Paleogene Baltic and Bitterfeld ambers are currently the most important source of Cenozoic fossil lichens. The lichen assemblage suggests that the climate of the European amber forests 41?23 million years (Ma) ago was relatively humid and most likely temperate. In addition to the climatic inference, the lichen fossils reveal information about the divergence and age of lineages and the interactions in the past epiphytic communities. As most crustose lichens are relatively obscured and closely attached in their substrate, calicioid taxa (Calicium, Chaenotheca) were previously the only crustose lichens known as amber inclusions. However, our recent survey demonstrated that further crustose lichens are preserved in European Paleogene ambers. Three of the fossil crustose lichens from Baltic (41?34 Ma) and Bitterfeld (23 Ma) amber belong to the extant genus Ochrolechia (Ochrolechiaceae, Lecanoromycetes) and one possessed conidiomata similar to those produced by modern fungi of the order Arthoniales (Arthoniomycetes). Most intriguingly, two of the fossil Ochrolechia specimens host lichenicolous fungi of the extant genus Lichenostigma (Lichenostigmatales, Arthoniomycetes). The finding confirms that Ochrolechia and Lichenostigma already diversified in the Paleogene. Our study of fruticose lichens from Paleogene amber revealed diverse finely pendulous and more robust morphologies. Among them, the genus Usnea is identifiable from late Eocene Baltic amber, based on general morphology and annular cortical fragmentation. The unique type of cortical cracking suggests the presence of a central cord that keeps the branch intact even when its cortex is split into segments. This evolutionary innovation has remained unchanged since the Palaeogene and likely contributed to the considerable ecological flexibility that allows Usnea species to flourish in a wide variety of ecosystems and climate regimes.
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