Paleoecology, Ploidy, Paleoatmospheric Composition, and Developmental Biology: A Review of the Multiple Uses of Fossil Stomata1[OPEN]

The presence of stomata is a diagnostic trait of all living and extinct land plants with the exception of liverworts. They are preserved widely in the fossil record from anatomically pristine stomatal complexes on permineralized and charcoalified stems of the earliest land plants dating back .400 million years to isolated guard cell pairs in quaternary aged palynological samples. Detailed study of fossil stomatal complexes has been used to track the evolution of genome size and to reconstruct atmospheric composition, to circumscribe new species to science, and to bring ancient landscapes to life by providing both habitat information and insights on fossil plant ecophysiological function and life form. This review explores how fossil stomata can be used to advance our understanding of plant, environment, and atmospheric evolution over the Phanerozoic. We compare the utility of qualitative (e.g. presence/absence of stomatal crypts) versus quantitative stomatal traits (e.g. amphistomaty ratio) in paleoecological reconstructions. A case study on Triassic-Jurassic Ginkgoales is provided to highlight the methodological difficulty of teasing apart the effect of genome size, ploidy, and environment on guard cell size evolution across mass extinction boundaries. We critique both empirical and mechanistic stomatal-based models for paleoCO2 reconstruction and highlight some key limitations and advantages of both approaches. Finally, we question if different stomatal developmental pathways have ecophysiological consequence for leaf gas exchange and ultimately the application of different stomatal-based CO2 proxy methods. We conclude that most studies currently only capture a fraction of the potential invaluable information that can be gleaned from fossilized stomata andhighlight future approaches to their study that better integrate across the disciplinary boundaries of paleobotany, developmental biology, paleoecology, and plant physiology. The fossil record of land plants (embryophytes) dates back unequivocally to the Middle Ordovician (;460 million years ago [mya]). This is supported by the presence of spore tetrads contained within an enveloping sporangium (Wellman et al., 2003). Since Wellman’s discovery, the fossil spore record has revealed older and older spores of variousmorphologies (naked, enveloped) and configurations (singular, paired, etc.) that may eventually push back even further the accepted date of the oldest land plant (Wellman and Strother, 2015). This early phase in land plant evolution is complex to interpret, however, since no stomata have been discovered so far on the earliest fossilized land plants, suggesting perhaps that theymay have been absent, as is the case for the early land plants’ algal predecessors. As soon as sheets of fossilized cuticle with true stomata started to appear in Siluran aged (443–419 mya) sediment samples, our ability to taxonomically separate charophyacean algae from land plants based on fragmentary fossil evidence improved greatly because the presence of stomata is the defining anatomical trait of all living and extinct land plant sporophytes with the exception of liverworts. Therefore, it is unsurprising that the use of fossilized stomata for taxonomic purposes and to elucidate the phylogeny of land plants as revealed by the fossil record has a long history in paleobotany. Stomatal traits that are considered of utility for fossil plant taxonomy and systematics are numerous, including stomatal presence or absence, size, geometry and orientation, and association with subsidiary cells (Table I), whether they are sunken, raised, or flush with epidermal cells or pluggedwithwax, are kidneyor dumbbell shaped, are overarched by papillate subsidiary/epidermal cells, or 1 This work was supported by a Science Foundation Ireland Principal Investigator Award 11/PI/1103, by European Research Council Award ERC-2011-StG 279962 to J.C.M., and by Swedish Research Council Starting Grant (VR NT-7 2016 04905) and the Bolin Centre for Climate Research, Stockholm University, to M.S. * Address correspondence to [email protected]. J.C.M. and M.S. cowrote the manuscript. [OPEN] Articles can be viewed without a subscription. www.plantphysiol.org/cgi/doi/10.1104/pp.17.00204