STABLE ISOTOPE SIGNALS FROM EXCEPTIONALLY-PRESERVED PLANT FOSSILS REVEAL PALEO-PHYSIOLOGICAL ADAPTATIONS TO DIFFERENT ATMOSPHERIC CONDITIONS

Physiological adaptations to paleoenvironment by ancient plants have traditionally been studied by morphological and anatomical means. The advent of new technology and rapidly-accumulated stable isotope records from both ancient and modern plants allow stable isotope data to be used for the inference of plant physiological evolution. Stable isotope (IRMS) analyses of exceptionally-preserved conifer fossils (Larix, Glyptostrobus, Pseudolarix, and Metasequoia) from Cenozoic fossil lagerstätten suggest that the offset of δ¹³C values between lipid molecules and bulk tissues was maintained in a similar fashion as their modern counterparts despite a marked positive shift (~4‰) of δ¹³C in the fossil material. Both morphological (light microscope and SEM) and bio-molecular (Py-GC-MS and NMR) studies suggested that these isotopic changes were unlikely caused by tissue decay, and constructed models demonstrated that the change of δ¹³C in atmospheric CO₂ (δ¹³C_atmo) alone cannot explain the positive shift of δ¹³C values seen in these Cenozoic fossils. Given our understanding of carbon isotope fractionations in higher plants, we propose a new explanation to this phenomenon: Plant physiological adaptations that resulted in the change of C_i/C_a ratios (the ratio of leaf internal to atmospheric CO₂ concentration), perhaps due to the high concentration of CO₂ during Paleogene, may be more likely responsible for the observed carbon isotope signatures in these exceptionally preserved fossils. Our data suggest that these morphologically static coniferous “living fossils” may have been more sensitive than we had expected to respond the change of atmospheric conditions over the geological time. We argue that such a physiological plasticity can be registered as stable carbon isotope signals in well-preserved fossils, and combining with morphological/anatomical data, stable isotope record is a powerful tool for better understandings of plant paleo-physiology.