CONICET | Buscador de Institutos y Recursos Humanos

Premise of Research: Despite their ecological significance in modern terrestrial ecosystems, knowledge about the evolution of arbuscular mycorrhizae based on the fossil record is still scarce, especially concerning the case of root nodules harboring arbuscular mycorrhizal fungi as observed in some extant gymnosperms and angiosperms. Exceptionally preserved conifer nodular roots were found in the Jurassic fossil-bearing chert deposits of the Deseado Massif (Santa Cruz, Argentina), raising the possibility to study them in association with arbuscular mycorrhizal fungi. The aim of this study is to describe the plant organs and their fungal partners, and to discuss the ecological significance of the interactions observed, particularly with respect to their occurrence in the hot-spring settings.Methodology: Thin sections of chert samples from the ?Cañadón Nahuel? locality of the La Matilde Formation, Deseado Massif (Santa Cruz, Argentina) were observed using light microscopy.Pivotal Results: The cortex of the nodules are occupied by several glomeromycotan fungal structures. The structures occur in a specific zone of the cortex - towards its center -, and includes intracellular hyphal coils and arbuscules. Glomoid spores, and coenocytic hyphae possibly penetrating the epidermal cells are also described and analyzed.Conclusions: The root nodules have affinities with the Araucariales, representing the oldest record of such structures for this conifer clade. This is also the first record nodules harboring arbuscular mycorrhizal fungi for the Jurassic; it extends our knowledge of the fossil record of this particular type of fungal association. The presence of the AM fungi may have influenced the distribution of the plants in the distal areas of the hot-spring influenced environments of Patagonia.