Despite escalating efforts to document and understand biodiversity, gaps in knowledge persist due to the inherent complexity of biological systems and the sheer scale of global biodiversity. Such knowledge shortfalls have been identified, described, and discussed for several well-studied organismal groups, leading to significant advancements in research and conservation. However, less-studied organismal groups (including most fungi) continue to suffer from pronounced knowledge gaps. Recently, Laboulbeniomycetes (phylum Ascomycota), a class of minute fungi obligately associated with arthropods either for dispersal or as epibionts, has been proposed as a model system to illustrate the extent of biodiversity and conservation shortfalls in poorly known organismal groups. Here, we explore biodiversity knowledge shortfalls for the Laboulbeniomycetes, examining gaps in species diversity (Linnean), distribution (Wallacean), populations (Prestonian), evolution (Darwinian), functional traits and ecological functions (Raunkiaeran), abiotic tolerance (Hutchinsonian), ecological interactions (Eltonian), natural history knowledge (Parkerian), persistence (Latimerian), conservation assessment (Scottian), and applied conservation, legal policies, fundings, and public awareness (Ostromian). Additionally, we introduce the Thaxterian shortfall, named after Roland Thaxter (1858–1932), professor at Harvard University and leading expert of Laboulbeniales, that quantifies the lack of taxonomic expertise in a given organismal group. We also discuss the implications of these shortfalls for conservation and future research avenues. We hope this chapter will serve as a catalyst for further discussion on biodiversity shortfalls in other taxa and pave the way towards a global momentum for the integration of poorly known organisms into conservation frameworks.