Morphological and functional stasis in mycorrhizal root nodules as exhibited by a Triassic conifer

Abstract

Mycorrhizal root nodules occur in the conifer families Araucariaceae, Podocarpaceae, and Sciadopityaceae. Although the fossil record of these families can be traced back into the early Mesozoic, the oldest fossil evidence of root nodules previously came from the Cretaceous. Here we report on cellularly preserved root nodules of the early conifer Notophytum from Middle Triassic permineralized peat of Antarctica. These fossil root nodules contain fungal arbuscules, hyphal coils, and vesicles in their cortex. Numerous glomoid-type spores are found in the peat matrix surrounding the nodules. This discovery indicates that mutualistic associations between conifer root nodules and arbuscular mycorrhizal fungi date back to at least the early Mesozoic, the period during which most of the modern conifer families first appeared. Notophytum root nodules predate the next known appearance of this association by 100 million years, indicating that this specialized form of mycorrhizal symbiosis has ancient origins.