Carbon and oxygen isotope fractionation in the Late Devonian heterocoral Oligophylloides: Implications for the skeletogenesis and evolution of the Heterocorallia

The order Heterocorallia is an enigmatic, anatomically distinct group of calcifying Palaeozoic corals with unclear affinities to other, either extinct or modern cnidarian lineages. Here, we present results of microstructure and stable isotope studies on exceptionally well-preserved skeletons of two species of the heterocoral Oligophylloides from the Famennian (Upper Devonian) of the Anti-Atlas, Morocco and the Holy Cross Mountains, Poland. The investigations provide new information on the skeletogenesis and isotope fractionation effects in Palaeozoic heterocorals. In order to estimate the possible role of diagenetic alteration on the observed carbon and oxygen isotope ratios, we have conducted several preservation tests, including transmitted-light, scanning-electron, fluorescence and cathodoluminescence microscopy, Raman spectroscopy, and trace element analyses. The leastand most-altered skeleton portions identified based on this approach are typified by clearly different isotopic signatures. The least-altered parts of the specimens reveal carbon and oxygen isotope ratios close to those of contemporaneous marine-equilibrated calcites. The moderately-pronounced, positive correlations between the 813C and 818O values observed for these coralla may reflect relatively minor vital fractionation effects, obscured to some degree by diagenetic overprint. The results suggest that the isotope fractionation mechanisms of the heterocorals differed markedly from the significant kinetic fractionation characteristic of the modern scleractinian corals. Rather, the moderate, if any, extent of non-equilibrium fractionation during skeletogenesis in the Heterocorallia resembled the typical, environmental and metabolic CO2-dominated fractionation effects shown by most other marine calcifiers. These include the extinct rugose corals, modern hydrocorals and, in particular, modern octocorals, with which the heterocorals show notable architectural and structural similarities.