Diversity-dependent evolutionary rates in early Palaeozoic zooplankton

The extent to which biological diversity affects rates of diversification is central to understanding macroevolutionary dynamics, yet no consensus has emerged on the importance of diversity-dependence of evolutionary rates. Here, we analyse the species-level fossil record of early Palaeozoic graptoloids, documented with high temporal resolution, to test directly whether rates of diversification were influenced by levels of standing diversity within this major clade of marine zooplankton. To circumvent the statistical regression-to-the-mean artefact, whereby higher- and lower-than-average values of diversity tend to be followed by negative and positive diversification rates, we construct a non-parametric, empirically scaled, diversity-independent null model by randomizing the observed diversification rates with respect to time. Comparing observed correlations between diversity and diversification rate to those expected from this diversity-independent model, we find evidence for negative diversity-dependence, accounting for up to 12% of the variance in diversification rate, with maximal correlation at a temporal lag of approximately 1 Myr. Diversity-dependence persists throughout the Ordovician and Silurian, despite a major increase in the strength and frequency of extinction and speciation pulses in the Silurian. By contrast to some previous work, we find that diversity-dependence affects rates of speciation and extinction nearly equally on average, although subtle differences emerge when we compare the Ordovician and Silurian

The Role of Temporal Abundance Structure and Habitat Preferences in the Survival of Conodonts during the Mid-Early Silurian Ireviken Mass Extinction Event

The Ireviken event was one of the most intense extinction episodes that occurred during the mid-Paleozoic era. It had a strong global effect on a range of clades, with conodonts, graptolites and chitinozoans affected most. Using geophysical proxies and conodont species parameters of their temporal abundance structure we investigate how they affected the selectivity of conodont species survival during this calamity. After performing bivariate logistic analyses on 34 species of conodonts, we find three variables that were statistically significantly associated with their odds of survival. These namely include spectral exponents that describe degrees of autocorrelation in a time series, the skewness of species abundance distribution, and average environmental preferences, which are mostly determined by ancient water depths at sampling sites. Model selection of multivariate logistic models found the best model includes species local abundance skewness and substrate preference. A similar pattern is revealed through the regression tree analysis. The apparent extinction selectivity points to a possible causes of environmental deterioration during the Ireviken event. The significant positive relationship between extinction risk and preferential existence in deeper environments suggests the open ocean causal mechanisms of biotic stress that occurred during the Ireviken event. Marine regressions, which were previously suggested as a causal factor in this extinction episode, on theoretical grounds should have had higher impact on species living in near-shore environments, through the processes of habitat loss which are associated with decreases of shelfal areas. In addition, the significant positive correlations found between skewness of abundance distributions and spectral exponent values and the probability of species survival suggest that community and ecosystem processes (which controlled species abundance fluctuation patterns) were significantly related to selectivity processes of this smaller mass extinction event

An Analysis of the Ireviken event in the Boree Creek Formation, New South Wales, Australia

Conodont data from a section through the Boree Creek Formation in New South Wales, the best sequence in Australia extending through the Llandovery–Wenlock boundary, represent strata of the Ireviken Event. A significant number of conodont datum planes within the sequence indicating step-wise extinctions can be recognised and correlated with the sequence through the Visby Beds, in Gotland, Sweden, the most comprehensively analysed Ireviken Event sequence globally. This intercontinental correlation, involving both north and south hemispheres, adds to the data set concerning cyclicity in Silurian marine strata and postulated models concerning causality.16 page(s