Searching for a nearest living equivalent for Bennettitales: a promising extinct plant group for stomatal proxy reconstructions of Mesozoic p CO 2

• Main Authors: Margret Steinthorsdottir (7459277), Caroline Elliott-Kingston (11170794), Mario Coiro (695370), Jennifer C. McElwain (6670754)
• Format: Dataset
• Language: unknown
• Published: 2021
• Subjects: Evolutionary Biology; Ecology; Sociology; Plant Biology; Environmental Sciences not elsewhere classified; Biological Sciences not elsewhere classified; Stomatal proxy; palaeo-pCO2 reconstruction; climate change; museum collections; herbarium specimens; atmospheric growth chambers; cross-calibration; Mesozoic; Bennettitales; Cycadales; Gnetales; basal angiosperms; Greenland; East Greenland
• Online Access: https://doi.org/10.6084/m9.figshare.15034935.v1

To understand Earth´s climate variability and improve predictions of future climate change, studying past climates is an important avenue to explore. A previously published record of p CO 2 , across the Triassic–Jurassic boundary (TJB, ~201 Ma) of East Greenland, showed that Bennettitales ( Anamozomites and Pterophyllum ) responded in parallel to the empirically proven p CO 2 -responders Ginkgoales, reducing their stomatal densities by half across the TJB, indicating a transient doubling of p CO 2 . The abundance of fossil Bennettitales in Mesozoic strata and natural history museum collections worldwide offers enormous potential for further stomatal proxy p CO 2 reconstructions, but a suitable nearest living equivalent (NLE) should ideally first be identified for this extinct plant group. Using specimens from herbarium collections, three species of cycads, historically considered the best NLE, were tested for p CO 2 response, as well as two species of tree ferns, grown in experimental growth chambers. None responded to changes in p CO 2 , and were consequently rejected as NLEs. Finally, two species of ferns were selected from the literature, and produced very similar p CO 2 compared to Ginkgoales. However, these understory ferns are not appropriate NLEs for Bennettitales due to differences in habitat and a distant evolutionary relationship. Future work should test additional plant groups, in particular seed plants such as basal angiosperms and Gnetales, for suitability as NLE for Bennettitales in p CO 2 reconstructions, for example through biogeochemical fingerprinting using infrared microspectroscopy. Until an appropriate NLE is identified, Bennettitales p CO 2 can be reconstructed based on cross-calibration of stomatal densities with those of co-occurring p CO 2 responders, such as Ginkgoales.