165 Morphological variaton wi thin Thamnodynastes pallidus (Linnaeus, 1758) (Serpentes: Dipsadidae: Xenodon tnae: Tachymenini) Romulo Pantoja Nóbrega 1 , Giovanna Gondim Montngelli 2 , Vivian Trevine 2,3 , Francisco Luis Franco 4 , Gustavo H.C. Vieira 1 , Gabriel C. Costa 5 & Daniel Oliveira Mesquita 1 1 Universidade Federal da Paraíba, Centro de Ciências Exatas e da Natureza, Departamento de Sistemátca e Ecologia. CEP: 58051-900 - João Pessoa, PB – Brasil 2 Museu de Zoologia da Universidade de São Paulo, Avenida Nazaré, 481 Ipiranga - CEP: 04263-000 - São Paulo, SP, Brasil 3 Insttuto de Biociências, Universidade de São Paulo, Departamento de Zoologia, Rua do Matão, Travessa 14, 101, Cidade Universitária - CEP 05508-090 - São Paulo, SP, Brazil 4 Insttuto Butantan, Divisão de Desenvolvimento Cientfco, Laboratório Especial de Coleções Zoológicas. Av. Dr. Vital Brasil, 1500,Bairro Butantã – CEP: 05503900 - São Paulo, SP – Brasil 5 Departamento de Ecologia, Centro de Biociências, Universidade Federal do Rio Grande do Norte, Campus Universitário - Lagoa Nova, Natal, RN, 59072-970, Brasil Herpetological Journal FULL PAPER Correspondence: Daniel Oliveira Mesquita (danmesq@dse.ufpb.br) Volume 26 (April 2016), 165–174 Published by the Britsh Herpetological Society InTroDucTIon M orphological variaton has been studied in a wide range of organisms, and its study reveals a broad number of causes to account for it. For example, divergent selectve (ecological) pressures can trigger and maintain phenotypic diversifcaton (Glor et al., 2003; Langerhans et al., 2004; Stuessy et al., 2006). Morphological differences due to sexual dimorphism (Bruner et al., 2005), and seasonal climate oscillations (Poroshin et al., 2010) have also been reported. Among snakes, the scenario is similar, and many examples of morphological variation within and among populations (and among distinct species) exist (e.g., Shine, 1986; Forsman & Shine, 1997; Pizzato & Marques, 2006; Martnez-Freiria et al., 2009; Pyron & Burbrink, 2009). Many studies relate morphology (colouraton and morphometrics) and its variaton to species evoluton and/or ecology. However, most studies have considered morphological variaton for taxonomic purposes. The genus Thamnodynastes is comprised of 19 valid species distributed throughout South America. Thamnodynastes pallidus is associated with the Amazon region and the Atlantc forest of northeastern Brazil, exhibitng a disjunct distributon. The characters employed in the defniton of this species are controversial, and its morphological variaton is poorly known. Some authors do not consider its distributon in the Atlantc Forest, atributng these specimens to T. almae. This study aims to compare the Amazonian and the Atlantc populatons of T. pallidus by performing an analysis of morphological (colouraton, morphometry, pholidosis and hemipenial morphology) and geographical variatons. We examined 70 specimens of T. pallidus from the Atlantc Forest, and 61 from the Amazon Forest. A logistc regression selected the number of infralabials, number of subcaudals, and snout length as the only predictors that could discriminate the two populatons. The distributon model shows regions with higher climatc suitability for T. pallidus spread across the Amazon basin and the Atlantc Forest. We provide sufcient evidence to characterise T. pallidus, and diferentate it from its congeners. Although we demonstrate the occurrence of variaton with respect to some meristc and hemipenial characters between and within each populaton, we conclude that these variatons are not sufcient to recognise them as distnct species. Key words: Amazon Forest, Atlantc Forest, hemipenis, pholidosis, South America, Squamata The genus Thamnodynastes is comprised of 19 valid species distributed throughout South America from lattudes 10°N (Northern Colombia) to 37°S (Southern Argentina) (Cei et al., 1992; Franco & Ferreira, 2002; Bailey et al., 2005; Bailey & Thomas, 2007). The genus is composed of medium-sized species (maximum SVL length: 620 mm), with 17 or 19 rows of dorsal scales on the midbody, and posterior dorsal-scale reducton; keeled or smooth scales; usually one apical pit present in dorsal scales; a divided cloacal scale (except for T. pallidus); elliptcal pupil; sexual dimorphism in ventral scales; and viviparity (Franco & Ferreira, 2002; Bailey et al., 2005). As a brief taxonomic history, T. pallidus was described by Linnaeus in 1758 as Coluber pallidus. In 1824, Wagler, in Spix described Natrix puncatissima based on a specimen from Bahia (Brazil). Subsequently, in 1830, Wagler described the genus Thamnodynastes, designatng the type-species as N. punctatssima Wagler, 1824, by monotypy. In 1899, Andersson synonymised