Systematic revision of the Spotted and Northern Dusky Salamanders (Plethodontidae: Desmognathus conanti and D. fuscus), with six new species from the eastern United States.

Spotted and Northern Dusky Salamanders (Desmognathus conanti and D. fuscus) have a long and complex taxonomic history. At least 10 other currently recognized species in the genus were either described from populations previously considered D. fuscus, described as or later considered subspecies thereof, or later considered synonyms thereof, before ultimately being recognized as distinct. Recent molecular analyses have also revealed extensive cryptic diversity within both species, which are polyphyletic assemblages of 13 distinct mitochondrial lineages with 5.7-10.3% uncorrected 'p' distances in the COI barcode locus. Based on phylogenomic data and population-clustering analyses considering admixture between lineages, 11 candidate species were circumscribed by recent authors. Those within D. conanti are also ecomorphologically variable, comprising both large, robust, keel-tailed populations, and small, gracile, round-tailed forms. Evaluating their distinctiveness based on genetic, geographic, and morphological evidence, we conclude that six of the candidates represent new species: Desmognathus anicetus sp. nov., D. bairdi sp. nov., D. campi sp. nov., D. catahoula sp. nov., D. lycos sp. nov., and D. tilleyi sp. nov. Consequently, we recognize eight total species from populations formerly associated with the nominal species D. conanti and D. fuscus, the re-delimited concepts of which also contain additional phylogeographic lineage diversity that may represent further distinct species. In addition to existing mitochondrial and nuclear phylogenetic, network, and clustering results, we present preliminary analyses of linear morphometrics to bolster diagnostic specificity based on phenotypic characteristics. These changes stabilize the previously paraphyletic taxonomy of species-level lineages within Desmognathus, though additional cryptic diversity may exist both within the species considered here, and elsewhere in the genus.