Physcomitrium pygmaeum is an ephemeral moss described in 1871 from a single collection from Utah, currently considered conspecific with Physcomitrium pyriforme. The interpretation of the taxon has been problematic due to its rarity in the field, the elusiveness of the type material, and an extremely scattered and inconsistent collection record. Here we present a comprehensive description and assessment of the taxon following the identification of the original material and lectotype designation, the examination of all herbarium specimens existing to the best of our knowledge, the collection of fresh material in Nevada, and the molecular barcoding of the latter using four plastid and two nuclear loci. Available information, albeit scant, suggests that this member of the North American bryoflora should be considered critically endangered following IUCN criteria.

PCR products were sequenced in an ABI Prism 3100 Genetic Analyzer, assembled with Geneious (https://www.geneious.com/), manually aligned in PhyDE v. 0.9971 (http://phyde.de/), and converted to NEXUS format.

This set includes two NEXUS alignments.  Ppygmaeum_nuc_7379_10039.nex is the partial concatenation of the 7379 (positions 1-301) and 10039 (positions 302-456) nuclear regions from the bait set published in https://doi.org/10.5061/dryad.8rq9465. Ppygmaeum_plast_4loci.nex is the concatenarion of loci atpB-rbcL (positions 1-729), psbA-trnH (730-1167), rps4 (1168-1810), and trnL-F (1811-2319). Taxon names and accession numbers are referenced to herbarium vouchers in the publication.