Looking for Arbuscular Mycorrhizal Fungi in the Fossil Record: An Illustrated Guide

The evolution and diversification of plants on land were profoundly influenced by mutually beneficial symbioses between the plants and certain fungi. The vast majority of fungi involved in these fossil associations are strikingly similar to present-day arbuscular mycorrhizal fungi (AMF), and their symbioses with plants closely resemble present-day arbuscular mycorrhizas (AM). Although fossil evidence of AM has variously been documented and illustrated throughout the late Palaeozoic to Cenozoic, the record in general remains exceedingly scant. However, we believe that more compelling evidence of AM and AMF in fossil plants can be gathered if paleobotanists are equipped with an accurate search image for mycorrhizal fungi and the core structural components of their associations with plants. This chapter presents an illustrated guide that provides researchers with a synopsis of important (i.e., recognizable in transmitted light) structural features of modern AM that facilitates the accurate identification of fossil members of this group of fungi and their discrimination from other, nonmycorrhizal fungi while examining structurally preserved plant fossils. Where available, fossil mycorrhizal fungi displaying the features included in this guide are also documented.