A new study proposes an explanation for why present-day shorelines are dominated by shells from clams and snails rather than brachiopods. The research attributes the shift to conditions during Earth’s largest mass extinction event, when oceans warm and oxygen levels decline. According to the study, many marine animals were unable to adapt to the combined stress of higher temperatures and reduced oxygen, leading to widespread die-offs. In contrast, species with body plans and metabolisms better suited to the changing environment survive and subsequently become more common, eventually dominating marine ecosystems. The findings are presented as a way to understand how major marine extinctions unfold and how modern marine life might respond to ongoing climate change. By focusing on the roles of ocean warming and oxygen loss, the study aims to connect environmental change during the extinction to differences in survival and recovery among marine groups. The work also suggests that biological traits that support coping with lower oxygen and warmer waters can be key determinants of which organisms persist after such global disturbances.