The Early Devonian Rhynie chert has yielded the oldest evidence of hyphal investments (= mantles) in fungal reproductive units, with four different investment morphologies documented to date. This paper describes an unusual fungal reproductive unit from the Rhynie chert that is spheroid–elongate in outline, up to 60 μm in diameter, and consists of a thin-walled central cavity enveloped in a plectenchymatous or pseudoparenchymatous investment of tightly abutting, interwoven hyphae; pores extend through the investment. Several specimens contain a single spherical structure in the cavity. Because the structure of the investment differs from that seen in all other fossil fungal reproductive units, the new genus Scepasmatocarpion and species S. fenestrulatum are proposed for the new form. The systematic affinities of S. fenestrulatum remain unresolved. Nevertheless, the fossils support the hypothesis that the diversity of the fungi that produced hyphal envelopes was extensive by the Early Devonian.