 Duncleistus was a large predator that lived during the Devonian period. It had a large mouth compared to its body size, which made it difficult to accurately estimate its size from fossils. Previous research suggested that Duncleistus could grow up to 9 meters long, but this estimate was based on the assumption that the relationship between mouth size and body size in sharks also applied to arthrodars. This assumption was incorrect, as arthrodars have much larger mouths than sharks, resulting in exaggerated estimates of their size. Additionally, the mouths of arthrodars are proportionally larger than those of extant macro-predatory sharks, suggesting they may have consumed larger prey than sharks. These findings indicate that the paleobiology and paleoecology of Duncleistus and other arthrodars may have differed significantly from that of extant macro-predatory sharks. This article was authored by Russell Engelman.