Taxonomic reassessment and conservation status of the beaded lizard, Heloderma horridum (Squamata: Helodermatidae)

The beaded lizard (Heloderma horridum) and Gila monster (H. suspectum) are large, highly venomous, anguimorph lizards threatened by human persecution, habitat loss and degrada- tion, and climate change. A recent DNA-based phylogenetic analysis of helodermatids (Douglas et al. 2010. Molecular Phylogenetics and Evolution 55: 153-167) suggests that the current infraspecific taxonomy (subspecies) of beaded lizards underestimates their biodiversity, and that species status for the various subspecies is warranted. Those authors discussed "conservation phylogenetics," which incorporates historical genetics in conservation decisions. Here, we reassess the taxonomy of beaded lizards utilizing the abovementioned molecular analysis, and incorporate morphology by performing a character mapping analysis. Furthermore, utilizing fossil-calibrated sequence diver- gence results, we explore beaded lizard diversification against a backdrop of the origin, diversifica - tion, and expansion of seasonally dry tropical forests (SDTFs) in Mexico and Guatemala. These for- ests are the primary biomes occupied by beaded lizards, and in Mesoamerica most are considered threatened, endangered, or extirpated. Pair-wise net sequence divergence (%) values were greatest between H. h. charlesbogerti and H. h. exasperatum (9.8%), and least between H. h. alvarezi and H. h. charlesbogerti (1%). The former clade represents populations that are widely separated in distribu- tion (eastern Guatemala vs. southern Sonora, Mexico), whereas in the latter clade the populations are much closer (eastern Guatemala vs. Chiapas, Mexico). The nominate subspecies (Heloderma h. horridum) differed from the other subspecies of H. horridum at 5.4% to 7.1%. After diverging from a most-recent common ancestor ~35 mya in the Late Eocene, subsequent diversification (cladogen - esis) of beaded lizards occurred during the late Miocene (9.71 mya), followed by a lengthy stasis of up to 5 my, and further cladogenesis extended into the Pliocene and Pleistocene. In both beaded lizards and SDTFs, the tempo of evolution and diversification was uneven, and their current distribu - tions are fragmented. Based on multiple lines of evidence, including a review of the use of trinomi- als in taxonomy, we elevate the four subspecies of beaded lizards to full species: Heloderma alvarezi (Chiapan beaded lizard), H. charlesbogerti (Guatemalan beaded lizard), H. exasperatum Rio Fuerte beaded lizard), and H. horridum (Mexican beaded lizard), with no changes in their vernacular names. Finally, we propose a series of research programs and conservation recommendations.