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Higher TaxaColubridae (Dipsadinae), Colubroidea, Caenophidia, Alethinophidia, Serpentes, Squamata (snakes)
Common NamesE: Guerrero Slender Leaf Litter Snake
S: Hojarasquera Delgada de Guerrero 
DistributionMexico (Guerrero, Oaxaca)

Type locality: near cueva de Tepozonales (17.2853 N, –99.3662 W; 432 m elevation), Cuajilotla, 20 km S Mochitlán, Guerrero, Mexico  
TypesHolotype: ENCB 18951, Adult female, obtained 17 July 2014 by L. F. Dávila Galavíz. 
DiagnosisDiagnosis: A member of Rhadinella, genus characterized by gracile body, head only slightly distinct from neck; inverted dark U-shaped marking on rostral; anterior supralabials usually with dark edges and pale centers; conspicuous or at least evidence of pale collar on neck; with reduced to small supracloacal tubercles in adult females; no subpreocular scale; posterior (2–7) teeth larger and thicker than preceding teeth, all enlarged teeth on same plane, no broad diastema; with basal nude pocket on hemipenis. Rhadinella dysmica is further characterized by the combination of a distinctive white collar, undivided at middorsum, extending 2.0–2.5 scale lengths at middorsum and 3–4 scale lengths on side of head; temporals 1 + 2; smooth dorsal scale rows 17, unreduced throughout body; ventrals 172; cloacal scute divided; divided subcaudals incomplete 63+N; dorsal coloration dark brown to blackish, almost totally obfuscating longitudinal striping, ground color barely extending to lateral edges of ventrals.
Rhadinella dysmica may be distinguished from many congeners (i.e., R. godmani, R. hannsteini, R. hempsteadae, R. kanalchuchan, R. kinkelini, R. lachrymans, R. montecristoi, R. pegosalyta, R. rogerromani, R. serperaster, and R. tolpanorum) by dark dorsal body coloration and absence of conspicuous striping. Additionally, the new species differs from R. godmani, R. hempsteadae, R. kanalchuchan, R. montecristoi, R. pegosalyta, and R. serperaster by having 17 dorsal scale rows (vs. more than 17 dorsal scale rows).
Rhadinella dysmica is morphologically similar to other species having mostly uniformly blackish dorsum, with or without indistinct dark vertebral line and pale dashes on some dorsal scales (i.e., R. schistosa, R. donaji, R. pilonaorum, and R. posadasi). In R. dysmica the color and configuration of the collar alone diagnostic from all other species of dark Rhadinella, extending from about posterior tips of parietals for several scale lengths, being white, and distinctly set off from mostly black head-cap. In R. shistosa collar U-shaped and usually broken medially (Fig. 3); in R. donaji white coloration of collar extends on to head, involving most of parietals and part of frontal; in R. pilonaorum collar orange with top of head mostly auburn mottled with black; in R. posadasi the collar is dark orange or auburn. Other features distinguishing these dark-colored species of Rhadinella include fewer ventrals (151–156 in females), fewer subcaudals (31–34 in females), and 1+1 temporals in Rhadinella shistosa; more subcaudals (100–111 in females) in R. pilonaorum; and fewer ventrals (143–146 in females) and more subcaudals (86–93 in females) in R. posadasi. Fourth and fifth infralabials contact posterior pair of genials in R. schistosa, R. pilonaorum, and R. posadasi (vs. the third and fourth in R. dysmica and R. donaji); distal portion of tails incomplete in holotypes of R. dysmica (a female) and R. donaji (a male), however with 66 subcaudals R. dysmica appears to have higher number than R. donaji, which has 40. (Campillo et al. 2016). 
CommentAbundance: very rare; described based on a single, female specimen; only known from 4 localities (Palacios-Aguilar et al. 2021).

Sympatry: Anolis dunni, Aspidoscelis guttata, Holcosus undulatus, Phyllodactylus lanei, Phyllodactylus tuberculosus, Ctenosaura pectinata, Sceloporus siniferus, Urosaurus bicarinatus, Boa imperator (sensu Hynková et al., 2009), and Leptodeira annulata.

Habitat: The individual was found near the entrance of the cave, crawling on the ground. 
EtymologyThe specific epithet is derived from the Greek ἐφέσπερος (dysmikos), meaning western, in reference to the westernmost distribution of this species of the genus. 
  • Ariano-Sánchez, D. y J. Campbell. 2018. A new species of Rhadinella (Serpentes: Dipsadidae) from the dry forest of Motagua Valley, Guatemala. Zootaxa 4442 (2): 338-344 - get paper here
  • CAMPILLO, GUSTAVO; LUIS FERNANDO DÁVILA-GALAVÍZ, OSCAR FLORES-VILLELA, JONATHAN A. CAMPBELL 2016. A new species of Rhadinella (Serpentes: Colubridae) from the Sierra Madre del Sur of Guerrero, Mexico. Zootaxa 4103 (2): 165-173 - get paper here
  • Mckay, J. Lindley; Olga Milenkaya, Christian Langner 2021. Neue Erkenntnisse zu Arealserweiterungen einiger Reptilienarten des mexikanischen Bundesstaates Oaxaca. Sauria 43 (2): 76-84
  • Palacios-Aguilar, Ricardo & OSCAR FLORES-VILLELA 2018. An updated checklist of the herpetofauna from Guerrero, Mexico. Zootaxa 4422 (1): 1-24 - get paper here
  • PALACIOS-AGUILAR, RICARDO; RUFINO SANTOS-BIBIANO, JONATHAN ATWOOD CAMPBELL & ELIZABETH BELTRÁN-SÁNCHEZ. 2021. Morphological variation and new distributional records of Rhadinella dysmica (Serpentes: Dipsadidae), with comparisons with other dark-colored congeners. Phyllomedusa: Journal of Herpetology 20(2): 151–159. - get paper here
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