Navigating uncertainty in maximum body size in marine metazoans.

Body size is a fundamental biological trait shaping ecological interactions, evolutionary processes, and our understanding of the structure and dynamics of marine communities on a global scale. Accurately defining a species' body size, despite the ease of measurement, poses significant challenges due to varied methodologies, tool usage, and subjectivity among researchers, resulting in multiple, often discrepant size estimates. These discrepancies, stemming from diverse measurement approaches and inherent variability, could substantially impact the reliability and precision of ecological and evolutionary studies reliant on body size data across extensive species datasets. This study examines the variation in reported maximum body sizes across 69,570 individual measurements of maximum size, ranging from <0.2 µm to >45 m, for 27,271 species of marine metazoans. The research aims to investigate how reported maximum size variations within species relate to organism size, taxonomy, habitat, and the presence of skeletal structures. The investigation particularly focuses on understanding why discrepancies in maximum size estimates arise and their potential implications for broader ecological and evolutionary studies relying on body size data. Variation in reported maximum sizes is zero for 38% of species, and low for most species, although it exceeds two orders of magnitude for some species. The likelihood of zero variation in maximum size decreased with more measurements and increased in larger species, though this varied across phyla and habitats. Pelagic organisms consistently had low maximum size range values, while small species with unspecified habitats had the highest variation. Variations in maximum size within a species were notably smaller than interspecific variation at higher taxonomic levels. Significant variation in maximum size estimates exists within marine species, and partially explained by organism size, taxonomic group, and habitat. Variation in maximum size could be reduced by standardized measurement protocols and improved meta-data. Despite the variation, egregious errors in published maximum size measurements are rare, and their impact on comparative macroecological and macroevolutionary research is likely minimal.

Reduced strength and increased variability of extinction selectivity during mass extinctions.

Two of the traits most often observed to correlate with extinction risk in marine animals are geographical range and body size. However, the relative effects of these two traits on extinction risk have not been investigated systematically for either background times or during mass extinctions. To close this knowledge gap, we measure and compare extinction selectivity of geographical range and body size of genera within five classes of benthic marine animals across the Phanerozoic using capture-mark-recapture models. During background intervals, narrow geographical range is strongly associated with greater extinction probability, whereas smaller body size is more weakly associated with greater extinction probability. During mass extinctions, the association between geographical range and extinction probability is reduced in every class and fully eliminated in some, whereas the association between body size and extinction probability varies in strength and direction across classes. While geographical range is universally the stronger predictor of survival during background intervals, variation among classes during mass extinction suggests a fundamental shift in extinction processes during these global catastrophes.

Breathless through Time: Oxygen and Animals across Earth's History.

AbstractOxygen levels in the atmosphere and ocean have changed dramatically over Earth history, with major impacts on marine life. Because the early part of Earth's history lacked both atmospheric oxygen and animals, a persistent co-evolutionary narrative has developed linking oxygen change with changes in animal diversity. Although it was long believed that oxygen rose to essentially modern levels around the Cambrian period, a more muted increase is now believed likely. Thus, if oxygen increase facilitated the Cambrian explosion, it did so by crossing critical ecological thresholds at low O2. Atmospheric oxygen likely remained at low or moderate levels through the early Paleozoic era, and this likely contributed to high metazoan extinction rates until oxygen finally rose to modern levels in the later Paleozoic. After this point, ocean deoxygenation (and marine mass extinctions) is increasingly linked to large igneous province eruptions-massive volcanic carbon inputs to the Earth system that caused global warming, ocean acidification, and oxygen loss. Although the timescales of these ancient events limit their utility as exact analogs for modern anthropogenic global change, the clear message from the geologic record is that large and rapid CO2 injections into the Earth system consistently cause the same deadly trio of stressors that are observed today. The next frontier in understanding the impact of oxygen changes (or, more broadly, temperature-dependent hypoxia) in deep time requires approaches from ecophysiology that will help conservation biologists better calibrate the response of the biosphere at large taxonomic, spatial, and temporal scales.

Mass extinctions alter extinction and origination dynamics with respect to body size.

Whether mass extinctions and their associated recoveries represent an intensification of background extinction and origination dynamics versus a separate macroevolutionary regime remains a central debate in evolutionary biology. The previous focus has been on extinction, but origination dynamics may be equally or more important for long-term evolutionary outcomes. The evolution of animal body size is an ideal process to test for differences in macroevolutionary regimes, as body size is easily determined, comparable across distantly related taxa and scales with organismal traits. Here, we test for shifts in selectivity between background intervals and the 'Big Five' mass extinction events using capture-mark-recapture models. Our body-size data cover 10 203 fossil marine animal genera spanning 10 Linnaean classes with occurrences ranging from Early Ordovician to Late Pleistocene (485-1 Ma). Most classes exhibit differences in both origination and extinction selectivity between background intervals and mass extinctions, with the direction of selectivity varying among classes and overall exhibiting stronger selectivity during origination after mass extinction than extinction during the mass extinction. Thus, not only do mass extinction events shift the marine biosphere into a new macroevolutionary regime, the dynamics of recovery from mass extinction also appear to play an underappreciated role in shaping the biosphere in their aftermath.