The origin and diversification of pteropods precede past perturbations in the Earth's carbon cycle

Pteropods are abundant aragonitic calcifiers, contributing up to 89% of total pelagic calcification. Because of their delicate shells, they are considered "canaries in the coalmine" to indicate impacts of ocean acidification. Their sensitivity to high CO₂ levels and limited fossil record has led to the widely held view that pteropods only became abundant after the PETM. Based on phylogenomic analyses, we show that all major pteropod groups have Cretaceous origins and, hence, they must have survived past perturbations in the Earth's carbon cycle. Although this suggests that pelagic aragonitic calcifiers have been more resilient to past ocean acidification than currently thought, it is unlikely that pteropods have experienced global change of the current magnitude and speed during their evolutionary history.