Bioerosion of the upper Jurassic lithographic limestone of Solnhofen (Germany) by mycorrhizal roots during the Cenozoic karstification

Bedding planes of the early Tithonian lithographic limestone of Solnhofen (southern Germany) contain extensive networks of filiform tubular structures. The size of the irregularly distributed network polygons varies between several centimeters to few millimeters, and the diameter of the circular tunnels directionally decreases from millimetric to sub-millimetric. The network represents a bioerosion trace fossil (i.e. boring) and is interpreted to result from the bioeroding activity of plant roots and their symbiotic interaction with fungi (i.e. mycorrhiza). The traces are filled with an earthy substrate that is rich in iron minerals and has an ochre-brown color, and many of the tunnels were loci for manganese dendrites preferably forming in the bedding plane. It is assumed that the formation of the traces by plant colonization and bioerosion took place during the Paleogene to Neogene, when the area was affected by karstification. The networks are comparable with mycorrhizal networks of extant plants, which comprise a symbiotic association of plants and fungi. Although such symbioses are important for plant ecosystems and were crucial for evolutionary processes, only their body-fossil record has been traced back to the Ordovician, whereas their trace-fossils expression was hitherto unknown. The structures are described as Radixichnus reticularis igen. et isp. nov., which is the first ichnotaxon interpreted as terrestrial root bioerosion trace fossils.