A new rare species of the Rhadinaeadecorata group from the Sierra Madre del Sur of Guerrero, Mexico (Squamata, Colubridae)

Abstract A new species of the Rhadinaeadecorata group is described based on two specimens from the Sierra Madre del Sur, Guerrero, Mexico. The new species differs from all other members of the genus Rhadinaea by having: eight supralabials; 149–151 (male) ventrals; 63–77 (male) subcaudals; two large pale nuchal blotches, forming an incomplete collar that occupies two scales laterally and is bissected along the dorsal midline; a postocular pale marking consisting of a well-defined, narrow line beginning behind the upper posterior margin of the eye and extending posteriorly nearly horizontally until connecting with the nuchal blotches; and the dark ground color of the flanks extending to the lateral portion of the ventrals. The large nuchal blotches distinguish the new species from the other members of the R.decorata group, except for R.cuneata and some individuals of R.hesperia (pale nuchal marking one-scale wide in R.marcellae, absent in the other species). The condition of the postocular pale marking distinguishes it from R.cuneata and R.hesperia (postocular pale marking wedge-shaped in R.cuneata, not connected with the pale post-cephalic markings in R.hesperia). Furthermore, the number of subcaudals and the coloration of the lateral portion of the ventrals distinguish it from R.omiltemana and R.taeniata, the remaining congeners found in Guerrero (85–90 in males of R.omiltemana and 91–121 in R.taeniata; dark color of the flanks not reaching ventrals in the former species, occasionally and faintly in R.taeniata). Additionally, a new combination for R.stadelmani is proposed. The new species is the first described in the genus Rhadinaea in more than 40 years.


Introduction
Snakes of the genus Rhadinaea Cope, 1863 (Colubridae: Dipsadinae) are distributed throughout Mesoamerica, ranging from the Sierra Madre Occidental of southern Sinaloa and Sierra Madre Oriental of northern Nuevo León in Mexico to northwestern Ecuador in South America, with an isolated species, R. flavilata (Cope, 1871), in the southeastern USA (Myers 1974). According to Myers (2011), the genus contains 20 species arranged in six species groups, mainly on the basis of their external morphology. These groups are (number of species in each group in parentheses) the R. calligaster (1), R. decorata (11), R. flavilata (2), R. taeniata (3), and R. vermiculaticeps (3) groups. Five species described after the publication of Myers's (1974) revision and originally placed in Rhadinaea have been transferred to Rhadinella (Myers 2011). Thus, no new species of Rhadinaea (sensu Myers 2011) has been described since Myers' systematic monograph of the group in 1974.
The Rhadinaea decorata group is characterized by the following combination of traits (Myers 1974): the hemipenis is single, without special features; a subpreocular is usually present; supracloacal ridges are usually present in males. The body is either striped or lined, with at least a hint of a narrow, linear dark marking involving dorsal scale rows 4 or 5, occasionally bordered above by a pale streak or series of small pale spots. A pale postocular marking extends from, or lays a short distance behind, the upper rear edge of the eye. The line may extend horizontally toward the neck or obliquely toward the corner of the mouth. Dorsal scales are arranged in 17-17-17 rows. Ventrals are 110-175 in males and 114-186 in females. Subcaudals are 56-137 in males and 60-120 in females. The tail comprises between 25% and 48% of the total length.
The Rhadinaea decorata group is the most diverse assemblage within the genus. Members of the group collectively range from the Mexican states of Tamaulipas and Sinaloa south and east to northwestern Ecuador, where they are mainly found in high mountains. All the species occur in Mexico and only R. decorata (Günther, 1858) is not endemic to the country (Myers 1974): R. bogertorum Myers, 1974 is known from northern Oaxaca (Myers 1974); R. myersi Rossman, 1965 from eastern Guerrero and southern Oaxaca (García-Vázquez et al. 2009); R. macdougalli Smith & Langebartel, 1949 from northern Oaxaca with an isolated population from Los Tuxtlas, Veracruz (Myers 1974, Pérez-Higareda et al. 2002; R. cuneata Myers, 1974 andR. forbesi Smith, 1942 from central Veracruz (Myers 1974); R. marcellae Taylor, 1949 from southern San Luis Potosí, Hidalgo, and northern Puebla (Nieto-Montes de Oca and Mendelson III 1997); R. quinquelineata Cope, 1886 from Hidalgo and northern Puebla (Myers 1974); R. gaigeae Bailey, 1937 from southern Tamaulipas, San Luis Potosí, Querétaro, and northern Hidalgo (Myers 1974); R. montana Smith, 1944 from central Nuevo León and western Tamaulipas (Myers 1974; R. hesperia Bailey, 1940 from the Sierra Madre del Sur of Guerrero and Oaxaca, the Pacific coast from southeastern Sinaloa to Oaxaca, and the Balsas Basin of Morelos and Puebla; and R. decorata from the Atlantic coast of southeastern San Luis Potosí, Mexico, southwards into northwestern Ecuador (Myers 1974).
