Chersodromus nigrum, Canseco-Márquez & Ramírez-González & Campbell, 2018

Canseco-Márquez, Luis, Ramírez-González, Cynthia G. & Campbell, Jonathan A., 2018, Taxonomic review of the rare Mexican snake genus Chersodromus (Serpentes: Dipsadidae), with the description of two new species, Zootaxa 4399 (2), pp. 151-169 : 162-165

Chersodromus nigrum sp. nov.

Fig. 8 , 9A–B , Table 1

Holotype. An adult male, MZFC 17619 (field no. ISZ 063), collected by Israel Solano Zavaleta on May 28 2005 at Xucayucan (19°53’47.9” N, 97°28’43.7” W; 1493 m asl), Municipality of Tlatlauquitepec , Puebla, Mexico.

Paratypes. Juvenile female, UTA R-63417 (field number UOGV 062 ; Fig. 9 A ) collected by Uri Omar García Vázquez on February 25 2002 at 5.5 km east from Tahitic (19° 55’ 33.9” N, 97° 31’ 42.4” W; 1530 m asl), Municipality of Zacapoaxtla , Puebla, Mexico; and an adult female MZFC 17661 (field number ISZ 059), same provenance as the holotype, obtained dead on road on May 27 2005 .

Diagnosis. Chersodromus nigrum can be distinguished from all Mexican species of snakes by having combination of prefrontals fused into single scale; postocular fused with supraocular; single anterior temporal present; dorsal scales keeled in 17 rows at midbody, unreduced posteriorly; supralabials 6, third and fourth entering orbit; infralabials 8, 1–5 contacting anterior chinshields); mental contacting anterior chinshields; venter mostly black.

This species is characterized by its large size (260-315 mm SVL). It differs from Chersodromus rubriventris and C. australis by having fused supraocular and postocular scales; distinguished from C. australis and usually C. liebmanni by having first 5 or 6 infralabials in contact with anterior chinshield (vs. 4–5), further differing from C. liebmanni by having six supralabials (vs. seven) ( Table 1).

Description of the holotype. Adult male; head length 9.07 mm; snout-vent length 260 mm; tail 74 mm; the tail comprises 22% of total length in males, 18–20% in females; head slightly distinct from neck; snout long, 2.5 times head length, rounded in dorsal aspect, projecting anteriorly beyond lower jaw; rostral 0.4 times broader than high, 2.7 times longer than internasal suture, with anterior end at level of anterior margin of nares; internasals broader than long (length vs breadth on L/R sides 0.63/0.57), rounded anteriorly, in contact laterally with anterior and posterior nasals; prefrontals fused, posterior corner contacting eye; postnasal plus loreal length 0.6 times length of snout, their common suture 0.2 times frontal length; frontal wider than long (length/breadth 0.8); supraocular fused with postocular; parietals 1.6 times longer than wide, approximately 0.5 times head length, parietal suture 1.1 times as long as frontal; large postocular on each side; nasal divided; postnasal 1.0 times as long as prenasal; combined length of prenasal and postnasal 2.2 times longer than loreal length; loreal 1.5 times wider than long, 3.7/ 3.4 times snout length, 1.1 times longer than horizontal diameter of eye, dorsal margin convex and ventral margin straight; eye small, 2.9 times snout length; supralabials six, first and second contacting postnasal, second and third contacting loreal, third and fourth contacting orbit, the upper posterior portion of fourth contacting postocular and anterior temporal, fifth largest, contacting anterior temporal; one anterior temporal broadly contacting fifth supralabial; two posterior temporal separating sixth supralabial from parietal; upper posterior temporal larger than lower; posterior temporals separated posteriorly from each other by seven nuchals contacting parietals; mental rounded anteriorly, 2.1 times wider than long, contacting anterior chinshields; infralabials eight, first five contacting anterior chinshields, sixth in contact with posterior chinshields; anterior chinshields longer than broad (1.7/2.2),1.9 times longer than posterior chinshields; posterior chinshields broadly contacting anterior chinshields; two preventral scales; infralabials smooth, anterior chinshield with pustules; dorsals in 17-17-17 rows, weakly keeled throughout body, without apical pits; ventrals 131; cloacal scute single; paired subcaudals 44.

Color (alcohol after formalin). Dorsal ground color black; dorsum of head with cream band crossing posterior two-thirds of parietals, including half of the sixth supralabial and most of fifth; narrow lower portions of supralabials 2–4 cream colored or with cream spots, otherwise first four supralabials mostly black; mental and first five infralabials completely black; sixth supralabials with dark spots or mottling; chinshields and throat with irregularly scattered black spots; ventral surface mostly dark with heavy black mottling on a white background, subcaudals completely black.

Maxillary dentition ( Fig. 8D ). Right maxilla extending anteriorly to the level of suture between second and third supralabials; dorsoventrally compressed; in lateral view bone robust throughout most of length, tapering slightly posteriorly; eight long, slender, recurved maxillary teeth very slightly decreasing in size posteriorly.

Hemipenis morphology ( Fig. 8 E–G ). Hemipenis cylindrical, slightly bilobed, covered homogeneously by spines, sulcus spermaticus centrifugal, bifurcate above mid-level of hemipenis body, walls of sulcus spermaticus robust and ornamented with spinules; intra-sulcar region covered with calyces. Two rows of hook-shaped spines present over most proximal area of hemipenis body. Basally, asulcate side with two spines resembling parallel hooks. Four rows inverted “V” shaped of spines, at level of bifurcation point of sulcus spermaticus. Distal portion of hemipenes covered by calyces with larges spines.

Variation. The two paratypes are females, one of which (MZFC 17661) has 315 mm SVL and 81 mm TL and is larger than the male holotype. The other female (UTA R-63417, Fig. 9A ) is 102 mm SVL and 21.9 mm TL. The tail comprises 28% of SVL in the male holotype, and 21–25 % in female paratypes. There are 137 ventrals and 32 subcaudals (broken tail) in MZFC 17661, and 129 ventrals and 44 subcaudals in UTA R-63417. Both specimens have five infralabials contacting the anterior chinshield. The collar in life varies from pale cream to yellowish ( Fig. 9A–B ).

Etymology. The specific name is from Latin nigrum , meaning black, alluding at to the coloration of the dorsal and ventral surface.

Distribution and habitat. This species is occurs in the cloud forests of the Sierra Madre Oriental of northern Puebla ( Fig. 1 and 9 C ) at elevations between 1493–1560 m asl. Other species of amphibians and reptiles recorded in the area include, Chiropterotriton sp., Charadrahyla taeniopus (Günther 1901) , Craugastor rhodopis (Cope 1867) , Rheohyla miotympanum (Cope 1862) , Diploglossus legnotus Campbell & Camarillo 1994 , Scincella silvicola (Taylor 1937) and Geophis sp.

Figures

FIGURE 1. Geographic distribution of the currently recognized species of the genus Chersodromus.

FIGURE 8. Dorsal (A), ventral (B), and lateral (C) views of the head (Scale = 3 mm); right maxilla (D, 2.42 mm); and from left to right: sulcate, lateral, and asulcate views of the hemipenes (E–G, Scale = 2.8 mm) of the holotype of Chersodromus nigrum sp. nov. (MZFC 17619).

FIGURE 9. Juvenile paratype (A, UTA R-63417) and adult (B) specimens in life of Chersodromus nigrum sp. nov., and its type locality near Xucayucan, Puebla (C).