Dasycladalean green algae show marked fluctuation in genus and species biodiversity from the Carboniferous to the Pliocene. Diversity lows (<10 species) alternate with peaks (>70 species) over periods of ∼20–50 Myr. Relatively few taxa are recorded for the earliest Carboniferous, Early Triassic, Early to Mid-Jurassic, Late Cretaceous, and Pliocene. Diversity maxima occur in the Permian, Early Cretaceous, and Paleocene. With the exception of the Late Cretaceous, biodiversity broadly tracked temperature from the Carboniferous to the Pliocene. Diversity minima generally correspond with low sea level, and diversity maxima with periods of intermediate sea level. Dasycladaleans were most diverse when their main habitats—warm shallow seas—were most extensive. This observation does not preclude the influence of additional important factors on dasycladalean evolutionary history, but it suggests a strong link between long-term patterns of dasycladalean diversity and global fluctuations in temperature and sea level.