Abstract
The pre-Cretaceous fossil record of leaf-inhabiting fungi is meagre. A structurally preserved pinnule fragment from the Upper Pennsylvanian of Grand-Croix (Saint-Étienne Basin, France) contains a fungus of uncertain affinity that occurs as an intracellular endophyte in the asymptomatic hypodermis. The fungus consists of branched, septate hyphae producing long-necked hyphal swellings and spherical to ovoid structures, probably conidia, which may occur singly or in short chains. This discovery provides new insights into the biology of leaf-inhabiting fungi some 300 Ma ago, and indicates that the mycological definition of fungal endophyte is difficult to apply to fossils. We suggest that, with fossils, the designation fungal endophyte should be used strictly in a descriptive sense for all fungi occurring within intact plant cells or tissues in which there are no visible disease symptoms.