The body size of marine ectotherms is often negatively correlated with ambient water temperature, as seen in many clades during the hyperthermal crisis of the end-Permian mass extinction (c. 252 Mya). However, in the case of ostracods, size changes during ancient hyperthermal events are rarely quantified. In this study, we evaluate the body size changes of ostracods in the Aras Valley section (NW Iran) in response to the drastic warming during the end-Permian mass extinction at three taxonomic levels (class, order, species). On the assemblage level, the warming triggers a complete species turnover in the Aras Valley section, with larger, newly emerging species dominating the immediate post-extinction assemblage for a short time. Individual ostracod species and instars do not show dwarfing or a change in body size as an adaptation to the temperature stress during the end-Permian crisis. This may indicate that ostracods might be exceptions to the temperature-size rule (TSR), using an adaptation mechanism that does not involve a decrease in body size. Many recent experimental studies show that accelerated development without size changes is a common response of modern ostracods to thermal stress.

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