The Permian/Triassic mass extinction marks a permanent phylogenetic shift in the composition of the sessile benthos, from one largely dominated by articulate brachiopods to one dominated by mollusks. Widespread evidence of oceanic hypoxia and anoxia at this time provides a possible selective kill mechanism that could help explain the large taxonomic losses in brachiopods compared to the morphologically and ecologically similar bivalve molluscs. Our study compared the oxygen consumption of an articulate brachiopod, Terebratalia transversa, with that of two pteriomorph bivalves, Glycymeris septentrionalis and Mytilus trossulus, under normoxia and hypoxia, as well as their tolerance to anoxia, to gain insight into the relative metabolic characteristics of each group. We found no significant difference in the oxygen consumption of the three species when normalized to the same dry-tissue mass. However, when calculated for animals of the same external linear dimensions, bivalve oxygen consumption was two to three times greater than that of brachiopods. Our results also showed no significant decrease in the oxygen consumption of the three species until measured at a partial pressure of oxygen ∼10% of normoxic values. Finally, T. transversa and M. trossulus showed no significant difference in their tolerance to complete anoxia, but both showed a much lower tolerance than another bivalve, Acila castrensis. Findings from this study suggest that oxygen limitation is unlikely to account for the observed selective extinction of brachiopods during the Permian/Triassic mass extinction. Results may provide valuable information for assessing hypotheses put forth to explain why articulate brachiopods continue to remain a relatively minor group in marine environments.