Scale by scale, river by river: On the taxonomy, osteology and distribution of the Amazonian snake Helicops acangussu

ABSTRACT Helicops acangussu was described in 2022 based on 27 specimens collected on the left bank of the Madeira River, state of Rondônia, Brazil. During visits to South American collections, we found 34 new specimens, most from regions about 1000 km away from the type locality, in the Amazon region of Colombia (municipality of Leticia) and Brazil (municipalities of Tabatinga and Presidente Figueiredo). The detailed examination of the material from these new populations allowed us to provide the first description of the skull, lower jaw and cervical osteology for the species, as well as to expand the external morphological variation of meristic and biometric characters and provide new data on the morphology of the hemipenis. We show that Helicops acangussu is a species with a solid taxonomy, but with pronounced intraspecific variation in hemipenis morphology and a wide distribution in the Amazon region.