The fossil record provides the only direct evidence of changes in biodiversity over time. Patterns in more inclusive taxonomic levels (e.g., families and orders) often become more complex because of interactions between biological traits and environmental conditions across different evolutionary lineages. Using supercomputing and artificial intelligence algorithms, we analyzed a high-resolution global dataset of fusuline foraminifera-the most diverse marine fossil group from the Carboniferous to the Permian (similar to 340 to 252 million years ago)-at an unprecedented temporal resolution of <45 thousand years. Our unbinned diversity reconstruction reveals unexpectedly simple diversity dynamics in this exceptionally well-preserved clade. We identify two (and likely a third) truncated exponential diversifications and four major diversity declines. During this interval, long-term cooling consistently promoted biodiversification, whereas warming events were closely linked to extinctions. These findings imply that the current rapid global warming, driven by anthropogenic CO2 emissions, represents a critical threat to modern ecosystems.