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Abstract Body size is an essential factor in an organism's survival, and when paired with paleoenvironmental proxies, size trends can provide insights into a lineage's evolutionary responses to changing environmental conditions. This study explores the diversity and body-volume trends of dacryoconarid tentaculitoids, globally abundant marine zooplankton, in the Devonian of the Appalachian Basin (eastern United States), spanning the late Givetian through the middle Frasnian punctata carbon isotope excursion. Using statistical approaches to model trends, we find evidence of a gradual, within-lineage reduction in styliolinid adult body sizes starting at the Givetian-Frasnian boundary. This reduction is followed by a significant decrease in both adult and initial chamber volumes during the punctata excursion. At the Givetian-Frasnian boundary, annulated forms (nowakiids) become rare and smooth forms (styliolinids) begin to dominate the assemblage. Using pre-existing geological and geochemical data sets, we consider environmental factors, including sea level, anoxia, nutrient availability, and temperature, as potential drivers of body-size reductions. Bottom-water anoxia most likely did not influence body-size trends of this pelagic group, but frequent water-column overturning in the Frasnian or other exchange between deep and shallow water may have affected taxonomic composition, favoring styliolinids. Sea-surface temperature correlates inversely with body size, suggesting that warming beginning in the early Frasnian may have contributed to gradual, long-term size reductions. Rising temperatures through the middle Frasnian may have led to the disappearance of dacryoconarids in the northern Appalachian Basin after the excursion.