Extending the fossil record for foliicolous Dothideomycetes: Bleximothyrium ostiolatum gen. et sp. nov., a unique fly-speck fungus from the Lower Cretaceous of Virginia, USA

Abstract

PREMISE: Fossils can reveal long-vanished characters that inform inferences about the timing and patterns of diversification of living fungi. Through analyzing well-preserved fossil scutella, shield-like covers of fungal sporocarps, we describe a new taxon of early Dothideomycetes with a combination of characters unknown among extant taxa. METHODS: Macerated clays from the Potomac Group, lower Zone 1, from the Lower Cretaceous (Aptian, 125–113 Ma) of Virginia USA yielded one gymnospermous leaf cuticle colonized by 21 sporocarps of a single fungal morphotype. We inferred a tree from nuclear ribosomal DNA of extant species, and coded morphological characters to evaluate alternative, equally parsimonious placements of the fossil in a molecular constraint tree of extant species. RESULTS: Bleximothyrium ostiolatum gen. et sp. nov. has an ostiolate scutellum of radiate, dichotomizing hyphae. Unlike otherwise similar extant and fossil taxa, B. ostiolatum has tangled hyphae at its scutellum margin. Scutella of B. ostiolatum are connected to superficial mycelium, to intercalary and lateral appressoria, and to extensive subcuticular “mycélium en palmettes”. The gymnospermous host has characters consistent with identity as a non-papillate ginkgophyte or cycad. CONCLUSIONS: Bleximothyrium ostiolatum is the oldest known fossil fly-speck fungus that occurs on plant cuticles and has the radiate, ostiolate scutellum known only from Dothideomycetes. Its combination of characters, its scutellum margin, and mycélium en palmettes are unknown in other extant and fossil species, and Bleximothyrium ostiolatum likely represents a new group of fly-speck fungi that may now be extinct.