Amphibia-Reptilia 41 (2020): 215-231 brill.com/amre The enigmatic Amazonian genus Eutrachelophis: morphological evidence and description of new taxa (Serpentes: Dipsadidae: Xenodontini) Hussam Zaher 1,∗ , Ana L.C. Prudente 2 Abstract. Eutrachelophis contains two species – Eu. bassleri and Eu. steinbachi – 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 Eutrachelophis 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 Eu. steinbachi in a new genus, due to the contrasting morphological disparities and lack of uniquely derived features shared with the remaining species of Eutrachelophis. 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 Eutrachelophis 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 Lygophis is morphologically intermediate between the highly specialized condition present in Eutrachelophis 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) Keywords: color patterns, head glands, hemipenis, skull, taxonomy. Introduction The Dipsadidae corresponds to the most species-rich snake family known to date. With a widespread distribution in Asia and the Amer- icas, the family reaches its highest diversity in the Neotropical region (Zaher et al., 2018), re- taining only relictual lineages in North Amer- ica and China (He et al., 2009; Wang et al., 2014; Zaher et al., 2019). Dipsadid monophyly is well-documented molecularly (e.g., Zaher et al., 2009; Grazziotin et al., 2012; Pyron et al., 2013; Wang et al., 2014; Zaher et al., 2019), being morphologically supported by two puta- tive hemipenial synapomorphies – body calyces 1 - Museu de Zoologia da Universidade de São Paulo, Avenida Nazaré 481, Ipiranga, São Paulo, CEP 04263-000, São Paulo, Brazil 2 - Museu Paraense Emílio Goeldi, Laboratório de Her- petologia, Avenida Perimetral 1901, CP 399, CEP 66040-170, Belém, Pará, Brazil ∗ Corresponding author; e-mail: hussam.zaher@gmail.com on the asulcate surface of the lobes and lat- eral enlarged spines disposed on the sides of the hemipenial body (Zaher, 1999; Zaher et al., 2009). Recently, Myers and McDowell (2014) de- scribed the dipsadid tribe Eutrachelophiini and its only genus Eutrachelophis to accommodate two rare species – Eu. bassleri Myers and Mc- Dowell, 2014 and Eu. steinbachi (Boulenger, 1905) – known to occur in the lowland rain forests of western (Ecuador, Peru and Bolivia) and eastern Amazonia (Brazil). Myers and Mc- Dowell (2014: 42) compared Eu. bassleri and Eu. steinbachi to a number of dipsadids and other colubroidean snakes and concluded that, despite striking differences in hemipenial mor- phology, great similarity in “external coloration, skull, dentition, head muscles, glands, and vis- ceral anatomy” warranted their allocation in the same genus. In the course of a series of surveys led by our institutions in the Brazilian states of Mato Grosso, Amazonas, and Acre, in the Amazonian © Koninklijke Brill NV, Leiden, 2019. DOI:10.1163/15685381-20191279 Downloaded from Brill.com11/06/2020 02:47:40PM via free access