Molecular characterization of acritarchs: applying infrared spectroscopy to better infer biological affinities with other organic-walled microfossils

Acritarchs are an informal, polyphyletic, and morphologically heterogenous group of organic-walled microfossils of unknown biological affinity. Some acritarchs share morphological similarities with certain microplankton resting stages (from e.g. dinoflagellates, prasinophycean-, chlorophyceanand zygnematophycean green algae), others with miospores, egg cases of zooplankton, or even skeletal fragments of higher organisms. For most extant organism groups able to produce organic-walled micro-remains, the structural, fossilizable molecular compounds are relatively well known. These are dinosporin in dinoflagellate cysts, algaenan and cellulose in green algae, sporopollenin in spores and pollen, proteinaceous polysaccharides in zooplankton eggs, chitin in higher animals and cellulose/lignin/cutin in higher plants. While considering the taphonomy of such compounds, molecular parallels can be drawn between acritarchs and microfossils with known biological affinities which, together with possible morphological parallels, provide a stronger argument for inferring biological assignments. Here we used attenuated total reflection microFourier transform infrared spectroscopy to collect a large dataset from a wide range of Quaternary to Palaeozoic micro-remains, including many acritarch species. These data reveal a – sometimes large – molecular variability in samples with taxonomically and morphologically heterogenous assemblages. This shows that chemo-specific signatures can survive diagenesis and can sometimes be used to better infer the biological affinity of acritarchs.