In recent years studies of the Rhynie chert have contributed appreciably to our understanding of the fungi that existed during the Lower Devonian. One of the recently documented groups is the ascomycetes, the largest group of true fungi with nearly two thousand genera. The principal morphological feature of ascomycetes is the sac-like ascus in which sexual ascospores are produced. Sections of Asteroxylon aerial stems and rhizomes have been discovered that contain numerous perithecia that appear just below the epidermis, often associated with a stomatal complex. In a few specimens perithecia occur at the base of enations. Perithecia are globose-spherical and possess a slightly elongate neck. The ostiole is lined with periphyses. Extending from a two-layered hymenium are elongate paraphyses and unitunicate asci. Ascospores may be uni-multicelled, elongate, and demonstrate germination at either end. Also present on some axes are unbranched conidiophores and cube-shaped arthrospores produced by segmentation of side branches. The fossil ascomycetes provide critical information about this unique group of true fungi, and offer important characters that can be used in establishing a major lineage of Euascomycetes with perithecial ascocarps.

Key words: Ascomycetes, Devonian, fungi, Rhynie chert