Examining morphological patterns is key to understanding mechanics and function in biological systems. In extinct taxa, there is a lack of observational or behavioral data to use to understand function, and groups without descendants limit the use of modern homologues. The only available evidence for function is the morphological characteristics of the preserved fossils. The purpose of this research is to analyze the functional consequences of variation in the lower jaw morphology in Late Devonian arthrodire placoderms. Landmark-based morphometric data were collected for 94 isolated arthrodire inferognathals (lower jaws) from the Gogo Formation in Western Australia and the Cleveland Shale in Ohio. These data were used to define the morphological differences and variation between two phylogenetically distinct morphotypes. Statistical results indicate fundamental shape differences between the two arthrodire morphotypes (coccosteomorphs and pachyosteomorphs). Differences in the shape of the blade portion of the jaws imply that the extreme coccosteomorph form (determined by Canonical Variate Analysis, CVA) had stiffer jaws with occlusion patterns similar to modern mammalian carnivores, while the extreme pachyosteomorphs (also determined by CVA) had higher mechanical advantage (force transfer) with occlusion patterns similar to modern herbivorous mammals and bamboo sharks. Arthrodires possibly filled a number of diverse feeding niches. Based on morphological disparity analyses, the mostly open basin pachyosteomorphs had greater overall morphological diversity than the reef-dwelling coccosteomorphs. Lower disparity in reef dwelling arthrodires may be due to sampling bias, environmental factors, or competition from other fish groups present in Devonian reef faunas.