Conodont 18O is increasingly used to reconstruct Paleozoic–Triassic seawater temperature changes. Less attention has been paid to 18O variation in time slices across paleoenvironments, within sample assemblages, or for reconstructing the thermal structure of Paleozoic oceans. Furthermore, there have been few independent tests of conodont ecologic models based on bio- and lithofacies distributions. Here we present the first test of ecologic models for conodonts based on 18O values of a Laurentian Lower Ordovician (Floian) shelf edge–upper slope assemblage in debrites of the proximal lower slope Shallow Bay Formation, Cow Head Group, western Newfoundland. Nine species yield a 1.6–1.8‰ intra-sample 18O variability based on mixed tissue and white matter-only analyses, equivalent to a ~7–8 °C range. Linear mixed models demonstrate statistically significant differences between the 18O of some species, supporting the interpretation that an isotopic and temperature gradient is preserved. By considering conodont 18O in a geologic context, we propose an integrated paleoecologic and paleoceanographic model with species tiered pelagically through the water column, and confirm the utility of conodonts for water mass characterization within Paleozoic oceans.