Extended Data Fig. 3. Oxygen and carbon isotope cross plot of modern and Paleozoic corals.

Scatterplot of oxygen and carbon isotope values measured on Paleozoic rugose and tabulate corals and their respective sedimentary matrix from the Hagen Balve Reef and Eifel region. Modern analogues of symbiont-bearing (light green squares) and symbiont-barren (dark green crosses) species from various locations were measured. A compilation of previously measured modern and fossil symbiont-bearing and symbiont-barren corals are also included from Swart (1983), and Frankowiak et al.³¹ and Zapalski 2014, where samples indicate distinct spaces for symbiont-bearing and symbiont-barren species as indicated by the respective field (adapted from Swart, 1983). Devonian values are shown as beige triangles, with darker and lighter brown tones indicating rugose and tabulate species, respectively. Notably, all Paleozoic samples from this study cluster within the symbiotic range and would indicate no difference between coral groups. Replicate measurements of the same species are taken together and the respective variation (±1 SD) is shown by horizontal (oxygen isotopes) or vertical lines (carbon isotopes).