Low-latitude Ordovician to Triassic brachiopod habitat temperatures (BHTs) determined from δ18O[brachiopod calcite]: A cold hard look at ice-house tropical oceans

► Application of the calcite thermometer modified for Paleozoic datasets. ► New δ18O results identify cool tropical oceans from mid-Ordovician to top-Permian. ► Recurring Paleozic land-based glaciations often extended to low latitudes. ► Abundant and diverse Paleozoic brachiopods thrived in cool tropical seas. ► Results consistent with Mg/Ca temperatures but not with clumped isotope results.