Searching for biological pigments and structural color in fossil plants

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Although the dazzling colors in living plants are well studied, those in fossil plants have received little attention. In the extant flora, plant color is primarily produced by biological pigments, also known as biochromes, which are expressed in a huge array of hues such as green, yellow, orange, red, pink, blue, violet, ultraviolet, tan, brown, gray, and white. Colors in plants can also arise from structural color or, rarely, from bioluminescence. Structural colors come from the physical and structural modification of surfaces through periodically arranged material on the micro- or nanoscale that make plants appear metallic blue, silver, or iridescent. It is highly likely that biochromes and structural color existed in plants in deep time. However, only a handful of cases of pigmentary color have been studied in fossil plants, and examples of structural color in fossil leaves or fruits have not been reported. Green Cenozoic leaves and a distinctly pink-colored Jurassic alga are among the few intriguing examples of fossil plants known to have pigmentation. Yet, given the highly accurate and sensitive instrumentation and methodology available today, it should be possible to extract, identify, and quantify plant pigments such as chlorophylls, carotenoids, and anthocyanins and their derivatives in fossil plants today. Several Cenozoic floras with colored leaves show promise for pigment research, such as the Miocene Clarkia flora in Idaho, USA, and the Eocene Geiseltal flora in eastern Germany. While derivatives of chlorophyll have been extracted and identified from green fossil leaves from both floras, it is likely that other pigments have also survived the 15 to 45 million years since deposition, which the newly purchased, highly sensitive and accurate Sciex QTRAP 6500+ System, a triple quadrupole/linear ion trap LCMS/MS mass spectrometer, in the Pharmaceutical Institute at the University of Bonn will be able to detect and measure. Non-destructive methods, such as confocal Raman spectroscopy, will be developed for the detection, identification, and chemical analysis of pigments. For field work and reconnaissance in museum collections, the non-destructive use of a hand-held spectrometer for the initial evaluation of promising specimens with pigmentary color remains will also be explored. Similarly, the use of a non-destructive, high-resolution approach, such as low-vacuum scanning electron microscopy, will be employed to search for nanostructures on the surface of fossil leaves that may have produced structural color. Because structural color is most common as a shimmery iridescence in the shade leaves of rainforest plants, we will analyze leaves from Cenozoic conservation lagerstätten such as the tropical Eocene flora from Messel near Frankfurt with the newly acquired, high-performance, variable-pressure TESCAN Vega 4 SEM at the Division of Paleontology at the University of Bonn.