FOSSIL ARBUSCULAR MYCORRHIZAE FROM THE EARLY DEVONIAN

The 400 million-year-old Rhynie chert has provided a wealth of informa tion not only of early land plants, but also of the fungi that inhabit ed this paleoecosystem. In this paper we report the first unequivocal evidence of arbuscules in an endomycorrhizal symbiosis. A new genus, G lomites, is characterized by extraradical, aseptate hyphae with a two- parted wall, and an intraradical, highly branched network of thin-wall ed hyphae. Hyphal branches produce terminal, elongate-globose multilay ered spores that lack a basal septum. Other hyphae penetrate cell wall s and form arbuscules. Arbuscules are morphologically identical to tho se of living arbuscular mycorrhizae (AM) in consisting of a basal trun k and highly dichotomous distal branches that form a bush-like tuft. A rbuscules are confined to a narrow band of specialized thin-walled cel ls in the outer cortex that continue to be meristematic. Features of t he fossil biotroph are compared with those of extant arbuscular mycorr hizae. Although interpretations regarding the evolution of mycorrhizal mutualisms continue to be speculative, the demonstration of arbuscule s in the Early Devonian indicates that nutrient transfer is an ancient phenomenon that may have been in existence when plants invaded the la nd.