During fieldwork conducted in the Sierra Madre del Sur, Guerrero, between 2006 and 2008 we collected an unusual individual of Rhadinaea in the vicinity of El Molote. In eleven trips to the locality we were unable to locate another specimen. Eight years after concluding our fieldwork another specimen with similar characteristics was collected ca. 3 km NW (in straight line) of where the first specimen was found. The snakes show a unique combination of characters and are apparently allopatric with respect to closely related species of Rhadinaea (see comparison section). These two individuals possess a unique combination of characteristics leading us to conclude they represent a new species that we describe below.

Materials and methods
Acronyms for herpetological collections follow Sabaj (2016), except for MZFC-HE for the Museo de Zoología of the Facultad de Ciencias, Universidad Nacional Autónoma de México. The specimens of the new species were fixed in 10% buffered formalin, subsequently transferred to 70% ethanol for permanent storage, and deposited in the herpetological collection of the MZFC-HE. We compared the new species with all the other species of Rhadinaea, based on the examination of 68 specimens belonging to 11 species, and also benefitted from data contained in the relevant literature (i.e. Myers 1974, Holm and Cruz 1994, Mendelson III and Kizirian 1995, Nieto-Montes de Oca and Mendelson III 1997, McCranie 2006, Myers 2011. We provide a list of the specimens examined in Supplementary material 1. We follow Myers (1974) for scale nomenclature. We performed scale counts under a dissecting microscope. We counted the ventrals as suggested by Myers (1974). We scored bilateral characters on both sides. When the condition of a given character was not identical on both sides, we give the conditions on the left and right sides, in that order, separated by a slash (/). We recorded measurements with a ruler (nearest 1 mm), digital callipers (nearest 0.1 mm), or an ocular micrometer to the nearest 0.1 mm. We measured head length from the tip of the snout to the angle of the jaw. We measured all scale dimensions at their maximum. We examined the hemipenial morphology in the new species after removal of the right hemipenis from the preserved holotype. The hemipenes were partially everted in the preserved specimens and therefore we could not record characters that are only visible in the retracted organs. We followed Pesantes (1994), Myers and Cadle (2003), Zaher and Prudente (2003), and Angarita-Sierra (2014) for hemipenial preparation. We follow Zaher (1999) and Myers and McDowell (2014) for hemipenial morphological terminology.

Results
Our review of the literature revealed the need to update the binomial name of Rhadinaea stadelmani Stuart & Bailey, 1941. The species was placed in the synonymy of Rhadinaea hempsteadae Stuart & Bailey, 1941 (= Rhadinella hempsteadae) by Myers (1974), but Mendelson III and Kizirian (1995) resurrected it based on additional material. Later, Myers (2011) resurrected the genus Rhadinella Smith, 1941 to accommodate the former members of the Rhadinaea godmani group, including Rhadinella hempsteadae. However, he did not comment on the generic placement of Rhadinaea stadelmani. Given the apparent close relationship of Rhadinaea stadelmani and Rhadinella hempsteadae and the morphological agreement of Rhadinaea stadelmani with the diagnosis of Rhadinella (Stuart and Bailey 1941, Myers 1974, 2011, Mendelson III and Kizirian 1995, we propose a new combination for Rhadinaea stadelmani as follows: Rhadinella stadelmani (Stuart & Bailey, 1941), comb. n.
Morphological examination of the specimens from El Molote supported their inclusion in the genus Rhadinaea (sensu Myers 2011), based on the following combination of traits (Myers 1974): hemipenis symmetrical, distally calyculate, unicapitate, spinose; sulcus spermaticus bifurcate; pupil round; full complement of colubrid head plates; subpreocular present; dorsal scales smooth, arranged in 17 rows with no posterior reduction; head with distinctive markings; body brown with longitudinal dark lines. Additionally, the specimens agree with Myers's (1974) definition of the R. decorata group presented above. However, the snakes share the presence of a unique set of character states that distinguish them from all known species of Rhadinaea (see below). The new species may be known subsequently as: Diagnosis. A colubrid snake of the Rhadinaea decorata group (sensu Myers 1974) that may be distinguished from all other members of the genus Rhadinaea by the following combination of character states: eight supralabials; 149-151 ventrals in males; 63-77 subcaudals in males; presence of two large pale nuchal blotches, forming an incomplete collar that occupies two scales laterally and bissected along the dorsal midline; postocular pale marking consisting of a well-defined and narrow line beginning anteriorly behind the upper posterior margin of the eye and extending posteriorly nearly horizontally until connecting with the nuchal blotches; and ground color of the flanks extending to the lateral portion of the ventrals.
