The ecological context of early vertebrate evolution has been characterized as a gradual shift from passive to more active feeding modes. This evolutionary scenario has been based largely on poorly constrained inferences of the feeding ecology of extinct stem-gnathostomes, among which heterostracans are the earliest. Pteraspidiform heterostracans possessed a feeding apparatus composed of rod-like oral plates with rows of rostrally facing denticles, previously interpreted as an adaptation for suspension feeding. Here, we test this hypothesis using computational fluid dynamics. We simulate water flow around 2D models consisting of rows of denticles both rostrally facing and reversed, to assess whether these orientations create recirculation patterns that are a hydrodynamic adaptation to suspension feeding. All tested models, independent of denticle orientation, show similar flow, velocity, and vorticity patterns. Recirculation patterns, highest velocity, and vorticity develop directly on top of the denticles and in spaces between the denticles. Therefore, we reject the hypothesis that denticle orientation is an adaptation for recirculation linked to suspension feeding. The denticles may instead have served to prevent material from lodging between the plates.