NO CONSISTENT ASSOCIATION BETWEEN TEST SIZE AND EXTINCTION RISK IN FORAMINIFERA

Body size scales with traits such as population density, generation time, and fecundity, suggesting that large size may have been a risk factor for extinction throughout the history of life. For example, extinctions in Pleistocene mammals were strongly biased with respect to size. However, the relationship between size and extinction remains unstudied for most taxa and most of Phanerozoic time. In this study, we assessed the relationship between size and extinction risk using newly-compiled size datasets for foraminiferan genera and species spanning the entire Phanerozoic. Foraminifera are an ideal study group because they have been abundant for much of the Phanerozoic, span over ten orders of magnitude in volume, and have an exceptional fossil record. We quantified the relationship between maximum test radius and extinction risk in stages from the late Devonian to the Holocene using logistic regression. We found that size is significantly associated with genus extinction risk for 23 of 58 stages in the species-level dataset and 22 of 48 stages in the genus-level dataset. In the species-level dataset, 13 stages show preferential extinction of larger taxa whereas 10 show bias toward extinction of smaller taxa. In the genus-level dataset, 17 stages show preferential extinction of larger taxa whereas 5 show bias toward extinction of smaller taxa. The strength of selectivity, measured as the natural logarithm of the odds ratio (β₁) from logistic regression, is not correlated with extinction rate and instances of significant association do not preferentially coincide with major biological or environmental events. Despite its biological importance, size shows no simple relationship to Phanerozoic survivorship patterns in foraminifers. These findings suggest size-biased extinction is not a general evolutionary rule but, rather, the product of specific types of extinction events and their associated selective pressures.