The beaded lizard (Heloderma horridum) has a fragmented distribution in Mesoamerica that extends from northwestern Mexico to eastern Guatemala. Locally known as the “escorpión” its natural history is surrounded by mystery, notoriety and misconception. Consequently, it is frequently slaughtered when encountered. Adding to this anthropogenic pressure, beaded lizard populations, with rare exceptions occur primarily in seasonally dry tropical forests, the most endangered biome in Mesoamerica owing to persistent deforestation for agriculture, cattle ranching, and a burgeoning human population. Drought and fires escalate the above threats and recent predictive models of climate change suggest persistence of seasonally dry tropical forests in this region is highly dubious.
Despite its large size and charismatic nature, knowledge of the ecology, geographic distribution, and status of beaded lizard populations remains limited.
Reiserer et al. (2013) reassess the taxonomic status of the beaded lizard populations using morphology, biogeography, and a recent molecular-based analysis conducted by Douglas et al. (2010).
The authors found the greatest divergence between H. h. charlesbogerti and H. h. exasperatum (9.8%), and the least between H. h. alvarezi and H. h. charlesbogerti (1%). The former clade represents populations that are widely separated in distribution (eastern Guatemala vs. southern Sonora, Mexico), while 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 by 5.4% to 7.1%.
The new study suggests beaded lizards diverged from a most-recent common ancestor about 35 million years ago in the Late Eocene, and subsequent diversification occurred during the late Miocene (9.71 mya), followed by a lengthy stasis of up to five million years, more recent diversification extended into the Pliocene and Pleistocene. In both beaded lizards and seasonally dry tropical forests, the tempo of evolution and diversification was uneven, and their current distributions are fragmented. Based on multiple lines of evidence, including a review of the use of trinomials in taxonomy, the authors elevated the four subspecies of beaded lizards to full species: Heloderma alvarezi (Chiapan beaded lizard), H. charlesbogerti (Guatemalan beaded lizard), H. exasperatum (Río Fuerte beaded lizard), and H. horridum (Mexican beaded lizard), with no changes in their vernacular names. The full article is available on-online.
Reiserer RS, Schuett GW, Beck DD. 2013. Taxonomic reassessment and conservation status of the beaded lizard, Heloderma horridum (Squamata: Helodermatidae). Amphibian & Reptile Conservation 7(1): 74–96.
Douglas ME, Douglas MR, Schuett GW, Beck DD, Sullivan BK. 2010. Conservation phylogenetics of helodermatid lizards using multiple molecular markers and a supertree approach. Molecular Phylogenetics and Evolution 55: 153–167.