Calcium oxalate (CaOx) is one of the most common bio-mineral in extant plants and is believed to serve a variety of functions such as calcium storage and herbivore defense. However, traces of CaOx crystals have rarely been identified in fossil plants, and they are primarily known from fossil gymnosperms, where empty cavities of former CaOx crystals or ghost crystals have been reported from leaf cuticles of some Late Cretaceous and Cenozoic conifers. Here we investigate fossil angiosperm leaves from the late Oligocene Rott Fossil Lagerstätte and report ghost crystals of various shapes, sizes and topology (distribution patterns), and cavities. These micromorphological structures of fossil leaves are compared to CaOx deposits in leaves of extant plants: globular structures in fossil leaves resemble CaOx druses (crystal aggregates) in fresh leaves in size and distribution; and angular or brick-shaped structures in the vascular system of fossil leaves closely resemble prismatic CaOx crystals in the vascular system of extant leaves in both size and topology. Chemically, CaOx druses have survived fossilization as cavities only, and were replaced by organic matter and ghost minerals containing Ca, Si, Al, S, and Fe. The identification of former CaOx remains in leaf fossils provides novel insights on the fate of plant bio-minerals during fossilization. More importantly, it provides an additional aspect of the ecophysiology of fossil plants thus improving the accuracy of palaeoecological reconstructions and can provide a broader perspective on the evolution of CaOx and their rule in plant ecology across geological timescales. Alternative interpretations of the fossil microstructures are discussed but ruled out.
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