IDENTIFYING CONTROLS ON PHANEROZOIC EXTINCTION AND DIVERSIFICATION PATTERNS

In attempting to explain long-term controls on patterns of extinction and diversification it is important to establish the necessary, logical connections between physical causes and biotic effects. This analysis requires multiple examinations of each cause-and-effect pair to document common associations and can only be achieved through the explicit comparison of phenomena that occur at different scales. The record of marine fossil life forms and our understanding of global environmental change over that same interval is now sufficiently complete to undertake these types of studies.

Evidence that Phanerozoic physical and biotic records are linked deterministically is provided by both records being directional in a similar manner. It is also striking that a number of paleoceanographical environmental proxies exhibit statistically significant, first-order patterns of Phanerozoic variation that strongly resemble the background extinction-intensity gradient. These results suggest that secular changes in the continent-derived nutrient flux to the oceans exerted a dominant control on ‘background’ extinction patterns. Furthermore, the pattern of second-order deviations from this general trend suggest that evolutionary developments in plant clades represent the mechanism through which these abiotic changes are first expressed in the biotic realm. The significance of these trends for understanding patterns of macroevolution can be seen in numerous examples of animal clade diversifications that coincide with plant clades diversifications; especially in the Early Paleozoic and Early Mesozoic.

On the larger end of the phenomenological spectrum, new statistical results show that the stage-level, ‘mass’ extinction record does not exhibit significant periodicity, but does show significant correspondence to the record of flood-basalt volcanism (Mesozoic and Cenozoic) and eustatic sea-level change (Paleozoic). The bolide impact record does not exhibit a significant overall association with stage level extinction events. Over the last 250 m.y. differences in the magnitudes of stage-level ‘mass’ extinction events seem to be best explained by a multi-factor model involving independent inputs from tectonic processes (e.g., hot-spot volcanism, eustatic sea level) perhaps augmented by extraterrestrially forced, global, environmental perturbations.