Chersodromus australis, 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 : 159-161

Chersodromus australis sp. nov.

Fig. 6 , 7A Table 1

Holotype. An adult female, MZFC 17618 (field number EPR 388), collected by Edmundo Pérez Ramos on June 27 1995 at San Isidro La Gringa (17°04.591´N, 94°03.844’ W; 350 m asl), Municipality of Santa María Chimalapa, Oaxaca, Mexico.

Diagnosis. Chersodromus australis can be distinguished from all Mexican species of snakes by having combination of prefrontals fused into single scale; postocular discrete from supraocular; single anterior temporal present; dorsal scales keeled in 17 rows at midbody, unreduced posteriorly; supralabials usually 6, third and fourth entering orbit; infralabials 7–8, usually 1–4 contacting anterior chinshields); mental contacting anterior chinshields; ventral surface of body cream colored.

Chersodromus australis differs of C. rubriventris by having 17 scales around body (vs. 15), by having eight infralabials (vs. six or seven), by having a cream colored belly (vs. orange), and by having the mental in contact with first pair of chinshields (vs. not in contact). Chersodromus australis differs from C. liebmanni by having six supralabials (vs. seven), and from the new species that follows by having a dark belly (vs. cream-colored) and in having four infralabials contacting anterior chinshields (vs. five or six) ( Table 1).

Description of the holotype. Adult female; head length 7.3 mm; snout-vent length 180 mm; tail 40 mm; the tail comprises 22% of total length; head slightly distinct from neck; snout long, 2.6 times head length, rounded in dorsal aspect, projecting anteriorly beyond lower jaw; rostral 0.6 times broader than high, 1.9 times longer than internasal suture, with posterior end approximately at level of anterior margin of nares; internasals broader than long (length vs width on L/R sides 0.59/0.56), rounded anteriorly, in contact laterally with anterior and posterior nasals; prefrontals fused together, posterior corner contacting eye; postnasal plus loreal 0.8 times length of snout, suture between them 0.2 times frontal length; frontal longer than wide (width/length 1.1); supraocular large, in broad contact with prefrontal and parietal, contacting postocular and frontal 1.0 times as long as horizontal diameter of eye; supraocular 1.0 times as long as loreal, forming about posterior half of dorsal margin of orbit, ventral margin not projecting anteriorly beyond margin of orbit, posteriorly exceeding 0.4 times length of posterior margin of orbit; parietals 1.4 times longer than broad, approximately 0.4 times head length, parietal suture 0.8 times as long as frontal; one postocular on right side, two on left side; nasal divided; postnasal 1.0 times as long as prenasal; combined length of prenasal and postnasal 1.8 times longer than loreal length; loreal 1.1 times wider that long, 2.6 times snout length, 1.0 times longer than horizontal diameter of eye, dorsal margin straight and ventral margin concave; eye small, 2.4 times snout length, vertical diameter 0.6 times its distance from lip; supralabials six, first and second in contact with postnasal, second and third in contact with loreal, third and fourth entering orbit, fifth largest, contacting anterior temporal; one anterior temporal in broad contact with fifth supralabial; two posterior temporals separating sixth supralabial from parietal; upper posterior temporal larger that lower; posterior temporals separated from each other by five nuchals contacting parietals; mental rounded anterior, 2.2 times broader than long, contacting anterior chinshields; infralabials eight, first four contacting anterior chinshields, fourth slightly and fifth in broad contact with posterior chinshields; anterior chinshields longer than broad (2.1/1.8), 1.5 times longer than posterior chinshields; posterior chinshields broadly contacting anterior chinshields; one preventral scale; infralabials and scales in chin region smooth; dorsals in 17-17-17 rows, keeled throughout body, without apical pits; ventrals 131; cloacal scute single; paired subcaudals 36.

Color (alcohol after formalin). Dorsal ground color black, including lateral borders of ventral scales; dorsum of head with a white band crossing posterior region of parietals, including all of fifth and sixth supralabials, middle of fourth, and lower part of supralabials three and four; head, first three and portion of fourth supralabial black; mental, first three and portion of fourth infralabial black; lateral edges of ventrals colored similarly to dorsum; remaining ventral surface, including throat, cream; subcaudals cream with few scattered small black spots.

Maxillary dentition ( Fig. 6D ). Left maxilla extending anteriorly to the level of suture between second and third supralabials; dorsoventrally compressed; in lateral view with dorsal flange at about midlevel, tapering at anterior and posterior ends; anterior portion of edentate, followed by seven long, slender, curved teeth; anterior five teeth subequal, posterior two slightly smaller.

Etymology. The specific name is derived from the Latin australis , meaning southern, alluding to the southernmost distribution of species for the genus.

Distribution and habitat. Chersodromus australis apparently has a relatively small distribution east of the Isthmus of Tehuantepec ( Fig. 1 ) in the region known locally as Chimalapas; it occurs at low elevations (350 m asl) in tropical rain forest ( Fig 7 B ). A single specimen was obtained on June 1995 under a rock at 16:00 hrs on a mountain near San Isidro la Gringa, municipality of Santa María Chimalapa. Other amphibians and reptiles recorded at the type locality included Incilius macrocristatus (Firchein & Smith 1957) , Norops pygmaeus (Álvarez del Toro & Smith 1956), Lepidophyma tuxtlae Werler & Shannon 1957 , Bothrops asper (Garman 1884) , and Tantillita lintoni (Smith 1940) .

Figures

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

FIGURE 6. Dorsal (A), ventral (B), and lateral (C) views of the head (Scale = 3 mm); and its left maxilla (D, 2.43 mm) of the holotype of Chersodromus australis sp. nov. (MZFC 17618).

FIGURE 7. Holotype of Chersodromus australis sp. nov. (MZFC 17618) (A), and its type-locality in Chimalapas region of Oaxaca (B).