A decrease in body size is a frequently observed response of modern marine ectotherms to climate-related stressors like warming, deoxygenation and acidification, from the mean body size in communities to a decrease in adult body size of individuals. Fossil assemblages also often exhibit a significantly smaller body size in the aftermath of environmental perturbations. This is referred to as the Lilliput effect. However, due to the divergent methods and differences in available data in paleontological and ecological studies, potential parallels between the drivers of fossil and modern body size changes remain obscure. Using a newly assembled dataset of body size changes from fossil, historical and modern body size studies, I present the first quantitative evidence that a negative change in body size is a general response of marine ectotherms to environmental crises throughout the entire Phanerozoic, warming-induced or not. However, the magnitude and volatility of size changes, especially on the species-level, are significantly higher during hyperthermals than nonhyperthermals, indicating differing mechanisms underlying the body size decrease. Specifically, I argue that the within-species body size changes during hyperthermal events could have the same underlying ecological and physiological mechanisms that are driving ectotherm body size decreases in today’s oceans. Studying fossil body size changes on very fine taxonomic and temporal scales from a viewpoint of evolutionary biology presents a valuable opportunity to uncover the specific mechanisms driving body size changes in individual situations. These insights can help generalize the sensitivities and body size responses of certain marine clades to climate-related stressors. I assess the body size patterns across the developmental stages of two ostracod species from the end-Permian Aras Valley section in Iran, as well as multiple Tethyan belemnite (extinct coleoid cephalopods) communities across the Pliensbachian-Toarcian environmental crisis. At the main extinction pulse, ostracods and belemnites show a drastic species turnover, with the postextinction assemblage being significantly larger in body size. The differing responses to climate-related stressors among the clades become apparent during the gradual warming leading up to both extinction events. Ostracod instar body size is not affected, perhaps suggesting that this group has a different response to warming, while two belemnite species decrease in body size. Altered growth rates are difficult, labor-intensive and usually destructive to measure in the fossil record, but often underlie changes in adult body size and morphology. Therefore, I employ 3D geometric morphometrics on the two known Lilliputian species of belemnites from the Pliensbachian-Toarcian GSSP. While the morphological growth trajectory of one species shows signs for a possible paedomorphosis, the adults of the other species gradually disappear and juveniles become morphologically robust, a change that does not recover after the warming pulse at the boundary. This morphological pattern is reminiscent of that produced through hypercalcification of modern juvenile cuttlefish in response to seawater acidification. In summary, this thesis demonstrates that a decrease in body size is a common response of marine ectotherms to environmental crises, with particularly strong changes within species during periods of significant warming. The parallels between the body size changes observed during ancient hyperthermal events and those seen in modern marine organisms provide compelling evidence that the mechanisms leading to body size responses in many marine clades have stayed consistent throughout the earth’s history. By analyzing fossil body size data at high resolutions and incorporating ontogenetic information, I uncover growth changes within fossil species that resemble ecophysiological processes observed in their modern relatives, as well as different clades’ distinctive responses to climate-related stressors. This research highlights the possibility and indeed necessity to inform hypotheses for testing the mechanisms underlying fossil body size patterns with known ecological processes. I conclude that warming has historically influenced marine life in similar ways to today, indicating that current body size reductions are consistent with long-term patterns throughout the Phanerozoic. These insights, derived from ancient crisis events, offer valuable perspectives for forecasting responses of marine clades to the ongoing climate crisis. They suggest that the currently observed body size trends are at least partially driven by warming and will unlikely be fully mitigated, even if direct anthropogenic impacts, like overfishing, ceased.
Paulina Nätscher (Thu,) studied this question.