Sprecher
Beschreibung
Mycorrhizae are symbiotic relations between land plants and soil fungi that play a vital role for plant- productivity, occur in most terrestrial ecosystems and plant species, and have a fossil record dating back at least into the Early Devonian. Extant Royal Ferns (Osmundaceae) are commonly mycorrhizal, but hardly any evidence of mycorrhizae is known from the rich and diverse fossil record of this family. Here I describe a putative arbuscular-mycorrhizal fungus (AMF) found within a silicified Claytosmunda beardmorensis rhizome from Late Triassic strata of the Fremouw Formation, Antarctica. Certain roots of the specimen contain an abundance of aseptate, inter- and intracellular hyphae that grow mainly along the longitudinal axis of the root and occasionally form coils within host cells. Hyphae are irregularly swollen and constricted and vary widely in diameter, with those at the periphery of the root cortex generally being wider than those closer to the root centre. Intercellular hyphae in a specific zone around the vascular bundle produce narrower side branches that enter host cells and form repeatedly ramified structures resembling arbuscules. Occurring in the same roots are thin- and thick-walled globular structures, some with multiple wall layers, that I interpret as storage vesicles and spores of the same fungus. This find represents the oldest fossil evidence for mycorrhizal symbiosis in the Osmundaceae, and the first example of mycorrhization in fossil ferns from Antarctica. Since C. beardmorensis is one of the oldest species of crown group Osmundaceae, its mycorrhizal status indicates that AMF-symbiosis is ancestral for the group.