Comparison. Rhadinaea nuchalis sp. n. may be distinguished from all other members of the genus Rhadinaea (except R. cuneata and some individuals of R. hesperia and R. omiltemana Günther, 1894) by the presence of two large pale nuchal blotches forming an incomplete collar that occupies two scales laterally and is bissected along the dorsal midline (pale nuchal marking one-scale long in R. laureata (Günther, 1868) and R. marcellae, absent in the other species). Rhadinaea nuchalis can be further distinguished from the members of the R. flavilata group by the presence of eight supralabials (usually seven in the R. flavilata group). Additionally, it differs from R. calligaster (Cope, 1875 Furthermore, Rhadinaea nuchalis can be distinguished from members of the R. vermiculaticeps group by having more ventrals in males (149-151 vs. 117-124 ventrals in males of the R. vermiculaticeps group). Rhadinaea nuchalis differs from R. cuneata by having less subcaudals in males (63-77 vs. 106-115) and by having a narrow postocular pale marking in the form of a nearly horizontal line (postocular pale marking wedge-shaped in R. cuneata). Specifically, R. nuchalis differs from other congeners that inhabit Guerrero except R. myersi by having fewer subcaudals in males (63-77 vs. 110-137 in R. hesperia; 85-90 in R. omiltemana; 91-121 in R. taeniata Peters, 1863). Additionally, it differs from R. myersi, R. omiltemana, and R. taeniata by having the dark ground color of the flanks extending to the lateral portion of the ventrals (dark ground color of the flanks not reaching ventrals in R. omiltemana, occasionally and faintly so in R. myersi and R. taeniata).
Description of holotype (Figs 1, 2). Male; adult; head length = 12.2 mm, snout-vent length (SVL) = 275 mm, tail length = 104 mm. Head distinct from neck; snout long, contained 2.5 times in head length, rounded from above, projecting anteriorly beyond lower jaw; rostral broader than high, portion visible from above 0.6 times as long as its distance from frontal, 0.5 times as long as internasal common suture, upper edge slightly above level of upper margin of nostrils; internasals  broader than long (width / length = 1.5), rounded laterally, contacting anterior and posterior nasals laterally, length and common suture ca. 0.8 and 0.4 times as long as prefrontal common suture, respectively; prefrontal contacting postnasal and loreal laterally, length ca. 0.3 times length of snout, common suture ca. 0.5 times frontal length; frontal longer than broad (width / length = 3.6/2.3), angulate posteriorly; supraocular large, contacting prefrontal, frontal, parietal, and upper postocular broadly, length ca. 1.3 times length of horizontal diameter of eye, 1.3/1.3 times as long as loreal, ventral margin projecting anteriorly and posteriorly beyond margins of orbit; parietals 1.6 times longer than broad, length approximately 0.4 times head length, common suture as long as frontal; nasal divided; prenasal 1.3/1 times as long as postnasal; prenasal and postnasal combined length ca. 2.3 times loreal length; loreal as high as long, contained 1.1 times in snout length, 0.4 times as long as horizontal diameter of eye, dorsal margin nearly straight; preocular single, 1.8 times higher than long; subpreocular present, tiny, separating preocular and fourth supralabial; postoculars two; upper postocular 1.4 times higher than long, 2.3 times longer than lower postocular; lower postocular 2.25 times longer than high; eye large, contained 4.1 times in snout length, vertical diameter 2.0 times distance from lip; supralabials 8/8, first and second contacting postnasal, second and third contacting loreal, fourth and fifth entering orbit, seventh largest, contacting anterior temporal, eight contacting lower posterior temporal; temporals 1 + 2; anterior temporal separating sixth and seventh supralabials from parietal; upper posterior temporals separated posteriorly by five nuchals; lower posterior temporal contacting anterior temporal and seventh supralabial anteriorly, eight supralabial ventrally. Mental 1.5 times broader than long, rounded anteriorly, separated from chinshields by first infralabials; infralabials 10/10, first to third contacting anterior chinshields, fourth to sixth separated from chinshields by interstitial skin, seventh to tenth separated by other scales; anterior chinshields 3.3 times longer than broad, as long as posterior chinshields; posterior chinshields separated from each other by two midgular scales.
