present a scrambled signal that is modulated by a large number of variables: the potential of particular life forms to generate evolutionary innovations, external constraints induced by the environment in its broad sense, the heterogeneity of the fossil record and the analytical artefacts due to sampling bias. A key question is how to characterise and quantify the separate input of any given factor in the overall diversity signal. This paper explores the structure of diversity data for spatangoid heart urchins and the sensitivity of recorded diversity to different factors of analytical bias (length of geological periods, proportion of palaeogeographical realms explored, accessible area of outcrops and historical determinism). Unexpectedly, recorded diversity of spatangoids is not proportional to the duration of stages. Bias implied by time scale is negligible compared to bias of sampling or historical determinism. Diversity at any given time is dependent on its recent history (autocorrelation). For spatangoids, a high correlation between diversity at time ti and ti−1 suggests that recorded diversity has an evolutionary significance. A nearly constant rate of diversification is hypothesised for the Cretaceous. A relative poor fossil record during the Turonian and the Coniacian interrupts the main trend of diversification. The number of species counted for a single time interval depends on the number of palaeogeographical realms considered. In conjunction with ecological and phylogenetic data, this relation suggests an evolutionary signal in which western Tethys acted as a centre of origination. Diversity at a single location is constrained ecologically and diversification is controlled by migration into new realms. Recorded diversity and available area of outcrop seem to be correlated, but alternative interpretations can be drawn, including large-scale bias in the fossil record or operation of similar causes (e.g., effect of sea-level fluctuation). Comparing recorded diversity with separate factors independently leads to conflicting results. A multivariate approach suggests that the main trend in recorded diversity might be partially related to evolutionary signal or biases connected with the heterogeneity of the fossil record. Results from other approaches (phylogeny, morphological disparity) are consistent with and emphasise the evolutionary significance of the recorded diversity of spatangoids.