Earth's history is punctuated by the "Big 5" mass extinctions, but smaller, regional-scale mass extinctions also dramatically impacted local faunal composition at times. As we are seeing today in the increase in coral bleaching events and ocean acidification, the marine realm is often on the front lines of climate change. Reconstructing how past ocean temperatures changed across extinction events allows us to understand the baseline climate state upon which extinction occurred and whether temperature changes may have caused, contributed to, or exacerbated extinction severity. My research uses stable and clumped isotope paleothermometry applied to marine mollusk fossils (fossil seashells) to do just this - study the state of the oceans during key extinction intervals of Earth's history. In this talk, I will describe the evolution in my thinking over my career relating to how clumped isotopes can be applied to reveal the most secrets from fossil shells and tell us not only about climate but about how the organism lived, built its shell, and died. I will present results relating to the Cretaceous-Paleogene global mass extinction and the Plio-Pleistocene regional molluscan mass extinction, including both typical applications of isotope paleothermometry and newer, more creative applications.