Color (in life; Figs 3-4). Dorsum of head ochre, extending to rostral anteriorly, to third dorsal scale posterior to parietals posteriorly along the dorsal midline, narrowing occupying one dorsal scale laterally; extending to upper half of nasal, lateral portion of prefrontal, upper third of preocular, lateral edge of supraocular, uppermost portion of upper postocular, lateral portion of parietal, and ventral portion of upper secondary temporal and dorsal scale posterior to it ventrolaterally; irregular dark markings present, except on ventral border anteriorl to orbit (creating the appearance of a faint preocular line). Postocular pale marking consisting of a nearly horizontal, blackbordered, cream line; beginning anteriorly on rear upper fourth of orbit; passing along upper portion of upper postocular, lateral portion of parietal, and upper portion of primary temporal, connecting with nuchal blotches posteriorly. Lateral stripe dark brown, black-bordered; ventrally bordering faint preocular line and postocular pale marking; extending anteriorly to posterior nasal, eighth supralabial posteriorly; occupying upper border of supralabials 1-6, upper half of supralabial 7, and entire surface of supralabial 8 except for anteroventral corner. Ground coloration below dark lateral stripe white, interspaces between each scale bright pink; supralabials 1-7, mental, infralabials, and anterior chinshields with irregular dark markings.  Hemipenes ( Figure 5). Hemipenes unilobed, unicapitate, length ≈ 5 mm. Sulcus spermaticus centripetal basally, centrolineal distally; bifurcating at level of distal end of basal third of capitulum, terminating distally at level of basal end of distal third of capitulum; sulcus spermaticus walls smooth, well-defined; intrasulcar region nude. Capitulum calyculate, longer on sulcate side (≈ 2.5 mm, vs. ≈ 1.2 mm on asulcate side); calyces papillate in most of capitulum, spinulate near base of capitulum. Hemipenial body covered in spines distally; enlarged spines 30, slightly curved, larger and more abundant on lateral surfaces of hemipenial body and asulcate side than on sulcate side; area of hemipenial body below spinose section covered in small spinules; spinules surrounding sulcus spermaticus walls, covering larger area on sulcate and asulcate sides than on lateral surfaces of hemipenial body, separated from enlarged spines by triangular nude patch on asulcate side. Basal-most portion of hemipenial body nude.
Variation. The paratype differs from the holotype by having the upper posterior temporal divided into two small scales on the right side, 10/9 infralabials, the posterior chinshields separated from first ventral by two rows of small scales, 149 ventrals, and 77 subcaudals. No remarkable differences in color pattern are present in the paratype with respect to the holotype.
Etymology. The specific name nuchalis comes from the Latin nucha, meaning nape. It makes reference to the large nuchal blotches present in the new species.  Distribution and ecology. Rhadinaea nuchalis sp. n. is known only at intermediate elevations from the vicinity of El Molote in the western portion of the Sierra Madre del Sur of Guerrero. The species appears to be allopatric with the other species of the R. decorata group. The closest Rhadinaea record to R. nuchalis is that of R. omiltemana from El Tambor, Coyuca de Benítez, Guerrero, approximately 8 km E in straight line from the type locality of R. nuchalis (Palacios-Aguilar et al. 2016). The second closest record is that of R. taeniata from 1.5 mi N San Vicente de Jesus, Guerrero, about 18 km SSW in straight line from the type locality of R. nuchalis (Myers 1974). Other close records are those of R. omiltemana from Omiltemi and Asoleadero, R. hesperia from Acahuizotla and Chilpancingo, and R. taeniata from Chilpancingo, all in Guerrero (Fig. 6). All the three are about 72 km, straight line, from the type locality of R. nuchalis The closest records of species in the R. decorata group, excluding those of R. hesperia, to the type locality of R. nuchalis are those of R. myersi from Malinaltepec, Guerrero, and southwestern Oaxaca (García-Vázquez et al. 2006); and those of R. macdougalli and R. bogertorum from northern Oaxaca (Myers 1974, Ramírez-Bautista et al. 1998. The region of El Molote is characterized by rugged topography and the presence of numerous permanent streams that flow into the Atoyac and Coyuca rivers, whose basins belong to the Costa Grande hydrologic region (Lozada et al. 2003). Additional descriptions of the climate and other ecological aspects of El Molote can be found in Meza and López (1997) and Pavón-Vázquez et al. (2011). Coffee plantations have replaced most of the original cloud forest. The forest is dense and tall in undisturbed places, with the canopy reaching 25-30 m in height (Fig. 7). Pinus ayacahuite, P. strobus var. chiapensis, and Ulmus mexicana are emergent species, and Alfaroa costaricensis, Sloanea sp., Quercus salicifolia, Cojoba arborea, Magnolia schiedeana, and Zanthoxylum melanostictum are dominant species (Lozada et al. 2003).

