The enigmatic Amazonian genus Eutrachelophis: morphological evidence and description of a new taxa (Serpentes: Dipsadidae: Xenodontini)

<p><i>Eutrachelophis</i> contains two species – <i>Eu. bassleri</i> and <i>Eu. steinbachi </i>– that are known from the lowland rainforests of western Amazonia (Ecuador, Peru, and Bolivia). Due to their unusual hemipenial morphology, they have been considered to belong to a separate tribe – Eutrachelophiini – of dipsadids. Here, we describe a new species of <i>Eutrachelophis</i> that fills an important morphological gap in the hemipenial pattern known for this genus. Although highly modified, apical disks are recognizable in the hemipenes of both species, supporting their inclusion in the tribe Xenodontini. We further allocate <i>Eu. steinbachi</i> in a new genus, due to the contrasting morphological disparities and lack of uniquely derived features shared with the remaining species of <i>Eutrachelophis</i>. The new genus can be distinguished from all other genera of Dipsadidae by the presence of deeply divided lobes with lobular projections that expand beyond the tip of the sulci, the latter ending on the middle surface of the lobes where it opens at the base of a small nude area considered to be the remnant of the Xenodontini apical disks. The two remaining species of <i>Eutrachelophis</i> retain well-developed Xenodontini apical disks that are expanded throughout the lobular region reaching each other’s edges at midline due to the fusion of both lobes in one unique structure. The condition observed in the genus <i>Lygophis</i> is morphologically intermediate between the highly specialized condition present in <i>Eutrachelophis</i> and the one known to characterize other Xenodontini, supporting the allocation of this genus in the tribe (Zoobank: www.zoobank.org/urn:lsid:zoobank.org:pub:96725BD0-D9E6-4A85-A4BD-D6BF988CFC5E).<b></b></p>