Morphology and ontogenetic development of Zwergimyces vestitus, a fungal reproductive unit enveloped in a hyphal mantle from the Lower Devonian Rhynie chert

Zwergimyces vestitus, a fungal reproductive unit enveloped in a mantle of interlaced circumferential hyphae from the Lower Devonian Rhynie chert, was formally described based on limited information obtained from six specimens. In this paper, we provide additional insights into the organization of Z. vestitus, together with specific developmental details, from > 100 new specimens. A marked expansion of the hyphal mantle may occur on one side of this reproductive unit. Moreover, the units appear to have been embedded in a loose meshwork of delicate hyphae. Variations in the organization of the mantle among the specimens suggest mantle formation took place by repeated branching of hyphae on the surface of the developing unit and by additional hyphae extending between the pre-existing mantle hyphae. What was initially described as one non-hyphal wall lining the inner surface of the mantle is now demonstrated as an outer sporangium wall, which continues into the subtending hypha, and an inner spore wall. The diagnoses for Zwergimyces and Z. vestitus are emended. Small spherules in several Z. vestitus specimens are interpreted as intrusive organisms. The new material of Z. vestitus provides the first insights into the ontogenetic development of one of the Rhynie chert mantled fungal reproductive units.