Stable isotopic values were measured on micrite, sparry calcite, dolomite, inarticulated brachiopods, and conodonts from the Lange Ranch section (central Texas) of the Lower Ordovician Tanyard Formation. The section spans the upper Cordylodus angulatus Zone through the lower Rossodus manitouensis Zone. An ∼2‰ negative δ13C shift from >0‰ to <−1.5‰VPDB through the section suggests the lower third of the Rossodus manitouensis Zone was sampled. Consistent with previous studies, the δ18O values of carbonates are low, ranging from −3.3‰ to −8.1‰VPDB. Phosphate δ18O values range from 15.4‰ to 17.1‰VSMOW. Paleotemperature estimates calculated from micrite δ18O values assuming an ice-free seawater δ18O value of −1‰VSMOW indicate Early Ordovician tropical seawater temperatures averaged 42°C, whereas δ18O values of co-occurring biogenic phosphate assuming the same seawater value yield paleotemperature estimates averaging 37°C. The phosphate values are interpreted as less affected by diagenesis than carbonate values and suggest Early Ordovician tropical paleotemperatures were not more than 10°C warmer or the oxygen isotopic composition of Early Ordovician hydrosphere was not more than 2‰ lower than present.