Climate sensitivity is defined as the global surface temperature response (ΔGST) to a given radiative forcing (ΔF) and, as such, forms an important factor in predicting the evolution of the climate system during the Anthropocene. The surface temperature at which the planet achieves radiative equilibrium is set by the interaction of numerous components of the Earth system, such that the magnitude and behavior of climate sensitivity under various levels of forcing remain difficult to constrain (Knutti et al., 2017). The target quantity of interest in most studies using climate models and instrumental observations is called the equilibrium climate sensitivity (ECS). ECS only considers the behavior of climate feedbacks with a centennial response timescale, including those that amplify (e.g., water vapor, sea ice, and clouds) or attenuate temperature changes (e.g., lapse rate adjustment), and is therefore considered the best measure for the expected global-scale atmospheric response to human perturbation within the near future (e.g., Sherwood et al., 2020).The geologic record is recognized as a powerful tool to elucidate the sensitivity of the climate system, as it contains numerous examples of a planet at steady state with elevated climate forcing created by changes in greenhouse gas concentrations, solar luminosity, and ocean area (Farnsworth et al., 2019;Rohling et al., 2012).

A negative correlation between body size and the latitudinal temperature gradient is well established for extant terrestrial endotherms but less so in the fossil record. Here we analyze the middle eocene site of Geiseltal (Germany), whose record is considered to span ca. 5 Myrs of gradual global cooling, and generate one of the most extensive mammalian paleogene body size datasets outside north America. the δ 18 o and δ 13 c isotopic analysis of bioapatite reveals signatures indicative of a humid, subtropical forest with no apparent climatic change across Geiseltal. Yet, body mass of hippomorphs and tapiromorphs diverges rapidly from a respective median body size of 39 kg and 124 kg at the base of the succession to 26 kg and 223 kg at the top. We attribute the divergent body mass evolution to a disparity in lifestyle, in which both taxa maximize their body size-related selective advantages. our results therefore support the view that intrinsic biotic processes are an important driver of body mass outside of abrupt climate events. Moreover, the taxonomy previously used to infer the duration of the Geiseltal biota is not reproducible, which precludes chronological correlation with eocene marine temperature curves.