Fossil Xenoxylon (Coniferopsida) wood from the Upper Cretaceous of Heilongjiang, China: Evidence of a tripartite plant-arthropod-fungal association

Abstract

Silicified conifer wood assignable to Xenoxylon meisteri from the Upper Cretaceous of northeastern China provides evidence of the presence of wood-degrading fungi and xylophagous arthropods, and also indicates the existence of a relationship between fungi and arthropods. Evidence of arthropod activities occurs in the form of vertical and horizontal borings that are lined on the inside with small coprolites. The borings are much larger (up to 2.4 mm in diameter) than the coprolites (74–127 μm in diameter), suggesting that two different animals, a larger one that made the borings and a smaller one that produced the coprolites, lived in them, either simultaneously or one after another. The wood in the surroundings of the borings contains different types of fungal remains. Especially abundant are slender, septate, and multi-branched hyphae on which thick-walled chlamydospores and other hyphal inflations are formed. Hyphae traversing from one host cell to another usually produce a swelling close to where they enter the cell wall. Decay holes in the infected wood suggest that this fungus was a wood-degrading saprotroph, probably a member of the Ascomycota. The fungus also occurs inside the arthropod borings, where it forms chains of barrel-shaped cells, likely conidia, suggesting that the arthropods may have served as dispersal vectors for the fungus. Xenoxylon meisteri provides the first indication of a tripartite plant-arthropod-fungal association from the Mesozoic of China, and gives new insights into the complex network of organismal relationships that existed in Cretaceous terrestrial ecosystems.