Identification key
An identification key to the species of Rhadinaea was included in Myers's (1974) revision of the genus. Examination of the known specimens of R. nuchalis would lead to couplet number 40 in Myers's (1974)

Discussion
Color pattern has been considered one of the most informative characters for distinguishing between species and species groups within Rhadinaea: "…once some idea has been gained of intraspecific variation, color pattern offers the most generally reliable method of identifying species because no two forms have identical patterns…" (Myers 1974). Our own examination of eleven species of Rhadinaea, including all of those distributed in Guerrero, and a review of the relevant literature for the whole genus revealed the existence of intraspecific variation in color pattern for some species, but not a single specimen of another species exhibits the combination of coloration characters present in R. nuchalis. Additionally, R. nuchalis can be distinguished from morphologically similar species, except for R. forbesi, and its geographically closest congeners by the presence of a low number of subcaudals in males (see Comparison), and from R. forbesi by having a greater number of ventrals in males (149-151, vs. 136-149 in males of R. forbesi).
DNA sequences of Rhadinaea have been included in studies looking at phylogenetic relationships above the generic level (e.g., Lawson et al. 2005, Pyron et al. 2013), but a molecular assessment of the monophyly of Rhadinaea (sensu Myers 2011) and its species groups is still pending. However, species groups within Rhadinaea are divergent morphologically and assignment of R. nuchalis to the R. decorata group is relatively straightforward based on external and hemipenial morphology (see above). Thus, the definition of the group provided by Myers (1974) does not need to be modified by the inclusion of R. nuchalis.
The presence of a continuous median dark line, the extension of the dark ground color of the flanks onto the ventrals, the darker coloration of the flanks with respect to the stripes surrounding the median dark line, the relative length of the capitulum and sulcus spermaticus, and the high number of hemipenial spines suggest that Rhadinaea nuchalis may be closely related to R. hesperia (Myers 1974), its geographically closest congener within the R. decorata group. However, R. hesperia can be differentiated from R. nuchalis by the number of subcaudals in males (see Comparison) and ventral coloration (white anteriorly, grading to deep reddish posteriorly vs. white, suffused lightly with bright pink from head to level of mid-body in R. nuchalis). Additionally, although R. hesperia occasionally has a pair of slightly enlarged white blotches on the neck, in all cases the pale postocular marking does not coalesce with them.
The congeners geographically closest to Rhadinaea nuchalis outside the R. decorata group are R. omiltemana and R. taeniata, members of the R. taeniata group. They differ from R. nuchalis by having more subcaudals in males; the dark ground color of the flanks usually not reaching the ventrals, except for some individuals of R. taeniata in which case the color is faint (see Comparison); a broad (involving at least five dorsal scale rows) dorsal stripe darker than the flanks (median dark line flanked by two stripes paler than the flanks and occupying only the vertebral row and the innermost portions of adjacent scale rows in R. nuchalis); and a relatively small capitulum, comprising between two-sevenths and two-fifths of the length of the sulcate side of the hemipenis (capitulum comprising approximately half of length of the sulcate side in R. nuchalis).
The Sierra Madre del Sur of Guerrero has received scant attention from herpetologists and most expeditions have focused on the central region of the state, particularly in the Chilpancingo region (i.e. Armstrong 1979, Adler 1996). However, the west and east portions of the Sierra have been largely ignored. Recent visits to the west portion have yielded a number of discoveries of new species (e.g., Campbell and Flores-Villela 2008, Campbell et al. 2009, Pavón-Vázquez et al. 2011, Feria-Ortiz and García-Vázquez 2012, Campbell et al. 2014. This suggests that the diversity of the Sierra Madre del Sur of Guerrero is higher than currently known (Palacios-Aguilar and Flores-Villela 2018), particularly for small and secretive snakes as exemplified by the present work and the recent descriptions of Epictia schneideri Wallach, 2016, Geophis occabus Pavón-Vázquez, García-Vázquez, Blancas-Hernández & Nieto-Montes de Oca, 2011, and Rhadinella dysmica Campillo, Dávila-Galavíz, Flores-Villela & Campbell, 2016.