Searching for a nearest living equivalent for Bennettitales: a promising extinct plant group for stomatal proxy reconstructions of Mesozoic<i>p</i>CO₂

To understand Earth´s climate variability and improve predictions of future change, studying past climates is an important avenue to explore. A previously published record pCO2, across the Triassic–Jurassic boundary (TJB, ~201 Ma) East Greenland, showed that Bennettitales (Anamozamites Pterophyllum) responded in parallel empirically proven pCO2-responders Ginkgoales, reducing their stomatal densities by half TJB, indicating a transient doubling pCO2. The abundance fossil Mesozoic strata natural history museum collections worldwide offers enormous potential for further proxy pCO2 reconstructions, but suitable nearest living equivalent (NLE) should ideally first be identified this extinct plant group. Using specimens from herbarium collections, three species cycads, historically considered best NLE, were tested response, as well two tree ferns, grown experimental growth chambers. None changes consequently rejected NLEs. Finally, ferns selected literature, produced very similar compared Ginkgoales. However, these understory are not appropriate NLEs due differences habitat distant evolutionary relationship. Future work test additional groups, particular seed plants such basal angiosperms Gnetales, suitability NLE example through biogeochemical fingerprinting using infrared microspectroscopy. Until identified, can reconstructed based on cross-calibration with those co-occurring responders,